Rheology and Compatibility of HPMC/HPS Complex
Key words: hydroxypropyl methylcellulose; hydroxypropyl starch; rheological properties; compatibility; chemical modification.
Hydroxypropyl methylcellulose (HPMC) is a polysaccharide polymer commonly used in the preparation of edible films. It is widely used in the field of food and medicine. The film has good transparency, mechanical properties and oil barrier properties. However, HPMC is a thermally induced gel, which leads to its poor processing performance at low temperature and high production energy consumption; in addition, its expensive raw material price limits its wide application including the pharmaceutical field. Hydroxypropyl starch (HPS) is an edible material widely used in the field of food and medicine. It has a wide range of sources and low price. It is an ideal material to reduce the cost of HPMC. Moreover, the cold gel properties of HPS can balance the viscosity and other rheological properties of HPMC. , to improve its processing performance at low temperature. In addition, HPS edible film has excellent oxygen barrier properties, so it can significantly improve the oxygen barrier properties of HPMC edible film.
HPS was added into HPMC for compounding, and the HPMC/HPS cold and hot reversed-phase gel compound system was constructed. The influence law of properties was discussed, the interaction mechanism between HPS and HPMC in solution, the compatibility and phase transition of the compound system were discussed, and the relationship between the rheological properties and structure of the compound system was established. The results show that the compound system has a critical concentration (8%), below the critical concentration, HPMC and HPS exist in independent molecular chains and phase regions; above the critical concentration, the HPS phase is formed in the solution as the gel center, The microgel structure, which is connected by the intertwining of HPMC molecular chains, exhibits a behavior similar to that of a polymer melt. The rheological properties of the compound system and the compound ratio conform to the logarithmic sum rule, and show a certain degree of positive and negative deviation, indicating that the two components have good compatibility. The compound system is a continuous phase-dispersed phase “sea-island” structure at low temperature, and the continuous phase transition occurs at 4:6 with the decrease of the HPMC/HPS compound ratio.
As an important component of food commodities, food packaging can prevent food from being damaged and polluted by external factors in the process of circulation and storage, thereby extending the shelf life and storage period of food. As a new type of food packaging material that is safe and edible, and even has a certain nutritional value, edible film has broad application prospects in food packaging and preservation, fast food and pharmaceutical capsules, and has become a research hotspot in the current food packaging related fields.
The HPMC/HPS composite membrane was prepared by casting method. The compatibility and phase separation of the composite system were further explored by scanning electron microscopy, dynamic thermomechanical property analysis and thermogravimetric analysis, and the mechanical properties of the composite membrane were studied. and oxygen permeability and other membrane properties. The results show that no obvious two-phase interface is found in the SEM images of all composite films, there is only one glass transition point in the DMA results of most of the composite films, and only one thermal degradation peak appears in the DTG curves of most of the composite films. HPMC has certain compatibility with HPS. The addition of HPS to HPMC significantly improves the oxygen barrier properties of the composite membrane. The mechanical properties of the composite membrane vary greatly with the compounding ratio and the relative humidity of the environment, and present a crossover point, which can provide a reference for product optimization for different application requirements.
The microscopic morphology, phase distribution, phase transition and other microstructures of the HPMC/HPS compound system were studied by simple iodine dyeing optical microscope analysis, and the transparency and mechanical properties of the compound system were studied by ultraviolet spectrophotometer and mechanical property tester. The relationship between the microscopic morphological structure and the macroscopic comprehensive performance of the HPMC/HPS compound system was established. The results show that a large number of mesophases are present in the compound system, which has good compatibility. There is a phase transition point in the compound system, and this phase transition point has a certain compound ratio and solution concentration dependence. The lowest point of transparency of the compound system is consistent with the phase transition point of HPMC from continuous phase to dispersed phase and the minimum point of tensile modulus. The Young’s modulus and elongation at break decreased with the increase of the solution concentration, which had a causal relationship with the transition of HPMC from the continuous phase to the dispersed phase.
A rheometer was used to study the effect of chemical modification of HPS on the rheological properties and gel properties of the HPMC/HPS cold and hot reversed-phase gel compound system. Capacities and phase transitions were studied, and the relationship between microstructure and rheological and gel properties was established. The research results show that the hydroxypropylation of HPS can reduce the viscosity of the compound system at low temperature, improve the fluidity of the compound solution, and reduce the phenomenon of shear thinning; the hydroxypropylation of HPS can narrow the linear viscosity of the compound system. In the elastic region, the phase transition temperature of the HPMC/HPS compound system is reduced, and the solid-like behavior of the compound system at low temperature and the fluidity at high temperature are improved. HPMC and HPS form continuous phases at low and high temperatures, respectively, and as dispersed phases determine the rheological properties and gel properties of the composite system at high and low temperatures. Both the abrupt change in the viscosity curve of the compounded system and the tan delta peak in the loss factor curve appear at 45 °C, which echoes the co-continuous phase phenomenon observed in the iodine-stained micrographs at 45 °C.
The effect of chemical modification of HPS on the crystalline structure and micro-divisional structure of the composite film was studied by synchrotron radiation small-angle X-ray scattering technology, and the mechanical properties, oxygen barrier properties and thermal stability of the composite film were systematically studied the influence of chemical structure changes of compound components on the microstructure and macroscopic properties of compound systems. The results of synchrotron radiation showed that the hydroxypropylation of HPS and the improvement of the compatibility of the two components could significantly inhibit the recrystallization of starch in the membrane and promote the formation of a looser self-similar structure in the composite membrane. The macroscopic properties such as mechanical properties, thermal stability and oxygen permeability of HPMC/HPS composite membrane are closely related to its internal crystalline structure and amorphous region structure. The combined effect of the two effects.
Chapter One Introduction
As an important component of food commodities, food packaging materials can protect food from physical, chemical and biological damage and pollution during circulation and storage, maintain the quality of food itself, facilitate food consumption, and ensure food. Long-term storage and preservation, and give food appearance to attract consumption and obtain value beyond material cost [1-4]. As a new type of food packaging material that is safe and edible, and even has a certain nutritional value, edible film has broad application prospects in food packaging and preservation, fast food and pharmaceutical capsules, and has become a research hotspot in the current food packaging related fields.
Edible films are films with a porous network structure, usually obtained by processing natural edible polymers. Many natural polymers existing in nature have gel properties, and their aqueous solutions can form hydrogels under certain conditions, such as some natural polysaccharides, proteins, lipids, etc. . Natural structural polysaccharides such as starch and cellulose, because of their special molecular structure of long-chain helix and stable chemical properties, can be suitable for long-term and various storage environments, and have been widely studied as edible film-forming materials . Edible films made from a single polysaccharide often have certain limitations in performance. Therefore, in order to eliminate the limitations of single polysaccharide edible films, obtain special properties or develop new functions, reduce product prices, and expand their applications, usually two kinds of polysaccharides are used. Or the above natural polysaccharides are compounded to achieve the effect of complementary properties . However, due to the difference in molecular structure between different polymers, there is a certain conformational entropy, and most polymer complexes are partially compatible or incompatible . The phase morphology and compatibility of the polymer complex will determine the properties of the composite material. The deformation and flow history during processing have a significant impact on the structure. Therefore, the macroscopic properties such as the rheological properties of the polymer complex system are studied. The interrelationship between microscopic morphological structures such as phase morphology and compatibility is important for regulating the performance, analysis and modification of composite materials, processing technology, guiding formula design and processing machinery design, and evaluating production. The processing performance of the product and the development and application of new polymer materials are of great significance.
In this chapter, the research status and application progress of edible film materials are reviewed in detail; the research situation of natural hydrogels; the purpose and method of polymer compounding and the research progress of polysaccharide compounding; the rheological research method of compounding system; The rheological properties and model construction of the cold and hot reverse gel system are analyzed and discussed, as well as the research significance, research purpose and research of this paper content.
1.1 Edible film
Edible film refers to the addition of plasticizers and cross-linking agents based on natural edible substances (such as structural polysaccharides, lipids, proteins), through different intermolecular interactions, through compounding, heating, coating, drying, etc. The film with porous network structure formed by treatment . It can provide various functions such as selectable barrier properties to gas, moisture, contents and external harmful substances, so as to improve the sensory quality and internal structure of food, and prolong the storage period or shelf life of food products .
1.1.1 Development History of Edible Films
The development of edible film can be traced back to the 12th and 13th centuries. At that time, the Chinese used a simple method of waxing to coat citrus and lemons, which effectively reduced the loss of water in the fruits and vegetables, so that the fruits and vegetables maintained their original luster, thereby prolonging the shelf life of fruits and vegetables, but excessively inhibiting the aerobic respiration of fruits and vegetables, resulting in fruit fermentative deterioration . In the 15th century, Asians had already started to make edible film from soy milk, and used it to protect food and increase the appearance of food [20]. In the 16th century, the British used fat to coat food surfaces to reduce the loss of food moisture. In the 19th century, sucrose was first used as an edible coating on nuts, almonds and hazelnuts to prevent oxidation and rancidity during storage . In the 1830s, commercial hot-melt paraffin films appeared for fruits such as apples and pears . At the end of the 19th century, Gelatin films are sprayed on the surface of meat products and other foods for food preservation . In the early 1950s, carnauba wax, etc., had been made into oil-in-water emulsions for coating and preservation of fresh fruits and vegetables . In the late 1950s, research on edible films applied to meat products began to develop, and the most extensive and successful example is the enema products processed from animal small intestines into casings .
Since the 1950s, it can be said that the concept of edible film has only been really proposed. Since then, many researchers have developed a strong interest in edible films. In 1991, Nisperes applied carboxymethyl cellulose (CMC) to the coating and preservation of bananas and other fruits, the fruit respiration was reduced, and the chlorophyll loss was delayed . Park et al. in 1994 reported the effective barrier properties of zein protein film to O2 and CO2, which improved the water loss, wilting and discoloration of tomatoes . In 1995, Lourdin used dilute alkaline solution to treat starch, and added glycerin to coat strawberries for freshness, which reduced the water loss rate of strawberries and delayed spoilage . Baberjee improved the edible film properties in 1996 by micro-liquefaction and ultrasonic treatment of the film-forming liquid, so the particle size of the film-forming liquid was significantly reduced and the homogeneous stability of the emulsion was improved . In 1998, Padegett et al. added lysozyme or nisin to soybean protein edible film and used it to wrap food, and found that the growth of lactic acid bacteria in food was effectively inhibited [30]. In 1999, Yin Qinghong et al. used beeswax to make a film coating agent for the preservation and storage of apples and other fruits, which could inhibit respiration, prevent shrinkage and weight loss, and inhibit microbial invasion .
For many years, corn-baking beakers for ice cream packaging, glutinous rice paper for candy packaging, and tofu skins for meat dishes are typical edible packaging. But commercial applications of edible films were virtually non-existent in 1967, and even wax-coated fruit preservation had very limited commercial use. Until 1986, a few companies began to provide edible film products, and by 1996, the number of edible film companies had grown to more than 600. At present, the application of edible film in food packaging preservation has been increasing, and has achieved an annual revenue of more than 100 million US dollars .
1.1.2 Characteristics and types of edible films
According to relevant research, edible film has the following outstanding advantages: edible film can prevent the decline and deterioration of food quality caused by the mutual migration of different food substances; some edible film components themselves have special nutritional value and Health care function; edible film has optional barrier properties to CO2, O2 and other gases; edible film can be used for microwave, baking, fried food and medicine film and coating; edible film can be used as antioxidants and preservatives and other carriers, thereby extending the shelf life of food; edible film can be used as a carrier for colorants and nutritional fortifiers, etc., to improve food quality and improve food sensory properties; edible film is safe and edible, and can be consumed together with food; Edible packaging films can be used for the packaging of small quantities or units of food, and form multi-layer composite packaging with traditional packaging materials, which improves the overall barrier performance of packaging materials .
The reason why edible packaging films have the above functional properties is mainly based on the formation of a certain three-dimensional network structure inside them, thus showing certain strength and barrier properties . The functional properties of the edible packaging film are significantly affected by the properties of its components, and the degree of internal polymer crosslinking, the uniformity and density of the network structure are also affected by different film-forming processes. There are obvious differences in performance [15, 35]. Edible films also have some other properties such as solubility, color, transparency, etc. Suitable edible film packaging materials can be selected according to the different use environments and the differences in the product objects to be packaged.
According to the forming method of edible film, it can be divided into films and coatings: (1) The pre-prepared independent films are usually called films . (2) The thin layer formed on the food surface by means of coating, dipping, and spraying is called coating . Films are mainly used for foods with different ingredients that need to be individually packaged (such as seasoning packets and oil packets in convenience foods), foods with the same ingredient but need to be packaged separately (such as small packages of coffee, milk powder, etc.), and medicines or health care products. Capsule material; coating is mainly used for the preservation of fresh food such as fruits and vegetables, meat products, coating of drugs and the assembly of controlled-release microcapsules .
According to the film-forming materials of edible packaging film, it can be divided into: polysaccharide edible film, protein edible film, lipid edible film, microbial edible film and composite edible film .
1.1.3 Application of edible film
As a new type of food packaging material that is safe and edible, and even has a certain nutritional value, edible film is widely used in the food packaging industry, the pharmaceutical field, the storage and preservation of fruits and vegetables, the processing and preservation of meat and aquatic products, the production of fast food, and the production of oil. It has broad application prospects in the preservation of foods such as fried baked candies.
1.1.3.1 Application in food packaging
The film-forming solution is covered on the food to be packaged by spraying, brushing, dipping, etc., to prevent the penetration of moisture, oxygen and aromatic substances, which can effectively reduce the loss of packaging and reduce the number of packaging layers; significantly reduce the outer layer of the food The complexity of the components of plastic packaging facilitates its recycling and processing, and reduces environmental pollution; it is applied to the separate packaging of some components of multi-component complex foods to reduce the mutual migration between different components, thereby reducing the pollution to the environment. Reduce the spoilage of food or the decline of food quality. The edible film is directly processed into packaging paper or packaging bags for food packaging, which not only achieves safety, cleanliness and convenience, but also reduces the pressure of white pollution on the environment.
Using corn, soybeans and wheat as the main raw materials, paper-like cereal films can be prepared and used for packaging of sausages and other foods. After use, even if they are discarded in the natural environment, they are biodegradable and can be turned into soil fertilizers to improve soil. . Using starch, chitosan and bean dregs as the main materials, edible wrapping paper can be prepared for packaging fast food such as fast-food noodles and French fries, which is convenient, safe and very popular; used for seasoning packets, solid soups The packaging of convenience foods such as raw materials, which can be directly cooked in the pot when used, can prevent food contamination, increase food nutrition, and facilitate cleaning . Dried avocado, potatoes, and broken rice are fermented and converted into polysaccharides, which can be used to prepare new edible inner packaging materials that are colorless and transparent, have good oxygen barrier properties and mechanical properties, and are used for the packaging of milk powder, salad oil and other products [19]. For military food, after the product is used, the traditional plastic packaging material is discarded in the environment and becomes a marker for enemy tracking, which is easy to reveal the whereabouts. In multi-component special foods such as pizza, pastry, ketchup, ice cream, yogurt, cakes and desserts, plastic packaging materials cannot be directly added to use, and edible packaging film shows its unique advantages, which can reduce the number of groups Fractional migration of flavor substances improves product quality and aesthetics [21]. Edible packaging film can be used in microwave food processing of batter system. Meat products, vegetables, cheese and fruits are pre-packaged by spraying, dipping or brushing, etc., frozen and stored, and only need to be microwaved for consumption.
Although few commercial edible packaging papers and bags are available, many patents have been registered on the formulation and application of potential edible packaging materials . The French food regulatory authorities have approved an industrialized edible packaging bag named “SOLUPAN”, which is composed of hydroxypropyl methylcellulose, starch and sodium sorbate, and is commercially available .
1.1.3.2 Application in Medicine
Gelatin, cellulose derivatives, starch and edible gum can be used to prepare soft and hard capsule shells of medicines and health products, which can effectively ensure the efficacy of medicines and health products, and are safe and edible; some medicines have inherent bitter taste, which is difficult to be used by patients. Accepted, edible films can be used as taste-masking coatings for such drugs; some enteric polymer polymers do not dissolve in the stomach (pH 1.2) environment, but are soluble in the intestinal (pH 6.8) environment and can be used in the intestinal Sustained-release drug coating; can also be used as a carrier for targeted drugs.
Blanco-Fernandez et al. prepared a chitosan acetylated monoglyceride composite film and used it for the sustained release of the antioxidant activity of vitamin E, and the effect was remarkable. Long-term antioxidant packaging materials . Zhang et al. blended starch with gelatin, added polyethylene glycol plasticizer, and used traditional. The hollow hard capsules were prepared by the dipping process of the composite film, and the transparency, mechanical properties, hydrophilic properties and phase morphology of the composite film were studied. good capsule material [52]. Lal et al. made kafirin into an edible coating for the enteric coating of paracetamol capsules, and studied the mechanical properties, thermal properties, barrier properties and drug release properties of the edible film. The results showed that the coating of sorghum Various hard capsules of gliadin film was not broken in the stomach, but released the drug in the intestine at pH 6.8 . Paik et al. prepared HPMC phthalate particles coated with indomethacin, and sprayed the edible film-forming liquid of HPMC on the surface of the drug particles, and studied the drug entrapment rate, average particle size of drug particles, edible film the results showed that the HPMCN-coated indomethacin oral drug could achieve the purpose of masking the bitter taste of the drug and targeting drug delivery . Oladzadabbasabadi et al. blended modified sago starch with carrageenan to prepare an edible composite film as a substitute for traditional gelatin capsules, and studied its drying kinetics, thermomechanical properties, physicochemical properties and barrier properties, The results show that the composite edible film has similar properties to gelatin and can be used in the production of pharmaceutical capsules .
1.1.3.3 Application in fruit and vegetable preservation
In fresh fruits and vegetables after picking, biochemical reactions and respiration are still vigorously going on, which will accelerate the tissue damage of fruits and vegetables, and it is easy to cause the loss of moisture in fruits and vegetables at room temperature, resulting in the quality of internal tissues and sensory properties of fruits and vegetables. decline. Therefore, preservation has become the most important issue in the storage and transportation of fruits and vegetables; traditional preservation methods have poor preservation effect and high cost. Coating preservation of fruits and vegetables is currently the most effective method in room temperature preservation. The edible film-forming liquid is coated on the surface of fruits and vegetables, which can effectively prevent the invasion of microorganisms, reduce the respiration, water loss and nutrient loss of fruit and vegetable tissues, delay the physiological aging of fruit and vegetable tissues, and keep fruit and vegetable tissues The original plump and smooth. Glossy appearance, so as to achieve the purpose of keeping fresh and prolonging the storage period . Americans use acetyl monoglyceride and cheese extracted from vegetable oil as the main raw materials to prepare edible film, and use it to cut fruits and vegetables to keep fresh, prevent dehydration, browning and the invasion of microorganisms, so that it can be maintained for a long time. Fresh state . Japan uses waste silk as raw material to prepare potato fresh-keeping film, which can achieve a fresh-keeping effect comparable to that of cold storage. Americans use vegetable oil and fruit as the main raw materials to make a coating liquid, and keep the cut fruit fresh, and found that the preservation effect is good.
Marquez et al. used whey protein and pectin as raw materials, and added glutaminase for cross-linking to prepare a composite edible film, which was used to coat fresh-cut apples, tomatoes and carrots, which can significantly reduce the weight loss rate. , inhibit the growth of microorganisms on the surface of fresh-cut fruits and vegetables, and prolong the shelf life on the premise of maintaining the taste and flavor of fresh-cut fruits and vegetables . Shi Lei et al. coated red globe grapes with chitosan edible film, which could reduce the weight loss and rot rate of grapes, maintain the color and brightness of grapes, and delay the degradation of soluble solids . Using chitosan, sodium alginate, sodium carboxymethylcellulose and polyacrylate as raw materials, Liu et al. prepared edible films by multilayer coating for fresh-keeping of fruits and vegetables, and studied their morphology, water solubility, etc. The results showed that the sodium carboxymethyl cellulose-chitosan-glycerol composite film had the best preservation effect . Sun Qingshen et al. studied the composite film of soybean protein isolate, which is used for the preservation of strawberries, which can significantly reduce the transpiration of strawberries, inhibit their respiration, and reduce the rate of rotten fruit . Ferreira et al. used fruit and vegetable residue powder and potato peel powder to prepare composite edible film, studied the water solubility and mechanical properties of composite film, and used coating method to preserve hawthorn. The results showed that the shelf life of hawthorn was prolonged. 50%, the weight loss rate decreased by 30-57%, and the organic acid and moisture did not change significantly . Fu Xiaowei et al. studied the preservation of fresh peppers by chitosan edible film, and the results showed that it could significantly reduce the respiration intensity of fresh peppers during storage and delay the aging of peppers . Navarro-Tarazaga et al. used beeswax-modified HPMC edible film to preserve plums. The results showed that beeswax could improve the oxygen and moisture barrier properties and mechanical properties of HPMC films. The weight loss rate of the plums was significantly reduced, the softening and bleeding of the fruit during storage were improved, and the storage period of the plums was prolonged . Tang Liying et al. used shellac alkali solution in starch modification, prepared edible packaging film, and studied its film properties; at the same time, using its film-forming liquid to coat mangoes for freshness can effectively reduce breathing It can prevent the browning phenomenon during storage, reduce the weight loss rate and prolong the storage period .
1.1.3.4 Application in the processing and preservation of meat products
Meat products with rich nutrients and high-water activity are easily invaded by microorganisms in the process of processing, transportation, storage and consumption, resulting in darkening of color and fat oxidation and other spoilage. In order to prolong the storage period and shelf life of meat products, it is necessary to try to inhibit the activity of enzymes in meat products and the invasion of microorganisms on the surface, and prevent the deterioration of color and odor caused by fat oxidation. At present, edible film preservation is one of the common methods widely used in meat preservation at home and abroad. Comparing it with the traditional method, it is found that the invasion of external microorganisms, the oxidative rancidity of fat and the loss of juice have been significantly improved in meat products packaged in edible film, and the quality of meat products has been significantly improved. Shelf life is extended.
The research on edible film of meat products began in the late 1950s, and the most successful application case was collagen edible film, which has been widely used in sausage production and processing . Emiroglu et al. added sesame oil to soybean protein edible film to make antibacterial film, and studied its antibacterial effect on frozen beef. The results showed that the antibacterial film can significantly inhibit the reproduction and growth of Staphylococcus aureus . Wook et al. prepared a proanthocyanidin edible film and used it to coat refrigerated pork for freshness. The color, pH, TVB-N value, thiobarbituric acid and microbial count of pork chops after storage for 14 days were studied. The results showed that the edible film of proanthocyanidins can effectively reduce the formation of thiobarbituric acid, prevent fatty acid spoilage, reduce the invasion and reproduction of microorganisms on the surface of meat products, improve the quality of meat products, and prolong the storage period and shelf life . Jiang Shaotong et al. added tea polyphenols and allicin to the starch-sodium alginate composite membrane solution, and used them to preserve the freshness of chilled pork, which could be stored at 0-4 °C for more than 19 days . Cartagena et al. reported the antibacterial effect of collagen edible film added with nisin antimicrobial agent on the preservation of pork slices, indicating that collagen edible film can reduce the moisture migration of refrigerated pork slices, delay the rancidity of meat products, and add 2 The collagen film with % nisin had the best preservation effect . Wang Rui et al. studied the changes of sodium alginate, chitosan and carboxymethyl fiber by comparative analysis of the pH, volatile base nitrogen, redness and total number of colonies of beef within 16 days of storage. The three kinds of edible films of sodium vitamin were used to preserve the freshness of chilled beef. The results showed that the edible film of sodium alginate had an ideal freshness preservation effect . Caprioli et al. wrapped cooked turkey breast with a sodium caseinate edible film and then refrigerated it at 4 °C. Studies have shown that the sodium caseinate edible film can slow down turkey meat during refrigeration. of rancidity .
1.1.3.5 Application in the preservation of aquatic products
The quality decline of aquatic products is mainly manifested in the reduction of free moisture, the deterioration of flavor and the deterioration of aquatic product texture. The decomposition of aquatic products, oxidation, denaturation and dry consumption caused by microbial invasion are all important factors affecting the shelf life of aquatic products . Frozen storage is a common method for the preservation of aquatic products, but there will also be a certain degree of quality degradation in the process, which is especially serious for freshwater fish.
The edible film preservation of aquatic products began in the late 1970s and has now been widely used. Edible film can effectively preserve frozen aquatic products, reduce water loss, and can also be combined with antioxidants to prevent fat oxidation, thereby achieving the purpose of extending shelf life and shelf life . Meenatchisundaram et al. prepared a starch-based composite edible film using starch as a matrix and added spices such as clove and cinnamon, and used it for the preservation of white shrimp. The results showed that the edible starch film can effectively inhibit the growth of microorganisms, slow down fat oxidation, prolong the shelf life of refrigerated white shrimp at 10 °C and 4 °C was as long as 14 and 12 days, respectively . Cheng Yuanyuan and others studied the preservative of pullulan solution and carried out the freshwater fish. Preservation can effectively inhibit the growth of microorganisms, slow down the oxidation of fish protein and fat, and have excellent preservation effect . Yunus et al. coated rainbow trout with a gelatin edible film to which bay leaf essential oil was added, and studied the effect of refrigerated preservation at 4 °C. The results showed that the gelatin edible film was effective in maintaining the quality of rainbow trout for up to 22 days. for a long time . Wang Siwei et al. used sodium alginate, chitosan and CMC as the main materials, added stearic acid to prepare edible film liquid, and used it to coat Penaeus vannamei for freshness. The study showed that the composite film of CMC and chitosan The liquid has a good preservation effect and can extend the shelf life by about 2 days . Yang Shengping and others used chitosan-tea polyphenol edible film for the refrigeration and preservation of fresh hairtail, which can effectively inhibit the reproduction of bacteria on the surface of hairtail, delay the formation of volatile hydrochloric acid, and extend the shelf life of hairtail to about 12 days .
1.1.3.6 Application in fried food
Deep fried food is a widely popular ready-to-eat food with a large output. It is wrapped with polysaccharide and protein edible film, which can prevent the color change of the food during the frying process and reduce the oil consumption. entry of oxygen and moisture [80]. Coating fried food with gellan gum can reduce oil consumption by 35%-63%, such as when frying sashimi, it can reduce oil consumption by 63%; when frying potato chips, it can reduce oil consumption by 35%-63%. Reduced fuel consumption by 60%, etc. [81].
Singthong et al. made edible films of polysaccharides such as sodium alginate, carboxymethyl cellulose and pectin, which were used for the coating of fried banana strips, and studied the oil absorption rate after frying. The results showed that pectin and carboxyl The fried banana strips coated with methylcellulose showed better sensory quality, among which the pectin edible film had the best effect on reducing oil absorption [82]. Holownia et al. coated HPMC and MC films on the surface of fried chicken fillets to study the changes in oil consumption, free fatty acid content and color value in frying oil. Pre-coating can reduce oil absorption and improve oil life [83]. Sheng Meixiang et al. made edible films of CMC, chitosan and soybean protein isolate, coated potato chips, and fried them at high temperature to study the oil absorption, water content, color, acrylamide content and sensory quality of potato chips. , the results showed that the soybean protein isolate edible film has a significant effect on reducing the oil consumption of fried potato chips, and the chitosan edible film has a better effect on reducing the acrylamide content [84]. Salvador et al. coated the surface of fried squid rings with wheat starch, modified corn starch, dextrin and gluten, which could improve the crispness of the squid rings and reduce the oil absorption rate [85].
1.1.3.7 Application in baked goods
Edible film can be used as a smooth coating to improve the appearance of baked goods; can be used as a barrier to moisture, oxygen, grease, etc. to improve the shelf life of baked goods, for example, chitosan edible film is used to surface coating bread It can also be used as an adhesive for crisp snacks and snacks, for example, roasted peanuts are often coated with adhesives to coat salt and seasonings [87].
Christos et al. made edible films of sodium alginate and whey protein and coated them on the surface of Lactobacillus rhamnosus probiotic bread. The study showed that the survival rate of probiotics was significantly improved, but the two types of bread showed Digestive mechanisms are very similar, so the coating of the edible film does not alter the texture, flavor and thermophysical properties of the bread [88]. Panuwat et al. added Indian gooseberry extract into methyl cellulose matrix to prepare an edible composite film, and used it to preserve the freshness of roasted cashews. The results showed that the composite edible film could effectively inhibit roasted cashews during storage. The quality deteriorated and the shelf life of roasted cashews was extended by up to 90 days [89]. Schou et al. made a transparent and flexible edible film with sodium caseinate and glycerin, and studied its mechanical properties, water permeability and its packaging effect on baked bread slices. The results showed that the edible film of sodium caseinate wrapped baked bread. After breading, its hardness can be reduced within 6 h of storage at room temperature [90]. Du et al. used apple-based edible film and tomato-based edible film added with plant essential oils to wrap roast chicken, which not only inhibited the growth of microorganisms before roasting the chicken, but also enhanced the flavor of the chicken after roasting [91]. Javanmard et al. prepared an edible film of wheat starch and used it to wrap baked pistachio kernels. The results showed that the edible starch film could prevent the oxidative rancidity of the nuts, improve the quality of the nuts, and prolong their shelf life [92]. Majid et al. used whey protein edible film to coat roasted peanuts, which can increase oxygen barrier, reduce peanut rancidity, improve roasted peanut brittleness, and prolong its storage period [93].
1.1.3.8 Application in confectionery products
The candy industry has high requirements for the diffusion of volatile components, so for chocolate and candies with polished surfaces, it is necessary to use water-soluble edible films to replace the coating liquid containing volatile components. The edible packaging film can form a smooth protective film on the surface of the candy to reduce the migration of oxygen and moisture [19]. The application of whey protein edible films in confectionery can significantly reduce the diffusion of its volatile components. When chocolate is used to encapsulate oily foods such as cookies and peanut butter, the oil will migrate to the outer layer of chocolate, making the chocolate sticky and causing a “reverse frost” phenomenon, but the inner material will dry out, resulting in a change in its flavor. Adding a layer of edible film packaging material with grease barrier function can solve this problem [94].
Nelson et al. used methylcellulose edible film to coat candies containing multiple lipids and showed very low lipid permeability, thereby inhibiting the frosting phenomenon in chocolate [95]. Meyers applied a hydrogel-wax bilayer edible film to chewing gum, which could improve its adhesion, reduce water volatilization, and prolong its shelf life [21]. Water prepared by Fadini et al. Decollagen-cocoa butter edible composite film was studied for its mechanical properties and water permeability, and it was used as a coating for chocolate products with good results [96].
1.1.4 Cellulose-Based Edible Films
Cellulose-based edible film is a kind of edible film made from the most abundant cellulose and its derivatives in nature as the main raw materials. Cellulose-based edible film is odorless and tasteless, and has good mechanical strength, oil barrier properties, transparency, flexibility and good gas barrier properties. However, due to the hydrophilic nature of cellulose, the resistance of cellulose-based edible film is Water performance is generally relatively poor [82, 97-99].
The cellulose-based edible film made of waste materials in food industry production can obtain edible packaging films with excellent performance, and can reuse waste materials to increase the added value of products. Ferreira et al. blended fruit and vegetable residue powder with potato peel powder to prepare a cellulose-based edible composite film, and applied it to the coating of hawthorn to preserve freshness, and achieved good results [62]. Tan Huizi et al. used the dietary fiber extracted from bean dregs as the base material and added a certain amount of thickener to prepare an edible film of soybean fiber, which has good mechanical properties and barrier properties [100], which is mainly used for packaging Fast food noodle seasoning, it is convenient and nutritious to dissolve the material package directly in hot water.
Water-soluble cellulose derivatives, such as methyl cellulose (MC), carboxymethyl cellulose (CMC) and hydroxypropyl methyl cellulose (HPMC), can form a continuous matrix and are commonly used in edible films development and research. Xiao Naiyu et al. used MC as the main film-forming substrate, added polyethylene glycol and calcium chloride and other auxiliary materials, prepared MC edible film by casting method, and applied it to the preservation of olecranon, which can prolong the olecranon’s mouth. The shelf life of peach is 4.5 days [101]. Esmaeili et al. prepared MC edible film by casting and applied it to the coating of plant essential oil microcapsules. The results showed that MC film has a good oil-blocking effect and can be applied to food packaging to prevent fatty acid spoilage [102]. Tian et al. modified MC edible films with stearic acid and unsaturated fatty acids, which could improve the water-blocking properties of MC edible films [103]. Lai Fengying et al. studied the effect of solvent type on the film-forming process of MC edible film and the barrier properties and mechanical properties of the edible film [104].
CMC membranes have good barrier properties to O2, CO2 and oils, and are widely used in the field of food and medicine [99]. Bifani et al. prepared CMC membranes and studied the effect of leaf extracts on the water barrier properties and gas barrier properties of the membranes. The results showed that the addition of leaf extracts could significantly improve the moisture and oxygen barrier properties of the membranes, but not for CO2. The barrier properties are related to the concentration of the extract [105]. de Moura et al. prepared chitosan nanoparticles reinforced CMC films, and studied the thermal stability, mechanical properties and water solubility of the composite films. The results show that chitosan nanoparticles can effectively improve the mechanical properties and thermal stability of CMC films. Sex [98]. Ghanbarzadeh et al. prepared CMC edible films and studied the effects of glycerol and oleic acid on the physicochemical properties of CMC films. The results showed that the barrier properties of the films were significantly improved, but the mechanical properties and transparency decreased [99]. Cheng et al. prepared a carboxymethyl cellulose-konjac glucomannan edible composite film, and studied the effect of palm oil on the physicochemical properties of the composite film. The results showed that the smaller lipid microspheres can significantly increase the composite film. The surface hydrophobicity and the curvature of the water molecule permeation channel can improve the moisture barrier performance of the membrane [106].
HPMC has good film-forming properties, and its film is flexible, transparent, colorless and odorless, and has good oil-barrier properties, but its mechanical properties and water-blocking properties need to be improved. The study by Zuniga et al. showed that the initial microstructure and stability of the HPMC film-forming solution can significantly affect the surface and internal structure of the film, and the way oil droplets enter during the formation of the film structure can significantly affect the light transmittance and surface activity of the film. The addition of the agent can improve the stability of the film-forming solution, which in turn affects the surface structure and optical properties of the film, but the mechanical properties and air permeability are not reduced [107]. Klangmuang et al. used organically modified clay and beeswax to enhance and modify HPMC edible film to improve the mechanical properties and barrier properties of HPMC film. The study showed that after beeswax and clay modification, the mechanical properties of HPMC edible film were comparable to those of edible film. The performance of moisture components was improved [108]. Dogan et al. prepared HPMC edible film, and used microcrystalline cellulose to enhance and modify the HPMC film, and studied the water permeability and mechanical properties of the film. The results showed that the moisture barrier properties of the modified film did not change significantly. , but its mechanical properties have been significantly improved [109]. Choi et al. added oregano leaf and bergamot essential oil into HPMC matrix to prepare edible composite film, and applied it to the coating preservation of fresh plums. The study showed that the edible composite film can effectively inhibit the respiration of plums, reducing the production of ethylene, reducing the rate of weight loss, and improving the quality of plums [110]. Esteghlal et al. blended HPMC with gelatin to prepare edible composite films and studied edible composite films. The physicochemical properties, mechanical properties and compatibility of HPMC gelatin showed that the tensile properties of HPMC gelatin composite films did not change significantly, which could be used in the preparation of medicinal capsules [111]. Villacres et al. studied the mechanical properties, gas barrier properties and antibacterial properties of HPMC-cassava starch edible composite films. The results showed that the composite films had good oxygen barrier properties and antibacterial effects [112]. Byun et al. prepared shellac-HPMC composite membranes, and studied the effects of the types of emulsifiers and shellac concentration on the composite membranes. The emulsifier reduced the water-blocking properties of the composite membrane, but its mechanical properties did not decrease significantly; the addition of shellac greatly improved the thermal stability of the HPMC membrane, and its effect increased with the increase of the shellac concentration [113].
1.1.5 Starch-Based Edible Films
Starch is a natural polymer for the preparation of edible films. It has the advantages of wide source, low price, biocompatibility and nutritional value, and is widely used in the food and pharmaceutical industries [114-117]. Recently, researches on pure starch edible films and starch-based edible composite films for food storage and preservation have emerged one after another [118]. High amylose starch and its hydroxypropylated modified starch are the main materials for the preparation of starch-based edible films [119]. The retrogradation of starch is the main reason for its ability to form a film. The higher the amylose content, the tighter the intermolecular bonding, the easier it is to produce retrogradation, and the better the film-forming property, and the final tensile strength of the film. bigger. Amylose can make water-soluble films with low oxygen permeability, and the barrier properties of high-amylose films will not decrease under high temperature environments, which can effectively protect the packaged food [120].
Starch edible film, colorless and odorless, has good transparency, water solubility and gas barrier properties, but it shows relatively strong hydrophilicity and poor moisture barrier properties, so it is mainly used in food oxygen and oil barrier packaging [121-123]. In addition, starch-based membranes are prone to aging and retrogradation, and their mechanical properties are relatively poor [124]. In order to overcome the above shortcomings, the starch can be modified by physical, chemical, enzymatic, genetic and additive methods to improve the properties of starch-based edible films [114].
Zhang Zhengmao et al. used ultra-fine starch edible film to coat strawberries and found that it can effectively reduce water loss, delay the reduction of soluble sugar content, and effectively prolong the storage period of strawberries [125]. Garcia et al. modified starch with different chain ratios to obtain modified starch film-forming liquid, which was used for fresh strawberry coating film preservation. The rate and decay rate were better than those of the uncoated group [126]. Ghanbarzadeh et al. modified starch by citric acid cross-linking and obtained chemically cross-linked modified starch film. Studies have shown that after cross-linking modification, the moisture barrier properties and mechanical properties of starch films were improved [127]. Gao Qunyu et al. carried out enzymatic hydrolysis treatment of starch and obtained starch edible film, and its mechanical properties such as tensile strength, elongation and folding resistance increased, and the moisture barrier performance increased with the increase of enzyme action time. significantly improved [128]. Parra et al. added a cross-linking agent to tapioca starch to prepare an edible film with good mechanical properties and low water vapor transmission rate [129]. Fonseca et al. used sodium hypochlorite to oxidize potato starch and prepared an edible film of oxidized starch. The study showed that its water vapor transmission rate and water solubility were significantly reduced, which can be applied to the packaging of high-water activity food [130].
Compounding starch with other edible polymers and plasticizers is an important method to improve the properties of starch-based edible films. Currently, the commonly used complex polymers are mostly hydrophilic colloids, such as pectin, cellulose, seaweed polysaccharide, chitosan, carrageenan and xanthan gum [131].
Maria Rodriguez et al. used potato starch and plasticizers or surfactants as the main materials to prepare starch-based edible films, showing that plasticizers can increase film flexibility and surfactants can reduce film stretchability [132]. Santana et al. used nanofibers to enhance and modify cassava starch edible films, and obtained starch-based edible composite films with improved mechanical properties, barrier properties, and thermal stability [133]. Azevedo et al. compounded whey protein with thermoplastic starch to prepare a uniform film material, indicating that whey protein and thermoplastic starch have strong interfacial adhesion, and whey protein can significantly improve starch availability. Water-blocking and mechanical properties of edible films [134]. Edhirej et al. prepared a tapioca starch-based edible film, and studied the effect of plasticizer on the physical and chemical structure, mechanical properties and thermal properties of the film. The results show that the type and concentration of plasticizer can significantly affect the tapioca starch film. Compared with other plasticizers such as urea and triethylene glycol, pectin has the best plasticizing effect, and the pectin-plasticized starch film has good water-blocking properties [135]. Saberi et al. used pea starch, guar gum and glycerin for the preparation of edible composite films. The results showed that pea starch played a major role in film thickness, density, cohesion, water permeability and tensile strength. Guar gum It can affect the tensile strength and elastic modulus of the membrane, and glycerol can improve the flexibility of the membrane [136]. Ji et al. compounded chitosan and corn starch, and added calcium carbonate nanoparticles to prepare a starch-based antibacterial film. The study showed that intermolecular hydrogen bonds were formed between starch and chitosan, and the mechanical properties of the film were and antibacterial properties were enhanced [137]. Meira et al. enhanced and modified corn starch edible antibacterial film with kaolin nanoparticles, and the mechanical and thermal properties of the composite film were improved, and the antibacterial effect was not affected [138]. Ortega-Toro et al. added HPMC to starch and added citric acid to prepare edible film. The study showed that the addition of HPMC and citric acid can effectively inhibit the aging of starch and reduce the water permeability of edible film, but the oxygen barrier properties drop [139].
1.2 Polymer hydrogels
Hydrogels are a class of hydrophilic polymers with a three-dimensional network structure that are insoluble in water but can be swelled by water. Macroscopically, a hydrogel has a definite shape, cannot flow, and is a solid substance. Microscopically, water-soluble molecules can be distributed in different shapes and sizes in the hydrogel and diffuse at different diffusion rates, so the hydrogel exhibits the properties of a solution. The internal structure of hydrogels has limited strength and is easily destroyed. It is in a state between a solid and a liquid. It has a similar elasticity to a solid, and is clearly different from a real solid.
1.2.1 Overview of polymer hydrogels
1.2.1.1 Classification of polymer hydrogels
Polymer hydrogel is a three-dimensional network structure formed by physical or chemical cross-linking between polymer molecules [143-146]. It absorbs a large amount of water in water to swell itself, and at the same time, it can maintain its three-dimensional structure and be insoluble in water. water.
There are many ways to classify hydrogels. Based on the difference in cross-linking properties, they can be divided into physical gels and chemical gels. Physical gels are formed by relatively weak hydrogen bonds, ionic bonds, hydrophobic interactions, van der Waals forces and physical entanglement between polymer molecular chains and other physical forces, and can be converted into solutions in different external environments. It is called reversible gel; chemical gel is usually a permanent three-dimensional network structure formed by cross-linking of chemical bonds such as covalent bonds in the presence of heat, light, initiator, etc. After the gel is formed, it is irreversible and permanent, also known as For the true condensate [147-149]. Physical gels generally do not require chemical modification and have low toxicity, but their mechanical properties are relatively poor and it is difficult to withstand large external stress; chemical gels generally have better stability and mechanical properties.
Based on different sources, hydrogels can be divided into synthetic polymer hydrogels and natural polymer hydrogels. Synthetic polymer hydrogels are hydrogels formed by chemical polymerization of synthetic polymers, mainly including polyacrylic acid, polyvinyl acetate, polyacrylamide, polyethylene oxide, etc.; natural polymer hydrogels are Polymer hydrogels are formed by cross-linking of natural polymers such as polysaccharides and proteins in nature, including cellulose, alginate, starch, agarose, hyaluronic acid, gelatin, and collagen [6, 7, 150], 151]. Natural polymer hydrogels usually have the characteristics of wide source, low price and low toxicity, and synthetic polymer hydrogels are generally easy to process and have large yields.
Based on different responses to the external environment, hydrogels can also be divided into traditional hydrogels and smart hydrogels. Traditional hydrogels are relatively insensitive to changes in the external environment; smart hydrogels can sense small changes in the external environment and produce corresponding changes in physical structure and chemical properties [152-156]. For temperature-sensitive hydrogels, the volume changes with the temperature of the environment. Usually, such polymer hydrogels contain hydrophilic groups such as hydroxyl, ether and amide or hydrophobic groups such as methyl, ethyl and propyl. The temperature of the external environment can affect the hydrophilic or hydrophobic interaction between gel molecules, hydrogen bonding and the interaction between water molecules and polymer chains, thereby affecting the balance of the gel system. For pH-sensitive hydrogels, the system usually contains acid-base modifying groups such as carboxyl groups, sulfonic acid groups or amino groups. In a changing pH environment, these groups can absorb or release protons, changing the hydrogen bonding in the gel and the difference between the internal and external ion concentrations, resulting in a volume change of the gel. For electric field, magnetic field and light-sensitive hydrogels, they contain functional groups such as polyelectrolytes, metal oxides, and photosensitive groups, respectively. Under different external stimuli, the system temperature or ionization degree is changed, and then the gel volume is changed by the principle similar to temperature or pH-sensitive hydrogel.
Based on different gel behaviors, hydrogels can be divided into cold-induced gels and thermal-induced gels [157]. Cold gel, referred to as cold gel for short, is a macromolecule that exists in the form of random coils at high temperature. During the cooling process, due to the action of intermolecular hydrogen bonds, helical fragments are gradually formed, thereby completing the process from solution. The transition to gel [158]; thermo-induced gel, referred to as thermal gel, is a macromolecule in solution state at low temperature. During the heating process, a three-dimensional network structure is formed through hydrophobic interaction, etc., thus completing the gelation transition [159], 160].
Hydrogels can also be divided into homopolymeric hydrogels, copolymerized hydrogels and interpenetrating network hydrogels based on different network properties, microscopic hydrogels and macroscopic hydrogels based on different gel sizes, and biodegradable properties. Differently divided into degradable hydrogels and non-degradable hydrogels.
1.2.1.2 Application of natural polymer hydrogels
Natural polymer hydrogels have the characteristics of good biocompatibility, high flexibility, abundant sources, sensitivity to the environment, high water retention and low toxicity, and are widely used in biomedicine, food processing, environmental protection, agriculture and forestry production and It has been widely used in industry and other fields [142, 161-165].
Application of natural polymer hydrogels in biomedical related fields. Natural polymer hydrogels have good biocompatibility, biodegradability, and no toxic side effects, so they can be used as wound dressings and directly contact human tissues, which can effectively reduce the invasion of microorganisms in vitro, prevent the loss of body fluids, and allow oxygen to pass through. Promotes wound healing; can be used to prepare contact lenses, with the advantages of comfortable wearing, good oxygen permeability, and auxiliary treatment of eye diseases [166, 167]. Natural polymers are similar to the structure of living tissues and can participate in the normal metabolism of the human body, so such hydrogels can be used as tissue engineering scaffold materials, tissue engineering cartilage repair, etc. Tissue engineering scaffolds can be classified into pre-shaped and injection-moulded scaffolds. Pre-molded stents utilize water the special three-dimensional network structure of the gel enables it to play a certain supporting role in biological tissues while providing a specific and sufficient growth space for cells, and can also induce cell growth, differentiation, and degradation and absorption by the human body [168]. Injection-molded stents utilize the phase transition behavior of hydrogels to rapidly form gels after being injected in a flowing solution state, which can minimize the pain of patients [169]. Some natural polymer hydrogels are environmentally sensitive, so they are widely used as drug-controlled release materials, so that the drugs encapsulated in them can be released to the required parts of the human body in a timed and quantitative manner, reducing the toxic and side effects of the drugs on the human body [ 170].
Application of natural polymer hydrogels in food-related fields. Natural polymer hydrogels are an important part of people’s three meals a day, such as some desserts, candies, meat substitutes, yogurt and ice cream. It is often used as a food additive in food commodities, which can improve its physical properties and give it a smooth taste. For example, it is used as a thickener in soups and sauces, as an emulsifier in juice, and as a suspending agent. In milk drinks, as a gelling agent in puddings and aspics, as a clarifying agent and foam stabilizer in beer, as a syneresis inhibitor in cheese, as a binder in sausages, as starch retrogradation Inhibitors are used in bread and butter [171-174]. From the Food Additives Handbook, it can be seen that a large number of natural polymer hydrogels are approved as food additives for food processing [175]. Natural polymer hydrogels are used as nutritional fortifiers in the development of health products and functional foods, such as dietary fibers, used in weight loss products and anti-constipation products [176, 177]; as prebiotics, they are used in colonic Health care products and products for preventing colon cancer [178]; natural polymer hydrogels can be made into edible or degradable coatings or films, which can be used in the field of food packaging materials, such as fruit and vegetable preservation, by coating them on fruits and vegetables On the surface, it can prolong the shelf life of fruits and vegetables and keep fruits and vegetables fresh and tender; it can also be used as packaging materials for convenience foods such as sausages and condiments to facilitate cleaning [179, 180].
Applications of natural polymer hydrogels in other fields. In terms of daily necessities, it can be added to creamy skin care or cosmetics, which can not only prevent the product from drying out in storage, but also lasting moisturizing and moisturizing the skin; it can be used for styling, moisturizing and slow release of fragrances in beauty makeup; It can be used in daily necessities such as paper towels and diapers [181]. In agriculture, it can be used to resist drought and protect seedlings and reduce labor intensity; as a coating agent for plant seeds, it can significantly increase the germination rate of seeds; when used in seedling transplanting, it can increase the survival rate of seedlings; pesticides, improve utilization and reduce pollution [182, 183]. In terms of environment, it is used as a flocculant and adsorbent for sewage treatment that often contains heavy metal ions, aromatic compounds and dyes to protect water resources and improve the environment [184]. In industry, it is used as dehydrating agent, drilling lubricant, cable wrapping material, sealing material and cold storage agent, etc. [185].
1.2.2 Hydroxypropyl methylcellulose thermogel
Cellulose is a natural macromolecular compound that has been studied earliest, has the closest relationship with humans, and is the most abundant in nature. It is widely present in higher plants, algae and microorganisms [186, 187]. Cellulose has gradually attracted widespread attention due to its wide source, low price, renewable, biodegradable, safe, non-toxic, and good biocompatibility [188].
1.2.2.1 Cellulose and its ether derivatives
Cellulose is a linear long-chain polymer formed by the connection of D-anhydroglucose structural units through β-1,4 glycosidic bonds [189-191]. Insoluble. Except for one end group at each end of the molecular chain, there are three polar hydroxyl groups in each glucose unit, which can form a large number of intramolecular and intermolecular hydrogen bonds under certain conditions; and cellulose is a polycyclic structure, and the molecular chain is semi-rigid. Chain, high crystallinity, and highly regular in structure, so it has the characteristics of high degree of polymerization, good molecular orientation, and chemical stability [83, 187]. Since the cellulose chain contains a large number of hydroxyl groups, it can be chemically modified by various methods such as esterification, oxidation, and etherification to obtain cellulose derivatives with excellent application properties [192, 193].
Cellulose derivatives are one of the earliest researched and produced products in the field of polymer chemistry. They are polymer fine chemical materials with a wide range of uses, which are chemically modified from natural polymer cellulose. Among them, cellulose ethers are widely used. It is one of the most important chemical raw materials in industrial applications [194].
There are many varieties of cellulose ethers, all of which generally have their unique and excellent properties, and have been widely used in many fields such as food and medicine [195]. MC is the simplest kind of cellulose ether with methyl group. With the increase of substitution degree, it can be dissolved in dilute alkaline solution, water, alcohol and aromatic hydrocarbon solvent in turn, showing unique thermal gel properties. [196]. CMC is an anionic cellulose ether obtained from natural cellulose by alkalization and acidification.
It is the most widely used and used cellulose ether, which is soluble in water [197]. HPC, a hydroxyalkyl cellulose ether obtained by alkalizing and etherifying cellulose, has good thermoplasticity and also exhibits thermal gel properties, and its gel temperature is significantly affected by the degree of hydroxypropyl substitution [198]. HPMC, an important mixed ether, also has thermal gel properties, and its gel properties are related to the two substituents and their ratios [199].
1.2.2.2 Hydroxypropyl methylcellulose structure
Hydroxypropyl methyl cellulose (HPMC), the molecular structure is shown in Figure 1-3, is a typical non-ionic water-soluble cellulose mixed ether. The etherification reaction of methyl chloride and propylene oxide is carried out to obtain [200,201], and the chemical reaction equation is shown in Figure 1-4.
There are hydroxy propoxy (-[OCH2CH(CH3)] n OH), methoxy (-OCH3) and unreacted hydroxyl groups on the structural unit of HPMC at the same time, and its performance is the reflection of the joint action of various groups. [202]. The ratio between the two substituents is determined by the mass ratio of the two etherifying agents, the concentration and mass of sodium hydroxide, and the mass ratio of etherifying agents per unit mass of cellulose [203]. Hydroxy propoxy is an active group, which can be further alkylated and hydroxy alkylated; this group is a hydrophilic group with a long-branched chain, which plays a certain role in plasticizing inside the chain. Methoxy is an end-capping group, which leads to the inactivation of this reaction site after the reaction; this group is a hydrophobic group and has a relatively short structure [204, 205]. Unreacted and newly introduced hydroxyl groups can continue to be substituted, resulting in a rather complex final chemical structure, and the HPMC properties vary within a certain range. For HPMC, a small amount of substitution can make its physicochemical properties quite different [206], for example, the physicochemical properties of high methoxy and low hydroxypropyl HPMC are close to MC; The performance of HPMC is close to that of HPC.
1.2.2.3 Properties of hydroxypropyl methylcellulose
(1) Thermogelability of HPMC
The HPMC chain has unique hydration-dehydration characteristics due to the introduction of hydrophobic-methyl and hydrophilic-hydroxypropyl groups. It gradually undergoes gelation conversion when heated, and returns to a solution state after cooling. That is, it has thermally induced gel properties, and the gelation phenomenon is a reversible but not identical process.
Regarding the gelation mechanism of HPMC, it is widely accepted that at lower temperatures (below the gelation temperature), HPMC in solution and polar water molecules are bound together by hydrogen bonds to form A so-called “birdcage”-like supramolecular structure. There are some simple entanglements between the molecular chains of the hydrated HPMC, other than that, there are few other interactions. When the temperature increases, HPMC first absorbs energy to break the intermolecular hydrogen bonds between water molecules and HPMC molecules, destroying the cage-like molecular structure, gradually losing the bound water on the molecular chain, and exposing hydroxypropyl and methoxy groups. As the temperature continues to increase (to reach the gel temperature), HPMC molecules gradually form a three-dimensional network structure through hydrophobic association, HPMC gels eventually form [160, 207, 208].
The addition of inorganic salts has some effect on the gel temperature of HPMC, some decrease the gel temperature due to salting out phenomenon, and others increase the gel temperature due to salt dissolution phenomenon [209]. With the addition of salts such as NaCl, the phenomenon of salting out occurs and the gel temperature of HPMC decreases [210, 211]. After salts are added to HPMC, water molecules are more inclined to combine with salt ions, so that the hydrogen bond between water molecules and HPMC is destroyed, the water layer around the HPMC molecules is consumed, and the HPMC molecules can be released quickly for hydrophobicity. Association, the temperature of gel formation gradually decreases. On the contrary, when salts such as NaSCN are added, the phenomenon of salt dissolution occurs and the gel temperature of HPMC increases [212]. The order of the decreasing effect of anions on the gel temperature is: SO42− > S2O32− > H2PO4− > F− > Cl− > Br− > NO3−> I− > ClO4− > SCN− , the order of cations on the gel temperature increase is: Li+ > Na+ > K+ > Mg2+ > Ca2+ > Ba2+ [213].
When some organic small molecules such as monohydric alcohols containing hydroxyl groups are added, the gel temperature increases with the increase of the addition amount, shows a maximum value and then decreases until phase separation occurs [214, 215]. This is mainly due to its small molecular weight, which is comparable to that of water molecules in order of magnitude, and can achieve molecular-level miscibility after compounding.
(2) Solubility of HPMC
HPMC has hot water insoluble and cold-water soluble properties similar to MC, but can be divided into cold dispersion type and hot dispersion type according to different water solubility [203]. Cold-dispersed HPMC can quickly disperse in water in cold water, and its viscosity increases after a period of time, and it is truly dissolved in water; heat-dispersed HPMC, on the contrary, shows agglomeration when adding water at a lower temperature, but it is more difficult to add. In high-temperature water, HPMC can be quickly dispersed, and the viscosity increases after the temperature decreases, becoming a real HPMC aqueous solution. The solubility of HPMC in water is related to the content of methoxy groups, which are insoluble in hot water above 85 °C, 65 °C and 60 °C from high to low. Generally speaking, HPMC is insoluble in organic solvents such as acetone and chloroform, but soluble in ethanol aqueous solution and mixed organic solutions.
(3) Salt tolerance of HPMC
The non-ionic nature of HPMC makes it unable to be ionized in water, so it will not react with metal ions to precipitate. However, the addition of salt will affect the temperature at which the HPMC gel is formed. When the salt concentration increases, the gel temperature of HPMC decreases; when the salt concentration is lower than the flocculation point, the viscosity of the HPMC solution can be increased, so in application, the purpose of thickening can be achieved by adding an appropriate amount of salt [210, 216].
(4) Acid and alkali resistance of HPMC
In general, HPMC has strong acid-base stability and is not affected by pH at pH 2-12. HPMC shows resistance to a certain degree of dilute acid, but shows a tendency to decrease in viscosity for concentrated acid; alkalis have little effect on it, but can slightly increase and then slowly decrease the solution viscosity [217, 218].
(5) Influence factor of HPMC viscosity
HPMC is pseudoplastic, its solution is stable at room temperature, and its viscosity is affected by molecular weight, concentration and temperature. At the same concentration, the higher the HPMC molecular weight, the higher the viscosity; for the same molecular weight product, the higher the HPMC concentration, the higher the viscosity; the viscosity of the HPMC product decreases with the increase of temperature, and reaches the gel formation temperature, with a sudden increase in viscosity due to gelation [9, 219, 220].
(6) Other properties of HPMC
HPMC has strong resistance to enzymes, and its resistance to enzymes increases with the degree of substitution. Therefore, the product has a more stable quality during storage than other sugar products [189, 212]. HPMC has certain emulsifying properties. Hydrophobic methoxy groups can be adsorbed on the surface of the oil phase in the emulsion to form a thick adsorption layer, which can act as a protective layer; water-soluble hydroxyl groups can be combined with water to improve the continuous phase. Viscosity, inhibits the coalescence of the dispersed phase, reduces the surface tension, and stabilizes the emulsion [221]. HPMC can be mixed with water-soluble polymers such as gelatin, methylcellulose, locust bean gum, carrageenan and gum arabic to form a uniform and transparent solution, and can also be mixed with plasticizers such as glycerin and polyethylene glycol. [200, 201, 214].
1.2.2.4 Problems existing in the application of hydroxypropyl methylcellulose
First, the high price limits the wide application of HPMC. Although HPMC film has good transparency, grease barrier properties and mechanical properties. However, its high price (about 100,000/ton) limits its wide application, even in higher-value pharmaceutical applications such as capsules. The reason why HPMC is so expensive is firstly because the raw material cellulose used to prepare HPMC is relatively expensive. In addition, two substituent groups, hydroxypropyl group and methoxy group, are grafted on HPMC at the same time, which makes its preparation process very difficult. Complex, so HPMC products are more expensive.
Second, the low viscosity and low gel strength properties of HPMC at low temperatures reduce its processability in various applications. HPMC is a thermal gel, which exists in a solution state with very low viscosity at low temperature, and can form a viscous solid-like gel at high temperature, so processing processes such as coating, spraying and dipping must be carried out at high temperature. Otherwise, the solution will easily flow down, resulting in the formation of non-uniform film material, which will affect the quality and performance of the product. Such high temperature operation increases the difficulty coefficient of operation, resulting in high production energy consumption and high production cost.
1.2.3 Hydroxypropyl starch cold gel
Starch is a natural polymer compound synthesized by photosynthesis of plants in the natural environment. Its constituent polysaccharides are usually stored in the seeds and tubers of plants in the form of granules together with proteins, fibers, oils, sugars and minerals. or in the root [222]. Starch is not only the main source of energy intake for people, but also an important industrial raw material. Because of its wide source, low price, green, natural and renewable, it has been widely used in food and medicine, fermentation, papermaking, textile and petroleum industries [223].
1.2.3.1 Starch and its derivatives
Starch is a natural high polymer whose structural unit is α-D-anhydroglucose unit. Different units are connected by glycosidic bonds, and its molecular formula is (C6H10O5) n. A part of the molecular chain in starch granules is connected by α-1,4 glycosidic bonds, which is linear amylose; another part of the molecular chain is connected by α-1,6 glycosidic bonds on this basis, which is branched amylopectin [224]. In starch granules, there are crystalline regions in which molecules are arranged in an orderly arrangement and amorphous regions in which molecules are arranged disorderly. part composition. There is no clear boundary between the crystalline region and the amorphous region, and amylopectin molecules can pass through multiple crystalline regions and amorphous regions. Based on the natural nature of starch synthesis, the polysaccharide structure in starch varies with plant species and source sites [225].
Although starch has become one of the important raw materials for industrial production due to its wide source and renewable properties, native starch generally has disadvantages such as poor water solubility and film-forming properties, low emulsifying and gelling abilities, and insufficient stability. To expand its application range, starch is usually physicochemically modified to adapt it to different application requirements [38, 114]. There are three free hydroxyl groups on each glucose structural unit in starch molecules. These hydroxyl groups are highly active and endow starch with properties similar to polyols, which provide the possibility for starch denaturation reaction.
After modification, some properties of native starch have been improved to a large extent, overcoming the use defects of native starch, so modified starch plays a pivotal role in the current industry [226]. Oxidized starch is one of the most widely used modified starches with relatively mature technology. Compared with native starch, oxidized starch is easier to gelatinize. Advantages of high adhesion. Esterified starch is a starch derivative formed by esterification of hydroxyl groups in starch molecules. A very low degree of substitution can significantly change the properties of native starch. The transparency and film-forming properties of starch paste are obviously improved. Etherified starch is the etherification reaction of hydroxyl groups in starch molecules to generate polystarch ether, and its retrogradation is weakened. Under the strong alkaline conditions that oxidized starch and esterified starch cannot be used, the ether bond can also remain relatively stable. prone to hydrolysis. Acid-modified starch, the starch is treated with acid to increase the amylose content, resulting in enhanced retrogradation and starch paste. It is relatively transparent and forms a solid gel upon cooling [114].
1.2.3.2 Hydroxypropyl starch structure
Hydroxypropyl starch (HPS), whose molecular structure is shown in Figures 1-4, is a non-ionic starch ether, which is prepared by the etherification reaction of propylene oxide with starch under alkaline conditions [ 223, 227, 228], and its chemical reaction equation is shown in Figure 1-6.
During the synthesis of HPS, in addition to reacting with starch to generate hydroxypropyl starch, propylene oxide can also react with the generated hydroxypropyl starch to generate polyoxypropyl side chains. degree of substitution. Degree of substitution (DS) refers to the average number of substituted hydroxyl groups per glucosyl group. Most of the glucosyl groups of starch contain 3 hydroxyl groups that can be replaced, so the maximum DS is 3. The molar degree of substitution (MS) refers to the average mass of substituents per mole of glucosyl group [223, 229]. The process conditions of the hydroxypropylation reaction, the starch granule morphology, and the ratio of amylose to amylopectin in the native starch all affect the size of the MS.
1.2.3.3 Properties of hydroxypropyl starch
(1) Cold gelation of HPS
For the hot HPS starch paste, especially the system with high amylose content, during the cooling process, the amylose molecular chains in the starch paste entangle with each other to form a three-dimensional network structure, and show obvious solid-like behavior. It becomes an elastomer, forms a gel, and can return to a solution state after reheating, that is, it has cold gel properties, and this gel phenomenon has reversible properties [228].
The gelatinized amylose is continuously coiled to form a coaxial single helical structure. The outside of these single helical structures is a hydrophilic group, and the inside is a hydrophobic cavity. At high temperature, HPS exists in aqueous solution as random coils from which some single helical segments stretch out. When the temperature is lowered, the hydrogen bonds between HPS and water are broken, the structural water is lost, and the hydrogen bonds between molecular chains are continuously formed, finally forming a three-dimensional network gel structure. The filling phase in the gel network of starch is the residual starch granules or fragments after gelatinization, and the intertwining of some amylopectin also contributes to the formation of gel [230-232].
(2) Hydrophilicity of HPS
The introduction of hydrophilic hydroxypropyl groups weakens the strength of hydrogen bonds between starch molecules, promotes the movement of starch molecules or segments, and reduces the melting temperature of starch microcrystals; the structure of starch granules is changed, and the surface of starch granules is rough As the temperature increases, some cracks or holes appear, so that water molecules can easily enter the interior of the starch granules, making the starch easier to swell and gelatinize, so the gelatinization temperature of the starch decreases. As the degree of substitution increases, the gelatinization temperature of hydroxypropyl starch decreases, and finally it can swell in cold water. After hydroxypropylation, the flowability, low temperature stability, transparency, solubility, and film-forming properties of starch pastes were improved [233–235].
(3) Stability of HPS
HPS is a non-ionic starch ether with high stability. During chemical reactions such as hydrolysis, oxidation, and cross-linking, the ether bond will not be broken and the substituents will not fall off. Therefore, the properties of HPS are relatively less affected by electrolytes and pH, ensuring that it can be used in a wide range of acid-base pH [236-238].
1.2.3.4 Application of HPS in the field of food and medicine
HPS is non-toxic and tasteless, with good digestion performance and relatively low hydrolyzate viscosity. It is recognized as a safe edible modified starch at home and abroad. As early as the 1950s, the United States approved hydroxypropyl starch for direct use in food [ 223, 229, 238]. HPS is a modified starch widely used in the food field, mainly used as a thickening agent, suspending agent and stabilizer.
It can be used in convenience foods and frozen foods such as beverages, ice cream, and jams; it can partially replace high-priced edible gums such as gelatin; it can be made into edible films and used as food coatings and packaging [229, 236].
HPS is commonly used in the field of medicine as fillers, binders for medicinal crops, disintegrants for tablets, materials for pharmaceutical soft and hard capsules, drug coatings, anti-condensing agents for artificial red blood cells and plasma thickeners, etc. [239].
1.3 Polymer Compounding
Polymer materials are widely used in all aspects of life and are indispensable and important materials. The continuous development of science and technology makes people’s requirements more and more diverse, and it is generally difficult for single-component polymer materials to meet the diverse application requirements of human beings. Combining two or more polymers is the most economical and effective method to obtain polymer materials with low price, excellent performance, convenient processing and wide application, which has attracted the attention of many researchers and has been paid more and more attention [ 240-242].
1.3.1 Purpose and method of polymer compounding
The main purpose of polymer compounding: (l) To optimize the comprehensive properties of materials. Different polymers are compounded, so that the final compound retains the excellent properties of a single macromolecule, learns from each other’s strengths and complements its weaknesses, and optimizes the comprehensive properties of polymer materials. (2) Reduce material cost. Some polymer materials have excellent properties, but they are expensive. Therefore, they can be compounded with other inexpensive polymers to reduce costs without affecting the use. (3) Improve material processing properties. Some materials have excellent properties but are difficult to process, and suitable other polymers can be added to improve their processing properties. (4) To strengthen a certain property of the material. In order to improve the performance of the material in a specific aspect, another polymer is used to modify it. (5) Develop new functions of materials.
Common polymer compounding methods: (l) Melting compounding. Under the shearing action of the compounding equipment, different polymers are heated to above the viscous flow temperature for compounding, and then cooled and granulated after compounding. (2) Solution reconstitution. The two components are stirred and blended by using a common solvent, or the dissolved different polymer solutions are stirred evenly, and then the solvent is removed to obtain a polymer compound. (3) Emulsion compounding. After stirring and mixing different polymer emulsions of the same emulsifier type, a coagulant is added to co-precipitate the polymer to obtain a polymer compound. (4) Copolymerization and compounding. Including graft copolymerization, block copolymerization and reactive copolymerization, the compounding process is accompanied by chemical reaction. (5) Interpenetrating network [10].
1.3.2 Compounding of natural polysaccharides
Natural polysaccharides are a common class of polymer materials in nature, which are usually chemically modified and exhibit a variety of excellent properties. However, single polysaccharide materials often have certain performance limitations, so different polysaccharides are often compounded to achieve the purpose of complementing the performance advantages of each component and expanding the scope of application. As early as the 1980s, research on the compounding of different natural polysaccharides has increased substantially [243]. The research on the natural polysaccharide compound system at home and abroad mostly focuses on the compound system of curdlan and non-curdlan and the compound system of two kinds of non-curd polysaccharide.
1.3.2.1 Classification of natural polysaccharide hydrogels
Natural polysaccharides can be divided into curdlan and non-curdlan according to their ability to form gels. Some polysaccharides can form gels by themselves, so they are called curdlan, such as carrageenan, etc.; others have no gelling properties themselves, and are called non-curd polysaccharides, such as xanthan gum.
Hydrogels can be obtained by dissolving natural curdlan in an aqueous solution. Based on the thermoreversibility of the resulting gel and the temperature dependence of its modulus, it can be subdivided into the following four different types [244]:
(1) Cryogel, polysaccharide solution can only obtain gel at low temperature, such as carrageenan.
(2) Thermally induced gel, polysaccharide solution can only obtain gel at high temperature, such as glucomannan.
(3) The polysaccharide solution can not only obtain gel at lower temperature, but also obtain gel at higher temperature, but present a solution state at intermediate temperature.
(4) The solution can only obtain gel at a certain temperature in the middle. Different natural curdlan has its own critical (minimum) concentration, above which gel can be obtained. The critical concentration of the gel is related to the continuous length of the polysaccharide molecular chain; the strength of the gel is greatly affected by the concentration and molecular weight of the solution, and generally, the strength of the gel increases as the concentration increases [245].
1.3.2.2 Compound system of curdlan and non-curdlan
Compounding non-curdlan with curdlan generally improves the gel strength of polysaccharides [246]. The compounding of konjac gum and carrageenan enhances the stability and gel elasticity of the composite gel network structure, and significantly improves its gel strength. Wei Yu et al. compounded carrageenan and konjac gum, and discussed the gel structure after compounding. The study found that after compounding carrageenan and konjac gum, a synergistic effect was produced, and a network structure dominated by carrageenan was formed, konjac gum is dispersed in it, and its gel network is denser than that of pure carrageenan [247]. Kohyama et al. studied the compound system of carrageenan/konjac gum, and the results showed that with the continuous increase of the molecular weight of konjac gum, the rupture stress of the composite gel continued to increase; konjac gum with different molecular weights showed similar gel formation. temperature. In this compound system, the formation of the gel network is undertaken by carrageenan, and the interaction between the two curdlan molecules results in the formation of weak cross-linked regions [248]. Nishinari et al. studied the gellan gum/konjac gum compound system, and the results showed that the effect of monovalent cations on the compound gel was more pronounced. It can increase the system modulus and gel formation temperature. Divalent cations can promote the formation of composite gels to a certain extent, but excessive amounts will cause phase separation and reduce the modulus of the system [246]. Breneer et al. studied the compounding of carrageenan, locust bean gum and konjac gum, and found that carrageenan, locust bean gum and konjac gum can produce synergistic effects, and the optimal ratio is locust bean gum/carrageenan 1:5.5, konjac gum/carrageenan 1:7, and when the three are compounded together, the synergistic effect is the same as that of carrageenan/konjac gum, indicating that there is no special compounding of the three. interaction [249].
1.3.2.2 Two non-curdlan compound systems
Two natural polysaccharides that do not have gel properties can exhibit gel properties through compounding, resulting in gel products [250]. Combining locust bean gum with xanthan gum produces a synergistic effect that induces the formation of new gels [251]. A new gel product can also be obtained by adding xanthan gum to konjac glucomannan for compounding [252]. Wei Yanxia et al. studied the rheological properties of the complex of locust bean gum and xanthan gum. The results show that the compound of locust bean gum and xanthan gum produces a synergistic effect. When the compound volume ratio is 4:6, the strongest synergistic effect [253]. Fitzsimons et al. compounded konjac glucomannan with xanthan gum at room temperature and under heating. The results showed that all compounds exhibited gel properties, reflecting the synergistic effect between the two. The compounding temperature and the structural state of xanthan gum did not affect the interaction between the two [254]. Guo Shoujun and others studied the original mix of pig feces bean gum and xanthan gum, and the results showed that pig feces bean gum and xanthan gum have a strong synergistic effect. The optimal compounding ratio of pig feces bean gum and xanthan gum compound adhesive is 6/4 (w/w). It is 102 times that of the single solution of soybean gum, and the gel is formed when the concentration of the compound gum reaches 0.4%. The compound adhesive has high viscosity, good stability and rheological properties, and is an excellent food-gums [255].
1.3.3 Compatibility of polymer composites
Compatibility, from a thermodynamic point of view, refers to achieving molecular-level compatibility, also known as mutual solubility. According to the Flory-Huggins model theory, the free energy change of the polymer compound system during the compounding process conforms to the Gibbs free energy formula:
△��� =△���—T△S (1-1)
Among them, △��� is the complex free energy, △��� is the complex heat, is the complex entropy; is the absolute temperature; the complex system is a compatible system only when the free energy changes △��� during the complex process [256].
The concept of miscibility arises from the fact that very few systems can achieve thermodynamic compatibility. Miscibility refers to the ability of different components to form homogeneous complexes, and the commonly used criterion is that the complexes exhibit a single glass transition point.
Different from thermodynamic compatibility, generalized compatibility refers to the ability of each component in the compound system to accommodate each other, which is proposed from a practical point of view [257].
Based on generalized compatibility, polymer compound systems can be divided into completely compatible, partially compatible and completely incompatible systems. A fully compatible system means that the compound is thermodynamically miscible at the molecular level; a partially compatible system means that the compound is compatible within a certain temperature or composition range; a completely incompatible system means that the compound is Molecular-level miscibility cannot be achieved at any temperature or composition.
Due to certain structural differences and conformational entropy between different polymers, most polymer complex systems are partially compatible or incompatible [11, 12]. Depending on the phase separation of the compound system and the level of mixing, the compatibility of the partially compatible system will also vary greatly [11]. The macroscopic properties of polymer composites are closely related to their internal microscopic morphology and the physical and chemical properties of each component. 240], so it is of great significance to study the microscopic morphology and compatibility of the compound system.
Research and Characterization Methods for Compatibility of Binary Compounds:
(1) Glass transition temperature T��� comparison method. Comparing the T��� of the compound with the T��� of its components, if only one T��� appears in the compound, the compound system is a compatible system; if there are two T���, and the two T��� positions of the compound are in the two groups The middle of the points T��� indicates that the compound system is a partially compatible system; if there are two T���, and they are located at the positions of the two components T���, it indicates that the compound system is an incompatible system.
T���The test instruments often used in the comparison method are dynamic thermomechanical analyzer (DMA) and differential scanning calorimeter (DSC). This method can quickly judge the compatibility of the compound system, but if the T��� of the two components is similar, a single T��� will also appear after compounding, so this method has certain shortcomings [10].
(2) Morphological observation method. First, observe the macroscopic morphology of the compound. If the compound has obvious phase separation, it can be preliminarily judged that the compound system is an incompatible system. Secondly, the microscopic morphology and phase structure of the compound are observed by microscope. The two components that are completely compatible will form a homogeneous state. Therefore, the compound with good compatibility can observe uniform phase distribution and small dispersed phase particle size. and blurry interface.
The test instruments often used in the topography observation method are optical microscope and scanning electron microscope (SEM). The topography observation method can be used as an auxiliary method in combination with other characterization methods。
(3) Transparency method. In a partially compatible compound system, the two components can be compatible within a certain temperature and composition range, and phase separation will occur beyond this range. In the process of the transformation of the compound system from a homogeneous system to a two-phase system, its light transmittance will change, so its compatibility can be studied by studying the transparency of the compound.
This method can only be used as an auxiliary method, because when the refractive indices of the two polymers are the same, the compound obtained by compounding the two incompatible polymers is also transparent.
(4) Rheological method. In this method, the sudden change of the viscoelastic parameters of the compound is used as the sign of phase separation, for example, the sudden change of the viscosity-temperature curve is used to mark the phase separation, and the sudden change of the apparent shear stress-temperature curve is used as the sign of phase separation. The compounding system without phase separation after compounding has good compatibility, and those with phase separation are incompatible or partially compatible system [258].
(5) Han’s curve method. Han’s curve is lg��� ’(���) lg G”, if the Han’s curve of the compound system has no temperature dependence, and the Han’s curve at different temperatures forms a main curve, the compound system is compatible; if the compound system is compatible The Han’s curve is temperature-dependent. If the Han’s curve is separated from each other at different temperatures and cannot form a main curve, the compound system is incompatible or partially compatible. Therefore, the compatibility of the compound system can be judged according to the separation of Han’s curve.
(6) Solution viscosity method. This method uses the change of solution viscosity to characterize the compatibility of the compound system. Under different solution concentrations, the viscosity of the compound is plotted against the composition. If it is a linear relationship, it means that the compound system is completely compatible; if it is a nonlinear relationship, it means that the compound system is partially compatible; if it is an S-shaped curve, then it shows that the compound system is completely incompatible [10].
(7) Infrared spectroscopy. After the two polymers are compounded, if the compatibility is good, there will be interactions such as hydrogen bonds, and the band positions of the characteristic groups on the infrared spectrum of each group on the polymer chain will shift. The offset of the characteristic group bands of the complex and each component can judge the compatibility of the complex system.
In addition, the compatibility of the complexes can also be studied by thermogravimetric analyzers, X-ray diffraction, small angle X-ray scattering, light scattering, neutron electron scattering, nuclear magnetic resonance and ultrasonic techniques [10].
1.3.4 Research progress of hydroxypropyl methylcellulose/hydroxypropyl starch compounding
1.3.4.1 Compounding of hydroxypropyl methylcellulose and other substances
Compounds of HPMC and other substances are mainly used in drug-controlled release systems and edible or degradable film packaging materials. In the application of drug-controlled release, the polymers often compounded with HPMC include synthetic polymers such as polyvinyl alcohol (PVA), lactic acid-glycolic acid copolymer (PLGA) and polycaprolactone (PCL), as well as proteins, Natural polymers such as polysaccharides. Abdel-Zaher et al. studied the structural composition, thermal stability and their relationship with performance of HPMC/PVA composites, and the results showed that there is some miscibility in the presence of the two polymers [259]. Zabihi et al. used HPMC/PLGA complex to prepare microcapsules for controlled and sustained release of insulin, which can achieve sustained release in the stomach and intestine [260]. Javed et al. compounded hydrophilic HPMC and hydrophobic PCL and used HPMC/PCL complexes as microcapsule materials for drug controlled and sustained release, which could be released in different parts of the human body by adjusting the compounding ratio [261]. Ding et al. studied the rheological properties such as viscosity, dynamic viscoelasticity, creep recovery, and thixotropy of HPMC/collagen complexes used in the field of controlled drug release, providing theoretical guidance for industrial applications [262]. Arthanari, Cai and Rai et al. [263-265] The complexes of HPMC and polysaccharides such as chitosan, xanthan gum, and sodium alginate were applied in the process of vaccine and drug sustained release, and the results showed a controllable drug release effect [263-265].
In the development of edible or degradable film packaging materials, the polymers often compounded with HPMC are mainly natural polymers such as lipids, proteins and polysaccharides. Karaca, Fagundes and Contreras-Oliva et al. prepared edible composite membranes with HPMC/lipid complexes, and used them in the preservation of plums, cherry tomatoes and citrus, respectively. The results showed that HPMC/lipid complex membranes had good the antibacterial effect of fresh-keeping [266-268]. Shetty, Rubilar, and Ding et al. studied the mechanical properties, thermal stability, microstructure, and interactions between components of edible composite films prepared from HPMC, silk protein, whey protein isolate, and collagen, respectively [269-271]. Esteghlal et al. formulated HPMC with gelatin to prepare edible films for use in bio-based packaging materials [111]. Priya, Kondaveeti, Sakata and Ortega-Toro et al. prepared HPMC/chitosan HPMC/xyloglucan, HPMC/ethyl cellulose and HPMC/starch edible composite films, respectively, and studied their thermal stability, mechanical properties properties, microstructure and antibacterial properties [139, 272-274]. The HPMC/PLA compound can also be used as a packaging material for food commodities, usually by extrusion [275].
In the development of edible or degradable film packaging materials, the polymers often compounded with HPMC are mainly natural polymers such as lipids, proteins and polysaccharides. Karaca, Fagundes and Contreras-Oliva et al. prepared edible composite membranes with HPMC/lipid complexes, and used them in the preservation of plums, cherry tomatoes and citrus, respectively. The results showed that HPMC/lipid complex membranes had good the antibacterial effect of fresh-keeping [266-268]. Shetty, Rubilar, and Ding et al. studied the mechanical properties, thermal stability, microstructure, and interactions between components of edible composite films prepared from HPMC, silk protein, whey protein isolate, and collagen, respectively [269-271]. Esteghlal et al. formulated HPMC with gelatin to prepare edible films for use in bio-based packaging materials [111]. Priya, Kondaveeti, Sakata and Ortega-Toro et al. prepared HPMC/chitosan HPMC/xyloglucan, HPMC/ethyl cellulose and HPMC/starch edible composite films, respectively, and studied their thermal stability, mechanical properties properties, microstructure and antibacterial properties [139, 272-274]. The HPMC/PLA compound can also be used as a packaging material for food commodities, usually by extrusion [275].
1.3.4.2 Compounding of starch and other substances
The research on the compounding of starch and other substances initially focused on various hydrophobic aliphatic polyester substances, including polylactic acid (PLA), polycaprolactone (PCL), polybutene succinic acid (PBSA), etc. 276]. Muller et al. studied the structure and properties of starch/PLA composites and the interaction between the two, and the results showed that the interaction between the two was weak and the mechanical properties of the composites were poor [277]. Correa, Komur and Diaz-Gomez et al. studied the mechanical properties, rheological properties, gel properties and compatibility of the two components of starch/PCL complexes, which were applied to the development of biodegradable materials, biomedical Materials and Tissue Engineering Scaffolding Materials [278-280]. Ohkika et al. found that the blend of cornstarch and PBSA is very promising. When the starch content is 5-30%, increasing the content of starch granules can increase the modulus and reduce the tensile stress and elongation at break [281,282]. Hydrophobic aliphatic polyester is thermodynamically incompatible with hydrophilic starch, and various compatibilizers and additives are usually added to improve the phase interface between starch and polyester. Szadkowska, Ferri, and Li et al. studied the effects of silanol-based plasticizers, maleic anhydride linseed oil, and functionalized vegetable oil derivatives on the structure and properties of starch/PLA complexes, respectively [283-285]. Ortega-Toro, Yu et al. used citric acid and diphenylmethane diisocyanate to compatibilize starch/PCL compound and starch/PBSA compound, respectively, to improve material properties and stability [286, 287].
In recent years, more and more researches have been done on the compounding of starch with natural polymers such as proteins, polysaccharides and lipids. Teklehaimanot, Sahin-Nadeen and Zhang et al studied the physicochemical properties of starch/zein, starch/whey protein and starch/gelatin complexes, respectively, and the results all achieved good results, which can be applied to food biomaterials and capsules [52, 288, 289]. Lozanno-Navarro, Talon and Ren et al. studied the light transmittance, mechanical properties, antibacterial properties and chitosan concentration of starch/chitosan composite films, respectively, and added natural extracts, tea polyphenols and other natural antibacterial agents to improve the antibacterial effect of the composite film. The research results show that the starch/chitosan composite film has great potential in the active packaging of food and medicine [290-292]. Kaushik, Ghanbarzadeh, Arvanitoyannis, and Zhang et al. studied the properties of starch/cellulose nanocrystals, starch/carboxymethylcellulose, starch/methylcellulose, and starch/hydroxypropylmethylcellulose composite films, respectively, and the main applications in edible/biodegradable packaging materials [293-295]. Dafe, Jumaidin and Lascombes et al. studied starch/food gum compounds such as starch/pectin, starch/agar and starch/carrageenan, mainly used in the field of food and food packaging [296-298]. The physicochemical properties of tapioca starch/corn oil, starch/lipid complexes were studied by Perez, De et al., mainly to guide the production process of extruded foods [299, 300].
1.3.4.3 Compounding of hydroxypropyl methylcellulose and starch
At present, there are not many studies on the compound system of HPMC and starch at home and abroad, and most of them are adding a small amount of HPMC into the starch matrix to improve the aging phenomenon of starch. Jimenez et al. used HPMC to reduce the aging of native starch to improve the permeability of starch membranes. The results showed that the addition of HPMC reduced the aging of starch and increased the flexibility of the composite membrane. The oxygen permeability of the composite membrane was significantly increased, but the waterproof performance did not. How much has changed [301]. Villacres, Basch et al. compounded HPMC and tapioca starch to prepare HPMC/starch composite film packaging materials, and studied the plasticizing effect of glycerin on the composite film and the effects of potassium sorbate and nisin on the antibacterial properties of the composite film. The results It shows that with the increase of HPMC content, the elastic modulus and tensile strength of the composite film are increased, the elongation at break is decreased, and the water vapor permeability has little effect; potassium sorbate and nisin can both improve the composite film. The antibacterial effect of two antibacterial agents is better when used together [112, 302]. Ortega-Toro et al. studied the properties of HPMC/starch hot-pressed composite membranes, and studied the effect of citric acid on the properties of composite membranes. The results showed that HPMC was dispersed in the starch continuous phase, and both citric acid and HPMC had an effect on the aging of starch. to a certain degree of inhibition [139]. Ayorinde et al. used HPMC/starch composite film for the coating of oral amlodipine, and the results showed that the disintegration time and release rate of the composite film were very good [303].
Zhao Ming et al. studied the effect of starch on the water retention rate of HPMC films, and the results showed that starch and HPMC had a certain synergistic effect, which resulted in an overall increase in water retention rate [304]. Zhang et al. studied the film properties of the HPMC/HPS compound and the rheological properties of the solution. The results show that the HPMC/HPS compound system has a certain compatibility, the compound membrane performance is good, and the rheological properties of HPS to HPMC Has a good balancing effect [305, 306]. There are few studies on the HPMC/starch compound system with high HPMC content, and most of them are in the shallow performance research, and the theoretical research on the compound system is relatively lacking, especially the gel of HPMC/HPS cold-heat reversed-phase composite gel. Mechanistic studies are still in a blank state.
1.4 Rheology of polymer complexes
In the process of processing polymer materials, flow and deformation will inevitably occur, and rheology is the science that studies the flow and deformation laws of materials [307]. Flow is a property of liquid materials, while deformation is a property of solid (crystalline) materials. A general comparison of liquid flow and solid deformation is as follows:
In practical industrial applications of polymer materials, their viscosity and viscoelasticity determine their processing performance. In the process of processing and molding, with the change of shear rate, the viscosity of polymer materials may have a large magnitude of several orders of magnitude. Change [308]. Rheological properties such as viscosity and shear thinning directly affect the control of pumping, perfusion, dispersion and spraying during the processing of polymer materials, and are the most important properties of polymer materials.
1.4.1 Viscoelasticity of polymers
Under the external force, the polymer liquid can not only flow, but also show deformation, showing a kind of “viscoelasticity” performance, and its essence is the coexistence of “solid-liquid two-phase” [309]. However, this viscoelasticity is not linear viscoelasticity at small deformations, but nonlinear viscoelasticity where the material exhibits large deformations and prolonged stress [310].
The natural polysaccharide aqueous solution is also called hydrosol. In the dilute solution, the polysaccharide macromolecules are in the form of coils separated from each other. When the concentration increases to a certain value, the macromolecular coils interpenetrate and overlap each other. The value is called the critical concentration [311]. Below the critical concentration, the viscosity of the solution is relatively low, and it is not affected by the shear rate, showing Newtonian fluid behavior; when the critical concentration is reached, the macromolecules that originally move in isolation begin to entangle with each other, and the solution viscosity significantly increases. increase [312]; while when the concentration exceeds the critical concentration, shear thinning is observed and the solution exhibits non-Newtonian fluid behavior [245].
Some hydrosols can form gels under certain conditions, and their viscoelastic properties are usually characterized by storage modulus G’, loss modulus G” and their frequency dependence. The storage modulus corresponds to the elasticity of the system, while The loss modulus corresponds to the viscosity of the system [311]. In dilute solutions, there is no entanglement between molecules, so over a wide range of frequencies, G′ is much smaller than G″, and showed strong frequency dependence. Since G′ and G″ are proportional to the frequency ω and its quadratic, respectively, when the frequency is higher, G′ > G″. When the concentration is higher than the critical concentration, G′ and G″ still have frequency dependence. When the frequency is lower, G′ < G″, and the frequency gradually increases, the two will cross, and reverse to G′ > in the high frequency region G”.
The critical point at which a natural polysaccharide hydrosol transforms into a gel is called the gel point. There are many definitions of gel point, and the most commonly used is the definition of dynamic viscoelasticity in rheology. When the storage modulus G′ of the system is equal to the loss modulus G″, it is the gel point, and G′ > G″ Gel formation [312, 313].
Some natural polysaccharide molecules form weak associations, and their gel structure is easily destroyed, and G’ is slightly larger than G”, showing a lower frequency dependence; while some natural polysaccharide molecules can form stable cross-linking regions, which The gel structure is stronger, G′ is much larger than G″, and has no frequency dependence [311].
1.4.2 Rheological behavior of polymer complexes
For a fully compatible polymer compound system, the compound is a homogeneous system, and its viscoelasticity is generally the sum of the properties of a single polymer, and its viscoelasticity can be described by simple empirical rules [314]. Practice has proved that the homogeneous system is not conducive to the improvement of its mechanical properties. On the contrary, some complex systems with phase-separated structures have excellent performance [315].
The compatibility of a partially compatible compound system will be affected by factors such as system compound ratio, shear rate, temperature and component structure, showing compatibility or phase separation, and the transition from compatibility to phase separation is inevitable. leading to significant changes in the viscoelasticity of the system [316, 317]. In recent years, there have been numerous studies on the viscoelastic behavior of partially compatible polymer complex systems. The research shows that the rheological behavior of the compound system in the compatibility zone presents the characteristics of the homogeneous system. In the phase separation zone, the rheological behavior is completely different from the homogeneous zone and extremely complex.
Understanding the rheological properties of the compounding system under different concentrations, compounding ratios, shear rates, temperatures, etc. is of great significance for the correct selection of processing technology, rational design of formulas, strict control of product quality, and appropriate reduction of production energy consumption. [309]. For example, for temperature-sensitive materials, the viscosity of the material can be changed by adjusting the temperature. And improve the processing performance; understand the shear thinning zone of the material, select the appropriate shear rate to control the processing performance of the material, and improve the production efficiency.
1.4.3 Factors affecting the rheological properties of the compound
1.4.3.1 Composition
The physical and chemical properties and internal structure of the compound system are a comprehensive reflection of the combined contributions of the properties of each component and the interaction between the components. Therefore, the physical and chemical properties of each component itself have a decisive role in the compound system. The degree of compatibility between different polymers varies widely, some are very compatible, and some are almost completely incompatible.
1.4.3.2 The ratio of compound system
The viscoelasticity and mechanical properties of the polymer compound system will change significantly with the change of the compound ratio. This is because the compound ratio determines the contribution of each component to the compound system, and also affects each component. interaction and phase distribution. Xie Yajie et al. studied chitosan/hydroxypropyl cellulose and found that the viscosity of the compound increased significantly with the increase of hydroxypropyl cellulose content [318]. Zhang Yayuan et al. studied the complex of xanthan gum and corn starch and found that when the ratio of xanthan gum was 10%, the consistency coefficient, yield stress and fluid index of the complex system increased significantly. Obviously [319].
1.4.3.3 Shear rate
Most polymer liquids are pseudoplastic fluids, which do not conform to Newton’s law of flow. The main feature is that the viscosity is basically unchanged under low shear, and the viscosity decreases sharply with the increase of shear rate [308, 320]. The flow curve of polymer liquid can be roughly divided into three regions: low shear Newtonian region, shear thinning region and high shear stability region. When the shear rate tends to zero, the stress and strain become linear, and the flow behavior of the liquid is similar to that of a Newtonian fluid. At this time, the viscosity tends to a certain value, which is called the zero-shear viscosity η0. η0 reflects the maximum relaxation time of the material and is an important parameter of polymer materials, which is related to the average molecular weight of the polymer and the activation energy of viscous flow. In the shear thinning zone, the viscosity gradually decreases with the increase of the shear rate, and the phenomenon of “shear thinning” occurs. This zone is a typical flow zone in the processing of polymer materials. In the high shear stability region, as the shear rate continues to increase, the viscosity tends to another constant, the infinite shear viscosity η∞, but this region is usually difficult to reach.
1.4.3.4 Temperature
Temperature directly affects the intensity of random thermal motion of molecules, which can significantly affect intermolecular interactions such as diffusion, molecular chain orientation, and entanglement. In general, during the flow of polymer materials, the movement of molecular chains is carried out in segments; as the temperature increases, the free volume increases, and the flow resistance of the segments decreases, so the viscosity decreases. However, for some polymers, as the temperature increases, hydrophobic association occurs between the chains, so the viscosity increases instead.
Various polymers have different degrees of sensitivity to temperature, and the same high polymer has different effects on the performance of its mechanism in different temperature ranges.
1.5 Research significance, research purpose and research content of this topic
1.5.1 Research significance
Although HPMC is a safe and edible material widely used in the field of food and medicine, it has good film-forming, dispersing, thickening, and stabilizing properties. HPMC film also has good transparency, oil barrier properties, and mechanical properties. However, its high price (about 100,000/ton) limits its wide application, even in higher-value pharmaceutical applications such as capsules. In addition, HPMC is a thermally induced gel, which exists in a solution state with low viscosity at low temperature, and can form a viscous solid-like gel at high temperature, so processing processes such as coating, spraying and dipping must It is carried out at high temperature, resulting in high production energy consumption and high production cost. Properties such as lower viscosity and gel strength of HPMC at low temperatures reduce the processability of HPMC in many applications.
In contrast, HPS is a cheap (about 20,000/ton) edible material that is also widely used in the field of food and medicine. The reason why HPMC is so expensive is that the raw material cellulose used to prepare HPMC is more expensive than the raw material starch used to prepare HPS. In addition, HPMC is grafted with two substituents, hydroxypropyl and methoxy. As a result, the preparation process is very complicated, so the price of HPMC is much higher than that of HPS. This project hopes to replace some of the expensive HPMCs with low-priced HPS, and reduce the product price on the basis of maintaining similar functions.
In addition, HPS is a cold gel, which exists in a viscoelastic gel state at low temperature and forms a flowing solution at high temperature. Therefore, adding HPS to HPMC can reduce the gel temperature of HPMC and increase its viscosity at low temperature. and gel strength, improving its processability at low temperatures. Moreover, HPS edible film has good oxygen barrier properties, so adding HPS into HPMC can improve the oxygen barrier properties of edible film.
In summary, the combination of HPMC and HPS: First, it has important theoretical significance. HPMC is a hot gel, and HPS is a cold gel. By compounding the two, there is theoretically a transition point between hot and cold gels. The establishment of HPMC/HPS cold and hot gel compound system and its mechanism research can provide a new way for the research of this kind of cold and hot reversed-phase gel compound system，established theoretical guidance. Secondly, it can reduce production costs and improve product profits. Through the combination of HPS and HPMC, the production cost can be reduced in terms of raw materials and production energy consumption, and the product profit can be greatly improved. Thirdly, it can improve the processing performance and expand the application. The addition of HPS can increase the concentration and gel strength of HPMC at low temperature, and improve its processing performance at low temperature. In addition, product performance can be improved. By adding HPS to prepare the edible composite film of HPMC/HPS, the oxygen barrier properties of the edible film can be improved.
The compatibility of the polymer compound system can directly determine the microscopic morphology and comprehensive properties of the compound, especially the mechanical properties. Therefore, it is very important to study the compatibility of the HPMC/HPS compound system. Both HPMC and HPS are hydrophilic polysaccharides with the same structural unit-glucose and modified by the same functional group hydroxypropyl, which greatly improves the compatibility of the HPMC/HPS compound system. However, HPMC is a cold gel and HPS is a hot gel, and the inverse gel behavior of the two leads to the phase separation phenomenon of the HPMC/HPS compound system. In summary, the phase morphology and phase transition of the HPMC/HPS cold-hot gel composite system are quite complex, so the compatibility and phase separation of this system will be very interesting.
The morphological structure and rheological behavior of polymer complex systems are interrelated. On the one hand, the rheological behavior during processing will have a great impact on the morphological structure of the system; on the other hand, the rheological behavior of the system can accurately reflect the changes in the morphological structure of the system. Therefore, it is of great significance to study the rheological properties of HPMC/HPS compound system for guiding production, processing and quality control.
The macroscopic properties such as morphological structure, compatibility and rheology of the HPMC/HPS cold and hot gel compound system are dynamic, and are affected by a series of factors such as solution concentration, compounding ratio, shear rate and temperature. The relationship between the microscopic morphological structure and the macroscopic properties of the composite system can be regulated by controlling the morphological structure and compatibility of the composite system.
1.5.2 Research purpose
The HPMC/HPS cold and hot reversed-phase gel compound system was constructed, its rheological properties were studied, and the effects of the physical and chemical structure of the components, compounding ratio and processing conditions on the rheological properties of the system were explored. The edible composite film of HPMC/HPS was prepared, and the macroscopic properties such as mechanical properties, air permeability and optical properties of the film were studied, and the influencing factors and laws were explored. Systematically study the phase transition, compatibility and phase separation of the HPMC/HPS cold and hot reversed-phase gel complex system, explore its influencing factors and mechanisms, and establish the relationship between microscopic morphological structure and macroscopic properties. The morphological structure and compatibility of the composite system are used to control the properties of composite materials.
1.5.3 Research content
In order to achieve the expected research purpose, this paper will do the following research:
(1) Construct the HPMC/HPS cold and hot reversed-phase gel compound system, and use a rheometer to study the rheological properties of the compound solution, especially the effects of concentration, compounding ratio and shear rate on the viscosity and flow index of the compound system. The influence and law of rheological properties such as thixotropy and thixotropy were investigated, and the formation mechanism of cold and hot composite gel was preliminarily explored.
(2) HPMC/HPS edible composite film was prepared, and scanning electron microscope was used to study the influence of the inherent properties of each component and the composition ratio on the microscopic morphology of the composite film; the mechanical property tester was used to study the inherent properties of each component, the composition of the composite film The influence of the ratio and environmental relative humidity on the mechanical properties of the composite film; the use of oxygen transmission rate tester and UV-Vis spectrophotometer to study the effects of the inherent properties of the components and the compound ratio on the oxygen and light transmission properties of the composite film The compatibility and phase separation of the HPMC/HPS cold-hot inverse gel composite system were studied by scanning electron microscopy, thermogravimetric analysis and dynamic thermomechanical analysis.
(3) The relationship between the microscopic morphology and mechanical properties of the HPMC/HPS cold-hot inverse gel composite system was established. The edible composite film of HPMC/HPS was prepared, and the influence of the compound concentration and compound ratio on the phase distribution and phase transition of the sample was studied by optical microscope and iodine dyeing method; The influence rule of compound concentration and compound ratio on the mechanical properties and light transmission properties of the samples was established. The relationship between the microstructure and mechanical properties of the HPMC/HPS cold-hot inverse gel composite system was investigated.
(4) Effects of HPS substitution degree on rheological properties and gel properties of HPMC/HPS cold-hot reversed-phase gel composite system. The effects of HPS substitution degree, shear rate and temperature on the viscosity and other rheological properties of the compound system, as well as the gel transition point, modulus frequency dependence and other gel properties and their laws were studied by using a rheometer. The temperature-dependent phase distribution and phase transition of the samples were studied by iodine staining, and the gelation mechanism of the HPMC/HPS cold-hot reversed-phase gel complex system was described.
(5) Effects of chemical structure modification of HPS on macroscopic properties and compatibility of HPMC/HPS cold-hot reversed-phase gel composite system. The edible composite film of HPMC/HPS was prepared, and the effect of HPS hydroxypropyl substitution degree on the crystal structure and micro-domain structure of the composite film was studied by synchrotron radiation small-angle X-ray scattering technology. The influence law of HPS hydroxypropyl substitution degree on the mechanical properties of composite membrane was studied by mechanical property tester; the influence law of HPS substitution degree on the oxygen permeability of composite membrane was studied by oxygen permeability tester; the HPS hydroxypropyl Influence of group substitution degree on thermal stability of HPMC/HPS composite films.
Chapter 2 Rheological study of HPMC/HPS compound system
Natural polymer-based edible films can be prepared by a relatively simple wet method [321]. First, the polymer is dissolved or dispersed in the liquid phase to prepare an edible film-forming liquid or film-forming suspension, and then concentrated by removing the solvent. Here, the operation is usually performed by drying at a slightly higher temperature. This process is typically used to produce prepackaged edible films, or to coat the product directly with a film-forming solution by dipping, brushing or spraying. The design of edible film processing requires the acquisition of accurate rheological data of the film-forming liquid, which is of great significance for the product quality control of edible packaging films and coatings [322].
HPMC is a thermal adhesive, which forms a gel at high temperature and is in a solution state at low temperature. This thermal gel property makes its viscosity at low temperature very low, which is not conducive to the specific production processes such as dipping, brushing and dipping. operation, resulting in poor processability at low temperatures. In contrast, HPS is a cold gel, a viscous gel state at low temperature, and a high temperature. A low viscosity solution state. Therefore, through the combination of the two, the rheological properties of HPMC such as viscosity at low temperature can be balanced to a certain extent.
This chapter focuses on the effects of solution concentration, compounding ratio and temperature on the rheological properties such as zero-shear viscosity, flow index and thixotropy of the HPMC/HPS cold-hot inverse gel compound system. The addition rule is used to preliminarily discuss the compatibility of the compound system.
2.2 Experimental method
2.2.1 Preparation of HPMC/HPS compound solution
First weigh HPMC and HPS dry powder, and mix according to 15% (w/w) concentration and different ratios of 10:0, 7:3, 5:5, 3:7, 0:10; then add 70 °C In C water, stir rapidly for 30 min at 120 rpm/min to fully disperse HPMC; then heat the solution to above 95 °C, stir rapidly for 1 h at the same speed to completely gelatinize HPS; gelatinization is completed After that, the temperature of the solution was rapidly reduced to 70 °C, and the HPMC was fully dissolved by stirring at a slow speed of 80 rpm/min for 40 min. (All w/w in this article are: dry basis mass of sample/total solution mass).
2.2.2 Rheological properties of HPMC/HPS compound system
2.2.2.1 Principle of rheological analysis
The rotational rheometer is equipped with a pair of up and down parallel clamps, and simple shear flow can be realized through the relative motion between the clamps. The rheometer can be tested in step mode, flow mode and oscillation mode: in step mode, the rheometer can apply transient stress to the sample, which is mainly used to test the transient characteristic response and steady-state time of the sample. Evaluation and viscoelastic response such as stress relaxation, creep and recovery; in flow mode, the rheometer can apply linear stress to the sample, which is mainly used to test the dependence of the viscosity of the sample on shear rate and the dependence of viscosity on temperature and thixotropy; in oscillation mode, the rheometer can generate sinusoidal alternating oscillating stress, which is mainly used for the determination of the linear viscoelastic region, thermal stability evaluation and gelation temperature of the sample.
2.2.2.2 Flow mode test method
A parallel plate fixture with a diameter of 40 mm was used, and the plate spacing was set to 0.5 mm.
1. Viscosity changes with time. The test temperature was 25 °C, the shear rate was 800 s-1, and the test time was 2500 s.
2. Viscosity varies with shear rate. Test temperature 25 °C, pre-shear rate 800 s-1, pre-shear time 1000 s; shear rate 10²-10³s.
The shear stress (τ ) and shear rate (γ) follows the Ostwald-de Waele power law:
̇τ=K.γ n (2-1)
where τ is the shear stress, Pa;
γ is the shear rate, s-1;
n is the liquidity index;
K is the viscosity coefficient, Pa·s-n.
The relationship between the viscosity (ŋ) of the polymer solution and the shear rate (γ) can be fitted by the carren modulus:
Among them, ŋ0 shear viscosity, Pa s;
ŋ∞ is the infinite shear viscosity, Pa s;
λis the relaxation time, s;
n is the shear thinning index；
3. Three-stage thixotropy test method. The test temperature is 25 °C, a. The stationary stage, the shear rate is 1 s-1, and the test time is 50 s; b. The shear stage, the shear rate is 1000 s-1, and the test time is 20 s; c. The structure recovery process , the shear rate is 1 s-1, and the test time is 250 s.
In the process of structure recovery, the recovery degree of the structure after different recovery time is expressed by the recovery rate of viscosity:
DSR=ŋt ⁄ ŋ╳100%
Among them, ŋt is the viscosity at the structural recovery time t s, Pa s;
hŋ is the viscosity at the end of the first stage, Pa s.
2.3 Results and Discussion
2.3.1 The effect of shear time on the rheological properties of the compound system
At a constant shear rate, the apparent viscosity may show different trends with increasing shear time. Figure 2-1 shows a typical curve of viscosity versus time in an HPMC/HPS compound system. It can be seen from the figure that with the extension of the shearing time, the apparent viscosity decreases continuously. When the shearing time reaches about 500 s, the viscosity reaches a stable state, which indicates that the viscosity of the compound system under high-speed shearing has a certain value. The time dependence of, that is, thixotropy is exhibited within a certain time range.
Therefore, when studying the variation law of the viscosity of the compound system with the shear rate, before the real steady-state shear test, a certain period of high-speed pre-shearing is required to eliminate the influence of thixotropy on the compound system. Thus, the law of viscosity variation with shear rate as a single factor is obtained. In this experiment, the viscosity of all samples reached a steady state before 1000 s at a high shear rate of 800 1/s with time, which is not plotted here. Therefore, in the future experimental design, pre-shearing for 1000 s at a high shear rate of 800 1/s was adopted to eliminate the effect of thixotropy of all samples.
2.3.2 The effect of concentration on the rheological properties of the compound system
Generally, the viscosity of polymer solutions increases with the increase of solution concentration. Figure 2-2 shows the effect of concentration on the shear rate dependence of the viscosity of HPMC/HPS formulations. From the figure, we can see that at the same shear rate, the viscosity of the compound system increases gradually with the increase of the solution concentration. The viscosity of HPMC/HPS compound solutions with different concentrations decreased gradually with the increase of shear rate, showing obvious shear thinning phenomenon, which indicated that the compound solutions with different concentrations belonged to pseudoplastic fluids. However, the shear rate dependence of viscosity showed a different trend with the change of solution concentration. When the solution concentration is low, the shear thinning phenomenon of the composite solution is small; with the increase of the solution concentration, the shear thinning phenomenon of the composite solution is more obvious.
2.3.2.1 Effect of concentration on zero shear viscosity of compound system
The viscosity-shear rate curves of the compound system at different concentrations were fitted by the Carren model, and the zero-shear viscosity of the compound solution was extrapolated (0.9960 < R₂< 0.9997). The effect of concentration on the viscosity of the compound solution can be further studied by studying the relationship between zero shear viscosity and concentration. From Figure 2-3, it can be seen that the relationship between the zero-shear viscosity and concentration of the compound solution follows a power law:
where k and m are constants.
In the double logarithmic coordinate, depending on the magnitude of the slope m, it can be seen that the dependence on the concentration presents two different trends. According to Dio-Edwards theory, at low concentration, the slope is higher (m = 11.9, R2 = 0.9942), which belongs to dilute solution; while at high concentration, the slope is relatively low (m = 2.8, R2 = 0.9822), which belongs to sub- Concentrated solution. Therefore, the critical concentration C* of the compound system can be determined to be 8% through the junction of these two regions. According to the common relationship between different states and concentrations of polymers in solution, the molecular state model of HPMC/HPS compound system in low temperature solution is proposed, as shown in Figure 2-3.
HPS is a cold gel, it is a gel state at low temperature, and it is a solution state at high temperature. At the test temperature (25 °C), HPS is a gel state, as shown in the blue network area in the figure; on the contrary, HPMC is a hot gel, At the test temperature, it is in a solution state, as shown in the red line molecule.
In the dilute solution of C < C*, the HPMC molecular chains mainly exist as independent chain structures, and the excluded volume makes the chains separate from each other; moreover, the HPS gel phase interacts with a few HPMC molecules to form a whole The form and HPMC independent molecular chains exist separately from each other, as shown in Figure 2-2a.
With the increasing concentration, the distance between the independent molecular chains and phase regions gradually decreased. When the critical concentration C* is reached, the HPMC molecules interacting with the HPS gel phase gradually increase, and the independent HPMC molecular chains begin to connect with each other, forming the HPS phase as the gel center, and the HPMC molecular chains are intertwined and connected with each other. The microgel state is shown in Figure 2-2b.
With the further increase of the concentration, C > C*, the distance between the HPS gel phases is further reduced, and the entangled HPMC polymer chains and the HPS phase region become more complex and the interaction is more intense, so the solution exhibits behavior similar to that of polymer melts, as shown in Fig. 2-2c.
2.3.2.2 Effect of concentration on fluid behavior of compound system
The Ostwald-de Waele power law (see formula (2-1)) is used to fit the shear stress and shear rate curves (not shown in the text) of the compound system with different concentrations, and the flow index n and viscosity coefficient K can be obtained. , the fitting result is as shown in Table 2-1.
Table 2-1 Flow behavior index (n) and fluid consistency index (K) of the HPS/HPMC solution with various concentration at 25 °C
The flow exponent of Newtonian fluid is n = 1, the flow exponent of pseudoplastic fluid is n < 1, and the farther n deviates from 1, the stronger the pseudoplasticity of the fluid, and the flow exponent of dilatant fluid is n > 1. It can be seen from Table 2-1 that the n values of the compound solutions with different concentrations are all less than 1, indicating that the compound solutions are all pseudoplastic fluids. At low concentrations, the n value of the reconstituted solution is close to 0, which indicates that the low-concentration compound solution is close to Newtonian fluid, because in the low-concentration compound solution, the polymer chains exist independently of each other. With the increase of the solution concentration, the n value of the compound system gradually decreased, which indicated that the increase of the concentration enhanced the pseudoplastic behavior of the compound solution. Interactions such as entanglement occurred between and with the HPS phase, and its flow behavior was closer to that of polymer melts.
At low concentration, the viscosity coefficient K of the compound system is small (C < 8%, K < 1 Pa·s n), and with the increase of concentration, the K value of the compound system gradually increases, indicating that the viscosity of the compound system decreased, which is consistent with the concentration dependence of zero shear viscosity.
2.3.3 Influence of compounding ratio on rheological properties of compounding system
Fig. 2-4 Viscosity vs. shear rate of HPMC/HPS solution with different blend ratio at 25 °C
Table 2-2 Flow behavior index (n) and fluid consistency index (K) of the HPS/HPMC solution with various blend ratio at 25 °
Figures 2-4 show the effect of compounding ratio on the shear rate dependence of HPMC/HPS compounding solution viscosity. It can be seen from the figure that the viscosity of the compound system with low HPS content (HPS < 20%) does not change substantially with the increase of shear rate, mainly because in the compound system with low HPS content , HPMC in solution state at low temperature is the continuous phase; the viscosity of the compound system with high HPS content gradually decreases with the increase of shear rate, showing obvious shear thinning phenomenon, which indicates that the compound solution is pseudoplastic fluid. At the same shear rate, the viscosity of the compound solution increases with the increase of HPS content, which is mainly because HPS is in a more viscous gel state at low temperature.
Using the Ostwald-de Waele power law (see formula (2-1)) to fit the shear stress-shear rate curves (not shown in the text) of the compound systems with different compound ratios, the flow exponent n and the viscosity coefficient K, the fitting results are shown in Table 2-2. It can be seen from the table that 0.9869 < R2 < 0.9999, the fitting result is better. The flow index n of the compound system decreases gradually with the increase of HPS content, while the viscosity coefficient K shows a gradually increasing trend with the increase of HPS content, indicating that the addition of HPS makes the compound solution more viscous and difficult to flow. This trend is consistent with Zhang’s research results, but for the same compounding ratio, the n value of the compounded solution is higher than Zhang’s result [305], which is mainly because pre-shearing was performed in this experiment to eliminate the effect of thixotropy is eliminated; the Zhang result is the result of the combined action of thixotropy and shear rate; the separation of these two methods will be discussed in detail in Chapter 5.
2.3.3.1 Influence of compounding ratio on zero shear viscosity of compounding system
The relationship between the rheological properties of the homogeneous polymer compound system and the rheological properties of the components in the system conforms to the logarithmic summation rule. For a two-component compound system, the relationship between the compound system and each component can be expressed by the following equation:
Among them, F is the rheological property parameter of the complex system;
F1, F2 are the rheological parameters of component 1 and component 2, respectively;
∅1 and ∅2 are the mass fractions of component 1 and component 2, respectively, and ∅1 ∅2 .
Therefore, the zero-shear viscosity of the compound system after compounding with different compounding ratios can be calculated according to the logarithmic summation principle to calculate the corresponding predicted value. The experimental values of the compound solutions with different compound ratios were still extrapolated by carren fitting of the viscosity-shear rate curve. The predicted value of the zero shear viscosity of the HPMC/HPS compound system with different compound ratios is compared with the experimental value, as shown in Figure 2-5.
The dotted line part in the figure is the predicted value of the zero shear viscosity of the compound solution obtained by the logarithmic sum rule, and the dotted line graph is the experimental value of the compound system with different compounding ratios. It can be seen from the figure that the experimental value of the compound solution exhibits a certain positive-negative-deviation relative to the compounding rule, indicating that the compound system cannot achieve thermodynamic compatibility, and the compound system is a continuous phase-dispersion at low temperature The “sea-island” structure of the two-phase system; and with the continuous reduction of the HPMC/HPS compounding ratio, the continuous phase of the compounding system changed after the compounding ratio was 4:6. The chapter discusses the research in detail.
It can be clearly seen from the figure that when the HPMC/HPS compound ratio is large, the compound system has a negative deviation, which may be because the high viscosity HPS is distributed in the dispersed phase state in the lower viscosity HPMC continuous phase middle. With the increase of HPS content, there is a positive deviation in the compound system, indicating that the continuous phase transition occurs in the compound system at this time. HPS with high viscosity becomes the continuous phase of the compound system, while HPMC is dispersed in the continuous phase of HPS in a more uniform state.
2.3.3.2 Influence of compounding ratio on fluid behavior of compounding system
Figures 2-6 show the flow index n of the compounded system as a function of HPS content. Since the flow index n is fitted from a log-logarithmic coordinate, n here is a linear sum. It can be seen from the figure that with the increase of HPS content, the flow index n of the compound system gradually decreases, indicating that HPS reduces the Newtonian fluid properties of the compound solution and improves its pseudoplastic fluid behavior. The lower part is the gel state with higher viscosity. It can also be seen from the figure that the relationship between the flow index of the compound system and the content of HPS conforms to a linear relationship (R2 is 0.98062), this shows that the compound system has good compatibility.
2.3.3.3 Influence of compounding ratio on viscosity coefficient of compounding system
Figure 2-7 shows the viscosity coefficient K of the compounded solution as a function of HPS content. It can be seen from the figure that the K value of pure HPMC is very small, while the K value of pure HPS is the largest, which is related to the gel properties of HPMC and HPS, which are in solution and gel state respectively at low temperature. When the content of the low-viscosity component is high, that is, when the content of HPS is low, the viscosity coefficient of the compound solution is close to that of the low-viscosity component HPMC; while when the content of the high-viscosity component is high, the K value of the compound solution increases with the increase of HPS content increased significantly, which indicated that HPS increased the viscosity of HPMC at low temperature. This mainly reflects the contribution of the viscosity of the continuous phase to the viscosity of the compound system. In different cases where the low-viscosity component is the continuous phase and the high-viscosity component is the continuous phase, the contribution of the continuous phase viscosity to the viscosity of the compound system is obviously different. When low-viscosity HPMC is the continuous phase, the viscosity of the compound system mainly reflects the contribution of the viscosity of the continuous phase; and when the high-viscosity HPS is the continuous phase, the HPMC as the dispersed phase will reduce the viscosity of the high-viscosity HPS. effect.
2.3.4 Thixotropy
Thixotropy can be used to evaluate the stability of substances or multiple systems, because thixotropy can obtain information on the internal structure and the degree of damage under shearing force [323-325]. Thixotropy can be correlated with temporal effects and shear history leading to microstructural changes [324, 326]. The three-stage thixotropic method was used to study the effect of different compounding ratios on the thixotropic properties of the compounding system. As can be seen from Figures 2-5, all samples exhibited different degrees of thixotropy. At low shear rates, the viscosity of the compound solution increased significantly with the increase of HPS content, which was consistent with the change of zero-shear viscosity with HPS content.
The structural recovery degree DSR of the composite samples at different recovery time is calculated by formula (2-3), as shown in Table 2-1. If DSR < 1, the sample has low shear resistance, and the sample is thixotropic; conversely, if DSR > 1, the sample has anti-thixotropy. From the table, we can see that the DSR value of pure HPMC is very high, almost 1, this is because the HPMC molecule is a rigid chain, and its relaxation time is short, and the structure is recovered quickly under high shear force. The DSR value of HPS is relatively low, which confirms its strong thixotropic properties, mainly because HPS is a flexible chain and its relaxation time is long. The structure did not fully recover within the testing time frame.
For the compound solution, in the same recovery time, when the HPMC content is greater than 70%, the DSR decreases rapidly with the increase of the HPS content, because the HPS molecular chain is a flexible chain, and the number of rigid molecular chains in the compound system increases with the addition of HPS. If it is reduced, the relaxation time of the overall molecular segment of the compound system is prolonged, and the thixotropy of the compound system cannot be recovered quickly under the action of high shear. When the content of HPMC is less than 70%, the DSR increases with the increase of the content of HPS, which indicates that there is an interaction between the molecular chains of HPS and HPMC in the compound system, which improves the overall rigidity of molecular segments in the compound system and shortens the relaxation time of the compound system is reduced, and the thixotropy is reduced.
In addition, the DSR value of the compounded system was significantly lower than that of pure HPMC, which indicated that the thixotropy of HPMC was significantly improved by compounding. The DSR values of most of the samples in the compound system were greater than those of pure HPS, indicating that the stability of HPS was improved to a certain extent.
It can also be seen from the table that at different recovery times, the DSR values all show the lowest point when the HPMC content is 70%, and when the starch content is greater than 60%, the DSR value of the complex is higher than that of pure HPS. The DSR values within 10 s of all samples are very close to the final DSR values, which indicates that the structure of the composite system basically completed most of the tasks of structure recovery within 10 s. It is worth noting that the composite samples with high HPS content showed a trend of increasing at first and then decreasing with the prolongation of recovery time, which indicated that the composite samples also showed a certain degree of thixotropy under the action of low shear, and their structure more unstable.
The qualitative analysis of the three-stage thixotropy is consistent with the reported thixotropic ring test results, but the quantitative analysis results are inconsistent with the thixotropic ring test results. The thixotropy of HPMC/HPS compound system was measured by thixotropic ring method with the increase of HPS content [305]. Degeneration first decreased and then increased. The thixotropic ring test can only speculate the existence of thixotropic phenomenon, but cannot confirm it, because the thixotropic ring is the result of the simultaneous action of shear time and shear rate [325-327].
2.4 Summary of this chapter
In this chapter, the thermal gel HPMC and the cold gel HPS were used as the main raw materials to construct a two-phase composite system of cold and hot gel. Influence of rheological properties such as viscosity, flow pattern and thixotropy. According to the common relationship between different states and concentrations of polymers in solution, the molecular state model of HPMC/HPS compound system in low temperature solution is proposed. According to the logarithmic summation principle of the properties of different components in the compound system, the compatibility of the compound system was studied. The main findings are as follows:
- Compound samples with different concentrations all showed a certain degree of shear thinning, and the degree of shear thinning increased with the increase of concentration.
- With the increase of concentration, the flow index of the compound system decreased, and the zero-shear viscosity and viscosity coefficient increased, indicating that the solid-like behavior of the compound system was enhanced.
- There is a critical concentration (8%) in the HPMC/HPS compound system, below the critical concentration, the HPMC molecular chains and the HPS gel phase region in the compound solution are separated from each other and exist independently; when the critical concentration is reached, in the compound solution A microgel state is formed with the HPS phase as the gel center, and the HPMC molecular chains are intertwined and connected to each other; above the critical concentration, the crowded HPMC macromolecular chains and their intertwining with the HPS phase region are more complex, and the interaction is more complex. more intense, so the solution behaves like a polymer melt.
- The compounding ratio has a significant impact on the rheological properties of the HPMC/HPS compound solution. With the increase of HPS content, the shear thinning phenomenon of the compound system is more obvious, the flow index gradually decreases, and the zero-shear viscosity and viscosity coefficient gradually increase. increases, indicating that the solid-like behavior of the complex is significantly improved.
- The zero-shear viscosity of the compound system exhibits a certain positive-negative-deviation relative to the logarithmic summation rule. The compound system is a two-phase system with a continuous phase-dispersed phase “sea-island” structure at low temperature, and, As the HPMC/HPS compounding ratio decreased after 4:6, the continuous phase of the compounding system changed.
- There is a linear relationship between the flow index and the compounding ratio of the compounded solutions with different compounding ratios, which indicates that the compounding system has good compatibility.
- For the HPMC/HPS compound system, when the low-viscosity component is the continuous phase and the high-viscosity component is the continuous phase, the contribution of the continuous phase viscosity to the viscosity of the compound system is significantly different. When the low-viscosity HPMC is the continuous phase, the viscosity of the compound system mainly reflects the contribution of the continuous-phase viscosity; while when the high-viscosity HPS is the continuous phase, the HPMC as the disperse phase will reduce the viscosity of the high-viscosity HPS. effect.
- Three-stage thixotropy was used to study the effect of compounding ratio on the thixotropy of the compounded system. The thixotropy of the compounded system showed a trend of first decreasing and then increasing with the decrease of the HPMC/HPS compounding ratio.
- The above experimental results show that through the compounding of HPMC and HPS, the rheological properties of the two components, such as viscosity, shear thinning phenomenon and thixotropy, have been balanced to a certain extent.
Chapter 3 Preparation and Properties of HPMC/HPS Edible Composite Films
Polymer compounding is the most effective way to achieve multi-component performance complementarity, develop new materials with excellent performance, reduce product prices, and expand the application range of materials [240-242, 328]. Then, due to certain molecular structure differences and conformational entropy between different polymers, most polymer compounding systems are incompatible or partially compatible [11, 12]. The mechanical properties and other macroscopic properties of the polymer compound system are closely related to the physicochemical properties of each component, the compounding ratio of each component, the compatibility between the components, and the internal microscopic structure and other factors [240, 329].
From the chemical structure point of view, both HPMC and HPS are hydrophilic curdlan, have the same structural unit – glucose, and are modified by the same functional group – hydroxypropyl group, so HPMC and HPS should have a good phase. Capacitance. However, HPMC is a thermally induced gel, which is in a solution state with very low viscosity at low temperature, and forms a colloid at high temperature; HPS is a cold-induced gel, which is a low temperature gel and is in a solution state at high temperature; the gel conditions and behavior are completely opposite. The compounding of HPMC and HPS is not conducive to the formation of a homogeneous system with good compatibility. Taking into account both chemical structure and thermodynamics, it is of great theoretical significance and practical value to compound HPMC with HPS to establish a cold-hot gel compound system.
This chapter focuses on the study of the inherent properties of the components in the HPMC/HPS cold and hot gel compound system, the compounding ratio and the relative humidity of the environment on the microscopic morphology, compatibility and phase separation, mechanical properties, optical properties, and thermal drop properties of the compound system. And the influence of macroscopic properties such as oxygen barrier properties.
3.1 Materials and Equipment
3.1.1 Main experimental materials
3.1.2 Main instruments and equipment
3.2 Experimental method
3.2.1 Preparation of HPMC/HPS edible composite film
The 15% (w/w) dry powder of HPMC and HPS was mixed with 3% (w/w) The polyethylene glycol plasticizer was compounded in deionized water to obtain the compounded film-forming liquid, and the edible composite film of HPMC/HPS was prepared by the casting method.
Preparation method: firstly weigh HPMC and HPS dry powder, and mix them according to different ratios; then add into 70 °C water, and stir rapidly at 120 rpm/min for 30 min to fully disperse HPMC; then heat the solution to Above 95 °C, stir quickly at the same speed for 1 h to completely gelatinize HPS; after gelatinization is completed, the temperature of the solution is rapidly reduced to 70 °C, and the solution is stirred at a slow speed of 80 rpm/min for 40 min. Fully dissolve HPMC. Pour 20 g of the mixed film-forming solution into a polystyrene petri dish with a diameter of 15 cm, cast it flat, and dry it at 37 °C. The dried film is peeled off from the disc to obtain an edible composite membrane.
Edible films were all equilibrated at 57% humidity for more than 3 days before testing, and the edible film portion used for mechanical property testing was equilibrated at 75% humidity for more than 3 days.
3.2.2 Micromorphology of the edible composite film of HPMC/HPS
3.2.2.1 Analysis principle of scanning electron microscope
The electron gun on the top of the Scanning Electron Microscopy (SEM) can emit a high amount of electrons. After being reduced and focused, it can form an electron beam with a certain energy and intensity. Driven by the magnetic field of the scanning coil, according to a certain time and space order Scan the surface of the sample point by point. Due to the difference in the characteristics of the surface micro-area, the interaction between the sample and the electron beam will generate secondary electron signals with different intensities, which are collected by the detector and converted into electrical signals, which are amplified by the video and input to The grid of the picture tube, after adjusting the brightness of the picture tube, a secondary electron image can be obtained that can reflect the morphology and characteristics of the micro-region on the surface of the sample. Compared with traditional optical microscopes, the resolution of SEM is relatively high, about 3nm-6nm of the surface layer of the sample, which is more suitable for the observation of micro-structure features on the surface of materials.
3.2.2.2 Test method
The edible film was placed in a desiccator for drying, and an appropriate size of edible film was selected, pasted on the SEM special sample stage with conductive adhesive, and then gold-plated with a vacuum coater. During the test, the sample was put into the SEM, and the microscopic morphology of the sample was observed and photographed at 300 times and 1000 times magnification under the electron beam acceleration voltage of 5 kV.
3.2.3 Light transmittance of HPMC/HPS edible composite film
3.2.3.1 Analysis principle of UV-Vis spectrophotometry
The UV-Vis spectrophotometer can emit light with a wavelength of 200~800nm and irradiate it on the object. Some specific wavelengths of light in the incident light are absorbed by the material, and molecular vibrational energy level transition and electronic energy level transition occur. Since each substance has different molecular, atomic and molecular spatial structures, each substance has its specific absorption spectrum, and the content of the substance can be determined or determined according to the level of absorbance at some specific wavelengths on the absorption spectrum. Therefore, UV-Vis spectrophotometric analysis is one of the effective means to study the composition, structure and interaction of substances.
When a beam of light hits an object, part of the incident light is absorbed by the object, and the other part of the incident light is transmitted through the object; the ratio of the transmitted light intensity to the incident light intensity is the transmittance.
The formula for the relationship between absorbance and transmittance is:
Among them, A is the absorbance;
T is the transmittance, %.
The final absorbance was uniformly corrected by absorbance × 0.25 mm/thickness.
3.2.3.2 Test method
Prepare 5% HPMC and HPS solutions, mix them according to different ratios, pour 10 g of the film-forming solution into a polystyrene petri dish with a diameter of 15 cm, and dry them at 37 °C to form a film. Cut the edible film into a 1mm×3mm rectangular strip, put it into the cuvette, and make the edible film close to the inner wall of the cuvette. A WFZ UV-3802 UV-vis spectrophotometer was used to scan the samples at the full wavelength of 200-800 nm, and each sample was tested 5 times.
3.2.4 Dynamic thermomechanical properties of HPMC/HPS edible composite films
3.2.4.1 Principle of dynamic thermomechanical analysis
Dynamic Thermomechanical Analysis (DMA) is an instrument that can measure the relationship between the mass and temperature of the sample under a certain shock load and programmed temperature, and can test the mechanical properties of the sample under the action of periodic alternating stress and time, temperature and temperature. frequency relationship.
High molecular polymers have viscoelastic properties, which can store mechanical energy like an elastomer on the one hand, and consume energy like mucus on the other hand. When the periodic alternating force is applied, the elastic part converts the energy into potential energy and stores it; while the viscous part converts the energy into heat energy and loses it. Polymer materials generally exhibit two states of low temperature glass state and high temperature rubber state, and the transition temperature between the two states is the glass transition temperature. The glass transition temperature directly affects the structure and properties of materials, and is one of the most important characteristic temperatures of polymers.
By analyzing the dynamic thermomechanical properties of polymers, the viscoelasticity of polymers can be observed, and important parameters that determine the performance of polymers can be obtained, so that they can be better applied to the actual use environment. In addition, dynamic thermomechanical analysis is very sensitive to glass transition, phase separation, cross-linking, crystallization and molecular motion at all levels of molecular segments, and can obtain a lot of information on the structure and properties of polymers. It is often used to study the molecules of polymers. movement behavior. Using the temperature sweep mode of the DMA, the occurrence of phase transitions such as the glass transition can be tested. Compared with DSC, DMA has higher sensitivity and is more suitable for the analysis of materials simulating actual usage.
3.2.4.2 Test method
Select clean, uniform, flat and undamaged samples, and cut them into 10mm×20mm rectangular strips. The samples were tested in tensile mode using Pydris Diamond dynamic thermomechanical analyzer from PerkinElmer, USA. The test temperature range was 25~150 °C, the heating rate was 2 °C/min, the frequency was 1 Hz, and the test was repeated twice for each sample. During the experiment, the storage modulus (E’) and loss modulus (E”) of the sample were recorded, and the ratio of the loss modulus to the storage modulus, that is, the tangent angle tan δ, could also be calculated.
3.2.5 Thermal stability of HPMC/HPS edible composite films
3.2.5.1 Principle of thermogravimetric analysis
Thermal Gravimetric Analyzer (TGA) can measure the change of the mass of a sample with temperature or time at a programmed temperature, and can be used to study the possible evaporation, melting, sublimation, dehydration, decomposition and oxidation of substances during the heating process. and other physical and chemical phenomena. The relationship curve between the mass of matter and temperature (or time) obtained directly after the sample is tested is called thermogravimetric (TGA curve). weight loss and other information. Derivative Thermogravimetric curve (DTG curve) can be obtained after the first-order derivation of the TGA curve, which reflects the change of the weight loss rate of the tested sample with temperature or time, and the peak point is the maximum point of the constant rate.
3.2.5.2 Test method
Select the edible film with uniform thickness, cut it into a circle with the same diameter as the thermogravimetric analyzer test disk, and then lay it flat on the test disk, and test it in a nitrogen atmosphere with a flow rate of 20 mL/min. The temperature range was 30–700 °C, the heating rate was 10 °C/min, and each sample was tested twice.
3.2.6.1 Principle of tensile property analysis
3.2.6 Tensile properties of HPMC/HPS edible composite films
The mechanical property tester can apply a static tensile load to the spline along the longitudinal axis under specific temperature, humidity and speed conditions until the spline is broken. During the test, the load applied to the spline and its deformation amount were recorded by the mechanical property tester, and the stress-strain curve during the tensile deformation of the spline was drawn. From the stress-strain curve, the tensile strength (ζt), elongation at break (εb) and elastic modulus (E) can be calculated to evaluate the tensile properties of the film.
The stress-strain relationship of materials can generally be divided into two parts: elastic deformation region and plastic deformation region. In the elastic deformation zone, the stress and strain of the material have a linear relationship, and the deformation at this time can be completely recovered, which is in line with Cook’s law; in the plastic deformation zone, the stress and strain of the material are no longer linear, and the deformation that occurs at this time is irreversibly, eventually the material breaks.
Tensile strength calculation formula：
Where: is tensile strength, MPa;
p is the maximum load or breaking load, N;
b is the sample width, mm;
d is the thickness of the sample, mm.
The formula for calculating elongation at break:
Where: εb is the elongation at break, %;
L is the distance between the marking lines when the sample breaks, mm;
L0 is the original gauge length of the sample, mm.
Elastic modulus calculation formula:
Among them: E is the elastic modulus, MPa;
ζ is stress, MPa;
ε is the strain.
3.2.6.2 Test method
Select clean, uniform, flat and undamaged samples, refer to the national standard GB13022-91, and cut them into dumbbell-shaped splines with a total length of 120mm, an initial distance between fixtures of 86mm, a distance between marks of 40mm, and a width of 10mm. The splines were placed at 75% and 57% (in an atmosphere of saturated sodium chloride and sodium bromide solution) humidity, and equilibrated for more than 3 days before measuring. In this experiment, the ASTM D638, 5566 mechanical property tester of Instron Corporation of the United States and its 2712-003 pneumatic clamp are used for testing. The tensile speed was 10 mm/min, and the sample was repeated 7 times, and the average value was calculated.
3.2.7 Oxygen permeability of HPMC/HPS edible composite film
3.2.7.1 Principle of oxygen permeability analysis
After the test sample is installed, the test cavity is divided into two parts, A and B; a high-purity oxygen flow with a certain flow rate is passed into the A cavity, and a nitrogen flow with a certain flow rate is passed into the B cavity; during the test process, the A cavity The oxygen permeates through the sample into the B cavity, and the oxygen infiltrated into the B cavity is carried by the nitrogen flow and leaves the B cavity to reach the oxygen sensor. The oxygen sensor measures the oxygen content in the nitrogen flow and outputs a corresponding electrical signal, thereby calculating the sample oxygen. transmittance.
3.2.7.2 Test method
Pick undamaged edible composite films, cut them into 10.16 x 10.16 cm diamond-shaped samples, coat the edge surfaces of the clamps with vacuum grease, and clamp the samples to the test block. Tested according to ASTM D-3985, each sample has a test area of 50 cm2.
3.3 Results and Discussion
3.3.1 Microstructure analysis of edible composite films
The interaction between the components of the film-forming liquid and the drying conditions-determine the final structure of the film and seriously affect various physical and chemical properties of the film [330, 331]. The inherent gel properties and compounding ratio of each component can affect the morphology of the compound, which further affects the surface structure and final properties of the membrane [301, 332]. Therefore, microstructural analysis of the films can provide relevant information on the molecular rearrangement of each component, which in turn can help us better understand the barrier properties, mechanical properties, and optical properties of the films.
The surface scanning electron microscope micrographs of HPS/HPMC edible films with different ratios are shown in Figure 3-1. As can be seen from Figure 3-1, some samples showed micro-cracks on the surface, which may be caused by the reduction of moisture in the sample during the test, or by the attack of the electron beam in the microscope cavity [122, 139]. In the figure, pure HPS membrane and pure HPMC. The membranes showed relatively smooth microscopic surfaces, and the microstructure of pure HPS membranes was more homogeneous and smoother than pure HPMC membranes, which may be mainly due to starch macromolecules (amylose molecules and amylopectin molecules) during the cooling process.) achieved better molecular rearrangement in aqueous solution. Many studies have shown that the amylose-amylopectin-water system in the cooling process
There may be a competitive mechanism between gel formation and phase separation. If the rate of phase separation is lower than the rate of gel formation, phase separation will not occur in the system, otherwise, phase separation will occur in the system [333, 334]. Moreover, when the amylose content exceeds 25%, the gelatinization of amylose and the continuous amylose network structure can significantly inhibit the appearance of phase separation [334]. The amylose content of HPS used in this paper is 80%, much higher than 25%, thus better illustrating the phenomenon that pure HPS membranes are more homogeneous and smoother than pure HPMC membranes.
It can be seen from the comparison of the figures that the surfaces of all the composite films are relatively rough, and some irregular bumps are scattered, indicating that there is a certain degree of immiscibility between HPMC and HPS. Moreover, the composite membranes with high HPMC content exhibited a more homogeneous structure than those with high HPS content. HPS-based condensation at 37 °C film formation temperature
Based on the gel properties, HPS presented a viscous gel state; while based on the thermal gel properties of HPMC, HPMC presented a water-like solution state. In the composite membrane with high HPS content (7:3 HPS/HPMC), the viscous HPS is the continuous phase, and the water-like HPMC is dispersed in the high-viscosity HPS continuous phase as the dispersed phase, which is not conducive to the uniform distribution of the dispersed phase ; In the composite film with high HPMC content (3:7 HPS/HPMC), the low-viscosity HPMC transforms into the continuous phase, and the viscous HPS is dispersed in the low-viscosity HPMC phase as the dispersed phase, which is conducive to the formation of a homogeneous phase. compound system.
It can be seen from the figure that although all composite films show rough and inhomogeneous surface structures, no obvious phase interface is found, indicating that HPMC and HPS have good compatibility. The HPMC/starch composite films without plasticizers such as PEG showed obvious phase separation [301], thus indicating that both the hydroxypropyl modification of starch and PEG plasticizers can improve the compatibility of the composite -system.
3.3.2 Optical properties analysis of edible composite films
The light transmission properties of the edible composite films of HPMC/HPS with different ratios were tested by UV-vis spectrophotometer, and the UV spectra are shown in Figure 3-2. The larger the light transmittance value, the more uniform and transparent the film is; conversely, the smaller the light transmittance value, the more uneven and opaque the film is. It can be seen from Figure 3-2(a) that all the composite films show a similar trend with the increase of the scanning wavelength in the full wavelength scanning range, and the light transmittance increases gradually with the increase of the wavelength. At 350nm, the curves tend to plateau.
Select the transmittance at the wavelength of 500nm for comparison, as shown in Figure 3-2(b), the transmittance of pure HPS film is lower than that of pure HPMC film, and with the increase of HPMC content, the transmittance decreases first, and then increased after reaching the minimum value. When the HPMC content increased to 70%, the light transmittance of the composite film was greater than that of pure HPS. It is well known that a homogeneous system will exhibit better light transmittance, and its UV-measured transmittance value is generally higher; inhomogeneous materials are generally more-opaque and have lower UV transmittance values. The transmittance values of the composite films (7:3, 5:5) were lower than those of pure HPS and HPMC films, indicating that there was a certain degree of phase separation between the two components of HPS and HPMC.
Fig. 3-2 UV spectra at all wavelengths (a), and at 500 nm (b), for HPS/HPMC blend films. The bar represents mean ±standard deviations. a-c: different letters are significantly different with various blend ratio (p < 0.05), applied in the full dissertation
3.3.3 Dynamic thermomechanical analysis of edible composite films
Figure 3-3 shows the dynamic thermomechanical properties of edible films of HPMC/HPS with different formulations. It can be seen from Fig. 3-3(a) that the storage modulus (E’) decreases with the increase of HPMC content. In addition, the storage modulus of all samples decreased gradually with increasing temperature, except that the storage modulus of pure HPS (10:0) film increased slightly after the temperature was increased to 70 °C. At high temperature, for the composite film with high HPMC content, the storage modulus of the composite film has an obvious downward trend with the increase of temperature; while for the sample with high HPS content, the storage modulus only decreases slightly with the increase of temperature.
Fig. 3-3 Storage modulus (E′) (a) and loss tangent (tan δ) (b) of HPS/HPMC blend films
It can be seen from Figure 3-3(b) that the samples with HPMC content higher than 30% (5:5, 3:7, 0:10) all show a glass transition peak, and with the increase of HPMC content, the glass transition the transition temperature shifted to high temperature, indicating that the flexibility of the HPMC polymer chain decreased. On the other hand, the pure HPS membrane exhibits a large envelope peak around 67 °C, while the composite membrane with 70% HPS content has no obvious glass transition. This may be because there is a certain degree of interaction between HPMC and HPS, thus restricting the movement of the molecular segments of HPMC and HPS.
3.3.4 Thermal stability analysis of edible composite films
Fig. 3-4 TGA curves (a) and their derivative (DTG) curves (b) of HPS/HPMC blend films
The thermal stability of the edible composite film of HPMC/HPS was tested by thermogravimetric analyzer. Figure 3-4 shows the thermogravimetric curve (TGA) and its weight loss rate curve (DTG) of the composite film. From the TGA curve in Figure 3-4(a), it can be seen that the composite membrane samples with different ratios show two obvious thermogravimetric change stages with the increase of temperature. The volatilization of the water adsorbed by the polysaccharide macromolecule results in a small phase of weight loss at 30–180 °C before the actual thermal degradation occurs. Subsequently, there is a larger phase of weight loss at 300~450 °C, here the thermal degradation phase of HPMC and HPS.
From the DTG curves in Figure 3-4(b), it can be seen that the thermal degradation peak temperatures of pure HPS and pure HPMC are 338 °C and 400 °C, respectively, and the thermal degradation peak temperature of pure HPMC is higher than that of HPS, indicating that HPMC Better thermal stability than HPS. When the HPMC content was 30% (7:3), a single peak appeared at 347 °C, which corresponds to the characteristic peak of HPS, but the temperature was higher than the thermal degradation peak of HPS; when the HPMC content was 70% ( 3:7), only the characteristic peak of HPMC appeared at 400 °C; when the content of HPMC was 50%, two thermal degradation peaks appeared on the DTG curve, 345 °C and 396 °C, respectively. The peaks correspond to the characteristic peaks of HPS and HPMC, respectively, but the thermal degradation peak corresponding to HPS is smaller, and both peaks have a certain shift. It can be seen that most of the composite membranes only show a characteristic single peak corresponding to a certain component, and they are offset compared to the pure component membrane, which indicates that there is a certain difference between the HPMC and HPS components. degree of compatibility. The thermal degradation peak temperature of the composite membrane was higher than that of pure HPS, indicating that HPMC could improve the thermal stability of HPS membrane to a certain extent.
3.3.5 Mechanical properties analysis of edible composite film
The tensile properties of HPMC/HPS composite films with different ratios were measured by mechanical property analyzer at 25 °C, relative humidity of 57% and 75%. Figure 3-5 shows the elastic modulus (a), elongation at break (b) and tensile strength (c) of HPMC/HPS composite films with different ratios under different relative humidity. It can be seen from the figure that when the relative humidity is 57%, the elastic modulus and tensile strength of pure HPS film are the largest, and the pure HPMC is the smallest. With the increase of HPS content, the elastic modulus and tensile strength of the composite films increased continuously. The elongation at break of pure HPMC membrane is much larger than that of pure HPS membrane, and both are greater than that of composite membrane.
When the relative humidity was higher (75%) compared to 57% relative humidity, the elastic modulus and tensile strength of all samples decreased, while the elongation at break increased significantly. This is mainly because water, as a generalized plasticizer, can dilute HPMC and HPS matrix, reduce the force between polymer chains, and improve the mobility of polymer segments. At high relative humidity, the elastic modulus and tensile strength of pure HPMC films were higher than those of pure HPS films, but the elongation at break was lower, a result that was completely different from the results at low humidity. It is worth noting that the variation of the mechanical properties of the composite films with component ratios at a high humidity of 75% is completely opposite to that at a low humidity compared to the case at a relative humidity of 57%. Under high humidity, the moisture content of the film increases, and water not only has a certain plasticizing effect on the polymer matrix, but also promotes the recrystallization of starch. Compared with HPMC, HPS has a stronger tendency to recrystallize, so the effect of relative humidity on HPS is much greater than that of HPMC.
Fig. 3-5 Tensile properties of HPS/HPMC films with different HPS/HPMC ratios equilibrated under different relative humility (RH) conditions. *: different number letters are significantly different with various RH, applied in the full dissertation
3.3.6 Analysis of Oxygen Permeability of Edible Composite Films
Edible composite film is used as food packaging material to extend the shelf life of food, and its oxygen barrier performance is one of the important indicators. Therefore, the oxygen transmission rates of edible films with different ratios of HPMC/HPS were measured at a temperature of 23 °C, and the results are shown in Figure 3-6. It can be seen from the figure that the oxygen permeability of pure HPS membrane is significantly lower than that of pure HPMC membrane, indicating that HPS membrane has better oxygen barrier properties than HPMC membrane. Due to the low viscosity and the existence of amorphous regions, HPMC is easy to form a relatively loose low-density network structure in the film; compared with HPS, it has a higher tendency to recrystallize, and it is easy to form a dense structure in the film. Many studies have shown that starch films have good oxygen barrier properties compared with other polymers [139, 301, 335, 336].
Fig. 3-6 Oxygen permeability of HPS/HPMC blend films
The addition of HPS can significantly reduce the oxygen permeability of HPMC membranes, and the oxygen permeability of composite membranes decreases sharply with the increase of HPS content. The addition of the oxygen-impermeable HPS can increase the tortuosity of the oxygen channel in the composite membrane, which in turn leads to a decrease in the oxygen permeation rate and ultimately lower oxygen permeability. Similar results have been reported for other native starches [139,301].
3.4 Summary of this chapter
In this chapter, using HPMC and HPS as the main raw materials, and adding polyethylene glycol as a plasticizer, the edible composite films of HPMC/HPS with different ratios were prepared by the casting method. The influence of the inherent properties of the components and the compounding ratio on the microscopic morphology of the composite membrane was studied by scanning electron microscopy; the mechanical properties of the composite membrane were studied by the mechanical-properties tester. The influence of the inherent properties of the components and the compounding ratio on the oxygen barrier properties and light transmittance of the composite film was studied by oxygen transmittance tester and UV-vis spectrophotometer. Scanning electron microscopy, thermogravimetric analysis and dynamic thermal analysis were used. Mechanical analysis and other analytical methods were used to study the compatibility and phase separation of the cold-hot gel compound system. The main findings are as follows:
- Compared with pure HPMC, pure HPS is easier to form a homogeneous and smooth microscopic surface morphology. This is mainly due to the better molecular rearrangement of starch macromolecules (amylose molecules and amylopectin molecules) in the starch aqueous solution during the cooling process.
- Compounds with high HPMC content are more likely to form homogeneous membrane structures. This is mainly based on the gel properties of HPMC and HPS. At the film-forming temperature, HPMC and HPS show a low-viscosity solution state and a high-viscosity gel state, respectively. The high-viscosity dispersed phase is dispersed in the low-viscosity continuous phase. , it is easier to form a homogeneous system.
- Relative humidity has a significant effect on the mechanical properties of HPMC/HPS composite films, and the degree of its effect increases with the increase of HPS content. At lower relative humidity, both the elastic modulus and tensile strength of the composite films increased with the increase of HPS content, and the elongation at break of the composite films was significantly lower than that of the pure component films. With the increase of relative humidity, the elastic modulus and tensile strength of the composite film decreased, and the elongation at break increased significantly, and the relationship between the mechanical properties of the composite film and the compounding ratio showed a completely opposite change pattern under different relative humidity. The mechanical properties of composite membranes with different compounding ratios show an intersection under different relative humidity conditions, which provides the possibility to optimize product performance according to different application requirements.
- The addition of HPS significantly improved the oxygen barrier properties of the composite membrane. The oxygen permeability of the composite membrane decreased sharply with the increase of HPS content.
- In the HPMC/HPS cold and hot gel compound system, there is a certain compatibility between the two components. No obvious two-phase interface was found in the SEM images of all the composite films, most of the composite films had only one glass transition point in the DMA results, and only one thermal degradation peak appeared in the DTG curves of most of the composite films. It shows that there is a certain descriptiveness between HPMC and HPS.
The above experimental results show that the compounding of HPS and HPMC can not only reduce the production cost of HPMC edible film, but also improve its performance. The mechanical properties, oxygen barrier properties and optical properties of the edible composite film can be achieved by adjusting the compounding ratio of the two components and the relative humidity of the external environment.
Chapter 4 Relationship between Micromorphology and Mechanical Properties of HPMC/HPS Compound System
Compared with the higher mixing entropy during metal alloy mixing, the mixing entropy during polymer compounding is usually very small, and the heat of compounding during compounding is usually positive, resulting in polymer compounding processes. The Gibbs free energy change in is positive (��� >), therefore, polymer formulations tend to form phase-separated two-phase systems, and fully compatible polymer formulations are very rare [242].
Miscible compound systems can usually achieve molecular-level miscibility in thermodynamics and form homogeneous compounds, so most polymer compound systems are immiscible. However, many polymer compound systems can reach a compatible state under certain conditions and become compound systems with certain compatibility [257].
The macroscopic properties such as mechanical properties of polymer composite systems depend to a large extent on the interaction and phase morphology of their components, especially the compatibility between components and the composition of continuous and dispersed phases [301]. Therefore, it is of great significance to study the microscopic morphology and macroscopic properties of the composite system and establish the relationship between them, which is of great significance to control the properties of composite materials by controlling the phase structure and compatibility of the composite system.
In the process of studying the morphology and phase diagram of the complex system, it is very important to choose appropriate means to distinguish different components. However, the distinction between HPMC and HPS is quite difficult, because both have good transparency and similar refractive index, so it is difficult to distinguish the two components by optical microscopy; in addition, because both are organic carbon-based material, so the two have similar energy absorption, so it is also difficult for scanning electron microscopy to accurately distinguish the pair of components. Fourier transform infrared spectroscopy can reflect the changes in the morphology and phase diagram of the protein-starch complex system by the area ratio of the polysaccharide band at 1180-953 cm-1 and the amide band at 1750-1483 cm-1 [52, 337], but this technique is very complex and typically requires synchrotron radiation Fourier transform infrared techniques to generate sufficient contrast for HPMC/HPS hybrid systems. There are also techniques to achieve this separation of components, such as transmission electron microscopy and small-angle X-ray scattering, but these techniques are usually complex [338]. In this subject, the simple iodine dyeing optical microscope analysis method is used, and the principle that the end group of the amylose helical structure can react with iodine to form inclusion complexes is used to dye the HPMC/HPS compound system by iodine dyeing, so that HPS The components were distinguished from the HPMC components by their different colors under the light microscope. Therefore, iodine dyeing optical microscope analysis method is a simple and effective research method for the morphology and phase diagram of starch-based complex systems.
In this chapter, the microscopic morphology, phase distribution, phase transition and other microstructures of the HPMC/HPS compound system were studied by means of iodine dyeing optical microscope analysis; and mechanical properties and other macroscopic properties; and through the correlation analysis of the microscopic morphology and macroscopic properties of different solution concentrations and compounding ratios, the relationship between the microstructure and macroscopic properties of the HPMC/HPS compound system was established, in order to control the HPMC/HPS. Provide the basis for the properties of composite materials.
4.1 Materials and Equipment
4.1.1 Main experimental materials
4.2 Experimental method
4.2.1 Preparation of HPMC/HPS compound solution
Prepare HPMC solution and HPS solution at 3%, 5%, 7% and 9% concentration, see 2.2.1 for preparation method. Mix HPMC solution and HPS solution according to 100:0, 90:10, 80:20, 70:30, 60:40, 50:50, 45:55, 40:60, 30:70, 20:80, 0:100 Different ratios were mixed at a speed of 250 rmp/min at 21 °C for 30 min, and mixed solutions with different concentrations and different ratios were obtained.
4.2.2 Preparation of HPMC/HPS composite membrane
See 3.2.1.
4.2.3 Preparation of HPMC/HPS composite capsules
Refer to the solution prepared by the method in 2.2.1, use a stainless-steel mold for dipping, and dry it at 37 °C. Pull out the dried capsules, cut off the excess, and put them together to form a pair.
4.2.4 HPMC/HPS composite film optical microscope
4.2.4.1 Principles of Optical Microscopy Analysis
The optical microscope uses the optical principle of magnifying imaging by a convex lens, and uses two converging lenses to expand the opening angle of the nearby tiny substances to the eyes, and enlarge the size of the tiny substances that cannot be discerned by the human eye until the size of the substances can be discerned by the human eye.
4.2.4.2 Test method
The HPMC/HPS compound solutions of different concentrations and compounding ratios were taken out at 21 °C, dropped on a glass slide, cast into a thin layer, and dried at the same temperature. The films were stained with 1% iodine solution (1 g of iodine and 10 g of potassium iodide were placed in a 100-mL volumetric flask, and dissolved in ethanol), placed in the field of light microscope for observation and photographed.
4.2.5 Light transmittance of HPMC/HPS composite film
4.2.5.1 Analysis principle of UV-vis spectrophotometry
Same as 3.2.3.1.
4.2.5.1 Test method
See 3.2.3.2.
4.2.6 Tensile properties of HPMC/HPS composite films
4.2.6.1 Principle of tensile property analysis
Same as 3.2.3.1.
4.2.6.1 Test method
The samples were tested after equilibrating at 73% humidity for 48 h. See 3.2.3.2 for the test method.
4.3 Results and Discussion
4.3.1 Product transparency observation
Figure 4-1 shows edible films and capsules prepared by compounding HPMC and HPS in a 70:30 compounding ratio. As can be seen from the figure, the products have good transparency, which indicates that HPMC and HPS have similar refractive indices, and a homogeneous compound can be obtained after compounding the two.
4.3.2 Optical microscope images of HPMC/HPS complexes before and after staining
Figure 4-2 shows the typical morphology before and after dyeing of HPMC/HPS complexes with different compounding ratios observed under an optical microscope. As can be seen from the figure, it is difficult to distinguish the HPMC phase and the HPS phase in the unstained figure; the dyed pure HPMC and pure HPS show their own unique colors, which is because the reaction of HPS and iodine through iodine staining Its color becomes darker. Therefore, the two phases in the HPMC/HPS compound system are simply and clearly distinguished, which further proves that HPMC and HPS are not miscible and cannot form a homogeneous compound. As can be seen from the figure, as the HPS content increases, the area of the dark area (HPS phase) in the figure keeps increasing as expected, thus confirming that two-phase rearrangement occurs during this process. When the content of HPMC is higher than 40%, HPMC presents the state of continuous phase, and HPS is dispersed in the continuous phase of HPMC as the dispersed phase. In contrast, when the content of HPMC is lower than 40%, HPS presents a state of continuous phase, and HPMC is dispersed in the continuous phase of HPS as a dispersed phase. Therefore, in the 5% HPMC/HPS compound solution, with the increasing HPS content, the opposite happened when the compound ratio was HPMC/HPS 40:60. The continuous phase changes from the initial HPMC phase to the later HPS phase. By observing the phase shape, it can be seen that the HPMC phase in the HPS matrix is spherical after dispersion, while the dispersed shape of the HPS phase in the HPMC matrix is more irregular.
Moreover, by calculating the ratio of the area of the light-colored area (HPMC) to the dark-colored area (HPS) in the HPMC/HPS complex after dyeing (without considering the mesophase situation), it was found that the area of HPMC (light color)/HPS (dark color) in the figure The ratio is always greater than the actual HPMC/HPS compound ratio. For example, in the staining diagram of HPMC/HPS compound with a compound ratio of 50:50, the area of HPS in the interphase area is not calculated, and the ratio of light/dark area is 71/29. This result confirms the existence of a large number of mesophases in the HPMC/HPS composite system.
It is well known that fully compatible polymer compounding systems are quite rare because during the polymer compounding process, the heat of compounding is usually positive and the entropy of compounding usually changes little, thus resulting in free energy during compounding change to a positive value. However, in the HPMC/HPS compound system, HPMC and HPS are still promising to show a greater degree of compatibility, because HPMC and HPS are both hydrophilic polysaccharides, have the same structural unit – glucose, and pass the same functional group is modified with hydroxypropyl. The phenomenon of multiple mesophases in the HPMC/HPS compound system also indicates that HPMC and HPS in the compound have a certain degree of compatibility, and a similar phenomenon occurs in the starch-polyvinyl alcohol blend system with plasticizer added. also appeared [339].
4.3.3 The relationship between the microscopic morphology and the macroscopic properties of the compound system
The relationship between the morphology, phase separation phenomenon, transparency and mechanical properties of the HPMC/HPS composite system was studied in detail. Figure 4-3 shows the effect of HPS content on the macroscopic properties such as transparency and tensile modulus of HPMC/HPS compound system. It can be seen from the figure that the transparency of pure HPMC is higher than that of pure HPS, mainly because the recrystallization of starch reduces the transparency of HPS, and the hydroxypropyl modification of starch is also an important reason for the reduction of transparency of HPS [340, 341]. It can be found from the figure that the transmittance of the HPMC/HPS compound system will have a minimum value with the difference of the HPS content. The transmittance of the compound system, in the range of HPS content below 70%, increases with it decreases with the increase of HPS content; when the HPS content exceeds 70%, it increases with the increase of HPS content. This phenomenon means that the HPMC/HPS compound system is immiscible, because the phase separation phenomenon of the system leads to the decrease of light transmittance. On the contrary, the Young’s modulus of the compound system also appeared a minimum point with the different proportions, and the Young’s modulus continued to decrease with the increase of HPS content, and reached the lowest point when the HPS content was 60%. The modulus continued to increase, and the modulus increased slightly. The Young’s modulus of the HPMC/HPS compound system showed a minimum value, which also indicated that the compound system was an immiscible system. The lowest point of light transmittance of HPMC/HPS compound system is consistent with the phase transition point of HPMC continuous phase to dispersed phase and the lowest point of Young’s modulus value in Figure 4-2.
4.3.4 The effect of solution concentration on the microscopic morphology of the compound system
Figure 4-4 shows the effect of solution concentration on the morphology and phase transition of the HPMC/HPS compound system. As can be seen from the figure, the low concentration of 3% HPMC/HPS compound system, in the compound ratio of HPMC/HPS is 40:60, the appearance of co-continuous structure can be observed; while in the high concentration of 7% solution, this co-continuous structure is observed in the figure with a compounding ratio of 50:50. This result shows that the phase transition point of the HPMC/HPS compound system has a certain concentration dependence, and the HPMC/HPS compound ratio of the phase transition increases with the increase of the compound solution concentration, and HPS tends to form a continuous phase. . In addition, the HPS domains dispersed in the HPMC continuous phase showed similar shapes and morphologies with the change of concentration; while the HPMC dispersed phases dispersed in the HPS continuous phase showed different shapes and morphologies at different concentrations. and with the increase of solution concentration, the dispersion area of HPMC became more and more irregular. The main reason for this phenomenon is that the viscosity of the HPS solution is much higher than that of the HPMC solution at room temperature, and the tendency of the HPMC phase to form a neat spherical state is suppressed due to the surface tension.
4.3.5 Effect of solution concentration on mechanical properties of compound system
Corresponding to the morphologies of Fig. 4-4, Fig. 4-5 shows the tensile properties of the composite films formed under different concentration solutions. It can be seen from the figure that the Young’s modulus and elongation at break of the HPMC/HPS composite system tend to decrease with the increase of solution concentration, which is consistent with the gradual transformation of HPMC from continuous phase to dispersed phase in Figure 4-4. The microscopic morphology is consistent. Since the Young’s modulus of HPMC homopolymer is higher than that of HPS, it is predicted that the Young’s modulus of HPMC/HPS composite system will be improved when HPMC is the continuous phase.
4.4 Summary of this chapter
In this chapter, HPMC/HPS compound solutions and edible composite films with different concentrations and compounding ratios were prepared, and the microscopic morphology and phase transition of the HPMC/HPS compound system were observed by optical microscope analysis of iodine staining to distinguish starch phases. The light transmittance and mechanical properties of the edible composite film of HPMC/HPS were studied by UV-vis spectrophotometer and mechanical property tester, and the effects of different concentrations and compounding ratios on the optical properties and mechanical properties of the compounding system were studied. The relationship between the microstructure and macroscopic properties of the HPMC/HPS compound system was established by combining the microstructure of the composite system, such as microstructure, phase transition and phase separation, and macroscopic properties such as optical properties and mechanical properties. The main findings are as follows:
- The optical microscope analysis method to distinguish starch phases by iodine staining is the most simple, direct and effective method for studying the morphology and phase transition of starch-based compound systems. With iodine staining, the starch phase appears darker and darker under light microscopy, while HPMC is not stained and therefore appears lighter in color.
- The HPMC/HPS compound system is not miscible, and there is a phase transition point in the compound system, and this phase transition point has a certain compound ratio dependence and solution concentration dependence.
- The HPMC/HPS compound system has good compatibility, and a large number of mesophases are present in the compound system. In the intermediate phase, the continuous phase is dispersed in the dispersed phase in the state of particles.
- The dispersed phase of HPS in HPMC matrix showed similar spherical shape at different concentrations; HPMC showed irregular morphology in HPS matrix, and the irregularity of the morphology increased with the increase of concentration.
- The relationship between the microstructure, phase transition, transparency and mechanical properties of the HPMC/HPS composite system was established. a. The lowest point of transparency of the compound system is consistent with the phase transition point of HPMC from the continuous phase to the dispersed phase and the minimum point of the decrease of tensile modulus. b. The Young’s modulus and elongation at break decrease with the increase of solution concentration, which is causally related to the morphological change of HPMC from continuous phase to dispersed phase in the compound system.
In summary, the macroscopic properties of the HPMC/HPS composite system are closely related to its microscopic morphological structure, phase transition, phase separation and other phenomena, and the properties of the composites can be regulated by controlling the phase structure and compatibility of the composite system.
Chapter 5 Influence of HPS Hydroxypropyl Substitution Degree on Rheological Properties of HPMC/HPS Compound System
It is well known that small changes in the chemical structure of starch can lead to dramatic changes in its rheological properties. Therefore, chemical modification offers the possibility to improve and control the rheological properties of starch-based products [342]. In turn, mastering the influence of starch chemical structure on its rheological properties can better understand the structural properties of starch-based products, and provide a basis for the design of modified starches with improved starch functional properties [235]. Hydroxypropyl starch is a professional modified starch widely used in the field of food and medicine. It is usually prepared by the etherification reaction of native starch with propylene oxide under alkaline conditions. Hydroxypropyl is a hydrophilic group. The introduction of these groups into the starch molecular chain can break or weaken the intramolecular hydrogen bonds that maintain the starch granule structure. Therefore, the physicochemical properties of hydroxypropyl starch are related to the degree of substitution of hydroxypropyl groups on its molecular chain [233, 235, 343, 344].
Many studies have investigated the effect of hydroxypropyl substitution degree on the physicochemical properties of hydroxypropyl starch. Han et al. studied the effects of hydroxypropyl waxy starch and hydroxypropyl cornstarch on the structure and retrogradation characteristics of Korean glutinous rice cakes. The study found that hydroxypropylation can reduce the gelatinization temperature of starch and improve the water holding capacity of starch. performance, and significantly inhibited the aging phenomenon of starch in Korean glutinous rice cakes [345]. Kaur et al. studied the effect of hydroxypropyl substitution on the physicochemical properties of different varieties of potato starch, and found that the degree of hydroxypropyl substitution of potato starch varied with different varieties, and its effect on the properties of starch with large particle size More significant; the hydroxypropylation reaction causes many fragments and grooves on the surface of starch granules; hydroxypropyl substitution can significantly improve the swelling properties, water solubility and solubility of starch in dimethyl sulfoxide, and improve starch the transparency of the paste [346]. Lawal et al. studied the effect of hydroxypropyl substitution on the properties of sweet potato starch. The study showed that after hydroxypropyl modification, the free swelling capacity and water solubility of starch were improved; the recrystallization and retrogradation of native starch were inhibited; Digestibility is improved [347]. Schmitz et al. prepared hydroxypropyl tapioca starch and found it to have higher swelling capacity and viscosity, lower aging rate, and higher freeze-thaw stability [344].
However, there are few studies on the rheological properties of hydroxypropyl starch, and the effects of hydroxypropyl modification on the rheological properties and gel properties of starch-based compound systems have been rarely reported so far. Chun et al. studied the rheology of low-concentration (5%) hydroxypropyl rice starch solution. The results showed that the effect of hydroxypropyl modification on the steady-state and dynamic viscoelasticity of starch solution was related to the degree of substitution, and a small amount of hydroxypropyl Propyl substitution can significantly change the rheological properties of starch solutions; the viscosity coefficient of starch solutions decreases with the increase of substitution degree, and the temperature dependence of its rheological properties increases with the increase of hydroxypropyl substitution degree. The amount decreases with increasing degree of substitution [342]. Lee et al. studied the effect of hydroxypropyl substitution on the physical properties and rheological properties of sweet potato starch, and the results showed that the swelling ability and water solubility of starch increased with the increase of the degree of hydroxypropyl substitution; The enthalpy value decreases with the increase of hydroxypropyl substitution degree; the viscosity coefficient, complex viscosity, yield stress, complex viscosity and dynamic modulus of starch solution all decrease with the increase of hydroxypropyl substitution degree, fluid index and loss factor It increases with the degree of hydroxypropyl substitution; the gel strength of starch glue decreases, the freeze-thaw stability increases, and the syneresis effect decreases [235].
In this chapter, the effect of HPS hydroxypropyl substitution degree on the rheological properties and gel properties of HPMC/HPS cold and hot gel compound system was studied. The transition situation is of great significance for in-depth understanding of the relationship between structure formation and rheological properties. In addition, the gelation mechanism of the HPMC/HPS reverse-cooling compound system was preliminarily discussed, in order to provide some theoretical guidance for other similar reverse-heat-cooling gel systems.
5.1 Materials and Equipment
5.1.1 Main experimental materials
5.1.2 Main instruments and equipment
5.2 Experimental method
5.2.1 Preparation of compound solutions
15% HPMC/HPS compound solutions with different compounding ratios (100/0, 50/50, 0/100) and HPS with different hydroxypropyl substitution degrees (G80, A939, A1081) were prepared. The preparation methods of A1081, A939, HPMC and their compound solutions are shown in 2.2.1. G80 and its compound solutions with HPMC are gelatinized by stirring under the conditions of 1500psi and 110°C in an autoclave, because G80 Native starch is high amylose (80%), and its gelatinization temperature is higher than 100 °C, which cannot be reached by the original water-bath gelatinization method [348].
5.2.2 Rheological properties of HPMC/HPS compound solutions with different degrees of HPS hydroxypropyl substitution
5.2.2.1 Principle of rheological analysis
Same as 2.2.2.1
5.2.2.2 Flow mode test method
A parallel plate clamp with a diameter of 60 mm was used, and the plate spacing was set to 1 mm.
- There is a pre-shear flow test method and a three-stage thixotropy. Same as 2.2.2.2.
- Flow test method without pre-shear and thixotropic ring thixotropy. The test temperature is 25 °C, a. Shearing at increasing speed, shear rate range 0-1000 s-1, shearing time 1 min; b. Constant shearing, shearing rate 1000 s-1, shearing time 1 min; c. Reduced speed shearing, the shear rate range is 1000-0s-1, and the shearing time is 1 min.
5.2.2.3 Oscillation mode test method
A parallel plate fixture with a diameter of 60 mm was used, and the plate spacing was set to 1 mm.
- Deformation variable sweep. Test temperature 25 °C, frequency 1 Hz, deformation 0.01-100 %.
- Temperature scan. Frequency 1 Hz, deformation 0.1 %, a. Heating process, temperature 5-85 °C, heating rate 2 °C/min; b. Cooling process, temperature 85-5 °C, cooling rate 2 °C/min. A silicone oil seal is used around the sample to avoid moisture loss during testing.
- Frequency sweep. Variation 0.1 %, frequency 1-100 rad/s. The tests were carried out at 5 °C and 85 °C, respectively, and equilibrated at the test temperature for 5 min before testing.
The relationship between the storage modulus G′ and loss modulus G″ of the polymer solution and the angular frequency ω follows a power law:
where n′ and n″ are the slopes of log G′-log ω and log G″-log ω, respectively;
G0′ and G0″ are the intercepts of log G′-log ω and log G″-log ω, respectively.
5.2.3 Optical microscope
5.2.3.1 Instrument principle
Same as 4.2.3.1
5.2.3.2 Test method
The 3% 5:5 HPMC/HPS compound solution was taken out at different temperatures of 25 °C, 45 °C, and 85 °C, dropped on a glass slide kept at the same temperature, and cast into a thin film. layer solution and dried at the same temperature. The films were stained with 1% iodine solution, placed in the field of light microscope for observation and photographed.
5.3 Results and Discussion
5.3.1 Viscosity and flow pattern analysis
5.3.1.1 Flow test method without pre-shear and thixotropic ring thixotropy
Using the flow test method without pre-shearing and the thixotropic ring thixotropic method, the viscosity of HPMC/HPS compound solution with different degrees of hydroxypropyl substitution HPS was studied. The results are shown in Figure 5-1. It can be seen from the figure that the viscosity of all samples shows a decreasing trend with the increase of shear rate under the action of shear force, showing a certain degree of shear thinning phenomenon. Most high-concentration polymer solutions or melts undergo strong disentanglement and molecular rearrangement under shear, thus exhibiting pseudoplastic fluid behavior [305, 349, 350]. However, the shear thinning degrees of HPMC/HPS compound solutions of HPS with different hydroxypropyl substitution degrees are different.
Fig. 5-1 Viscosities vs. shear rate of the HPS/HPMC solution with different hydropropyl substitution degree of HPS (without pre-shearing, the solid and hollow symbols present increasing rate and decreasing rate process, respectively)
It can be seen from the figure that the viscosity and shear thinning degree of the pure HPS sample are higher than those of the HPMC/HPS compound sample, while the shear thinning degree of the HPMC solution is the lowest, mainly because the viscosity of HPS at low temperature is significantly higher than that of HPMC. In addition, for the HPMC/HPS compound solution with the same compound ratio, the viscosity increases with the HPS hydroxypropyl substitution degree. This may be because the addition of hydroxypropyl groups in starch molecules breaks the intermolecular hydrogen bonds and thus leads to the disintegration of starch granules. Hydroxypropylation significantly reduced the shear thinning phenomenon of starch, and the shear thinning phenomenon of native starch was the most obvious. With the continuous increase of the hydroxypropyl substitution degree, the shear thinning degree of HPS gradually decreased.
All samples have thixotropic rings on the shear stress-shear rate curve, indicating that all samples have a certain degree of thixotropy. The thixotropic strength is represented by the size of the thixotropic ring area. The more thixotropic the sample is [351]. The flow index n and viscosity coefficient K of the sample solution can be calculated by the Ostwald-de Waele power law (see equation (2-1)).
Table 5-1 Flow behavior index (n) and fluid consistency index (K) during increasing rate and decreasing rate process and thixotropy loop area of the HPS/HPMC solution with different hydropropyl substitution degree of HPS at 25 °C
Table 5-1 shows the flow index n, viscosity coefficient K and thixotropic ring area of HPMC/HPS compound solutions with different degrees of hydroxypropyl substitution HPS in the process of increasing shearing and decreasing shearing. It can be seen from the table that the flow index n of all samples is less than 1, indicating that all sample solutions are pseudoplastic fluids. For the HPMC/HPS compound system with the same HPS hydroxypropyl substitution degree, the flow index n increases with the increase of HPMC content, indicating that the addition of HPMC makes the compound solution exhibit stronger Newtonian fluid characteristics. However, with the increase of HPMC content, the viscosity coefficient K decreased continuously, indicating that the addition of HPMC reduced the viscosity of the compound solution, because the viscosity coefficient K was proportional to the viscosity. The n value and K value of pure HPS with different hydroxypropyl substitution degrees in the rising shear stage both decreased with the increase of hydroxypropyl substitution degree, indicating that hydroxypropylation modification can improve the pseudoplasticity of starch and reduce the Viscosity of starch solutions. On the contrary, the value of n increases with the increase of the substitution degree in the decreasing shear stage, indicating that the hydroxypropylation improves the Newtonian fluid behavior of the solution after high-speed shearing. The n value and K value of the HPMC/HPS compound system were affected by both HPS hydroxypropylation and HPMC, which were the result of their combined action. Compared with the increasing shearing stage, the n values of all samples in the decreasing shearing stage became larger, while the K values became smaller, indicating that the viscosity of the compound solution was reduced after high-speed shearing, and the Newtonian fluid behavior of the compound solution was enhanced. .
The area of thixotropic ring decreased with the increase of HPMC content, indicating that the addition of HPMC reduced the thixotropy of the compound solution and improved its stability. For the HPMC/HPS compound solution with the same compounding ratio, the area of thixotropic ring decreases with the increase of HPS hydroxypropyl substitution degree, indicating that hydroxypropylation improves the stability of HPS.
5.3.1.2 Shearing method with pre-cutting and three-stage thixotropic method
The shear method with pre-shear was used to study the change of viscosity of HPMC/HPS compound solution with different degrees of hydroxypropyl substitution HPS with shear rate. The results are shown in Figure 5-2. It can be seen from the figure that the HPMC solution shows almost no shear thinning, while the other samples show shear thinning. This is consistent with the results obtained with the shearing method without pre-shearing. It can also be seen from the figure that at low shear rates, the highly hydroxypropyl substituted sample exhibits a plateau region.
Fig. 5-2 Viscosities vs. shear rate of the HPS/HPMC solution with different hydropropyl substitution degree of HPS (with pre-shearing)
The zero-shear viscosity (h0), flow index (n) and viscosity coefficient (K) obtained by fitting are shown in Table 5-2. From the table, we can see that for the pure HPS samples, the n values obtained by both methods increase with the degree of substitution, indicating that the solid-like behavior of the starch solution decreases as the degree of substitution increases. With the increase of HPMC content, the n values all showed a downward trend, indicating that HPMC reduced the solid-like behavior of the solution. This shows that the qualitative analysis results of the two methods are consistent.
Comparing the data obtained for the same sample under different test methods, it is found that the value of n obtained after pre-shearing is always greater than that obtained by the method without pre-shearing, which indicates that the composite system obtained by the pre-shearing method is a solid-like the behavior is lower than that measured by the method without pre-shearing. This is because the final result obtained in the test without pre-shear is actually the result of the combined action of shear rate and shear time, while the test method with pre-shear first eliminates the thixotropic effect by high shear for a certain period of time. Therefore, this method can more accurately determine the shear thinning phenomenon and flow characteristics of the compound system.
From the table, we can also see that for the same compounding ratio (5:5), the n value of the compounding system is close to 1, and the pre-sheared n increases with the degree of hydroxypropyl substitution It shows that HPMC is a continuous phase in the compound system, and HPMC has a stronger effect on starch samples with low hydroxypropyl substitution degree, which is consistent with the result that the n value increases with the increase of substitution degree without pre-shearing on the contrary. The K values of the compound systems with different degrees of substitution in the two methods are similar, and there is no particularly obvious trend, while the zero-shear viscosity shows a clear downward trend, because the zero-shear viscosity is independent of the shear rate. The intrinsic viscosity can accurately reflect the properties of the substance itself.
Fig. 5-3 Three interval thixotropy of the HPS/HPMC blend solution with different hydropropyl substitution degree of HPS
The three-stage thixotropic method was used to study the effect of different degrees of hydroxypropyl substitution of hydroxypropyl starch on the thixotropic properties of the compound system. It can be seen from Figure 5-3 that in the low shear stage, the solution viscosity decreases with the increase of HPMC content, and decreases with the increase of substitution degree, which is consistent with the law of zero shear viscosity.
The degree of structural recovery after different time in the recovery stage is expressed by the viscosity recovery rate DSR, and the calculation method is shown in 2.3.2. It can be seen from Table 5-2 that within the same recovery time, the DSR of pure HPS is significantly lower than that of pure HPMC, which is mainly because the HPMC molecule is a rigid chain, and its relaxation time is short, and the structure can be recovered in a short time. recover. While HPS is a flexible chain, its relaxation time is long, and the structure recovery takes a long time. With the increase of substitution degree, the DSR of pure HPS decreases with the increase of substitution degree, indicating that hydroxypropylation improves the flexibility of starch molecular chain and makes the relaxation time of HPS longer. The DSR of the compound solution is lower than that of pure HPS and pure HPMC samples, but with the increase of the substitution degree of HPS hydroxypropyl, the DSR of the compound sample increases, which indicates that the thixotropy of the compound system increases with the increase of HPS hydroxypropyl substitution. It decreases with increasing degree of radical substitution, which is consistent with the results without pre-shearing.
Table 5-2 Zero shear viscosity (h0), flow behavior index (n), fluid consistency index (K) during increasing rate and the degree of structure recovery (DSR) after a certain recover time for the HPS/HPMC solution with different hydropropyl substitution degree of HPS at 25 °C
In summary, the steady-state test without pre-shearing and the thixotropic ring thixotropy test can qualitatively analyze samples with large performance differences, but for the compounds with different HPS hydroxypropyl substitution degrees with small performance differences The research results of the solution are contrary to the real results, because the measured data are the comprehensive results of the influence of shear rate and shear time, and cannot truly reflect the influence of a single variable.
5.3.2 Linear viscoelastic region
It is well known that for hydrogels, the storage modulus G′ is determined by the hardness, strength and number of the effective molecular chains, and the loss modulus G′′ is determined by the migration, motion and friction of small molecules and functional groups. It is determined by frictional energy consumption such as vibration and rotation. Existence sign of the intersection of storage modulus G′ and loss modulus G″ (ie. tan δ = 1). The transition from solution to gel is called the gel point. The storage modulus G′ and the loss modulus G″ are often used to study the gelation behavior, the formation rate and structural properties of the gel network structure [352]. They can also reflect the internal structure development and molecular structure during the formation of the gel network structure. interaction [353].
Figure 5-4 shows the strain sweep curves of HPMC/HPS compound solutions with different degrees of hydroxypropyl substitution HPS at a frequency of 1 Hz and a strain range of 0.01%-100%. It can be seen from the figure that in the lower deformation area (0.01–1%), all samples except HPMC are G′ > G″, showing a gel state. For HPMC, G′ is in the whole shape The variable range is always less than G”, indicating that HPMC is in solution state. In addition, the deformation dependence of viscoelasticity of different samples is different. For the G80 sample, the frequency dependence of viscoelasticity is more obvious: when the deformation is greater than 0.3%, it can be seen that G’ gradually decreases, accompanied by a significant increase in G”. increase, as well as a significant increase in tan δ; and intersect when the deformation amount is 1.7%, which indicates that the gel network structure of G80 is severely damaged after the deformation amount exceeds 1.7%, and it is in a solution state.
Fig. 5-4 Storage modulus (G′) and loss modulus (G″) vs. strain for HPS/HPMC blends with the different hydroypropyl substitution degree of HPS (The solid and hollow symbols present G′ and G″, respectively)
Fig. 5-5 tan δ vs. strain for HPMC/HPS blend solution with the different hydropropyl substitution degree of HPS
It can be seen from the figure that the linear viscoelastic region of pure HPS is obviously narrowed with the decrease of hydroxypropyl substitution degree. In other words, as the HPS hydroxypropyl degree of substitution increases, the significant changes in the tan δ curve tend to appear in the higher deformation amount range. In particular, the linear viscoelastic region of G80 is the narrowest of all samples. Therefore, the linear viscoelastic region of G80 is used to determine
Criteria for determining the value of the deformation variable in the following series of tests. For the HPMC/HPS compound system with the same compounding ratio, the linear viscoelastic region also narrows with the decrease of the hydroxypropyl substitution degree of HPS, but the shrinking effect of the hydroxypropyl substitution degree on the linear viscoelastic region is not so obvious.
5.3.3 Viscoelastic properties during heating and cooling
The dynamic viscoelastic properties of HPMC/HPS compound solutions of HPS with different degrees of hydroxypropyl substitution are shown in Figure 5-6. As can be seen from the figure, HPMC exhibits four stages during the heating process: an initial plateau region, two structure-forming stages, and a final plateau region. In the initial plateau stage, G′ < G″, the values of G′ and G″ are small, and tend to decrease slightly with the increase of temperature, showing the common liquid viscoelastic behavior. The thermal gelation of HPMC has two distinct stages of structure formation bounded by the intersection of G′ and G″ (that is, the solution-gel transition point, around 49 °C), which is consistent with previous reports. Consistent [160, 354]. At high temperature, due to hydrophobic association and hydrophilic association, HPMC gradually forms a cross-network structure [344, 355, 356]. In the plateau region of the tail, the values of G′ and G″ are high, which indicates that the HPMC gel network structure is fully formed.
These four stages of HPMC appear sequentially in reverse order as the temperature decreases. The intersection of G′ and G″ shifts to the low temperature region at about 32 °C during the cooling stage, which may be due to hysteresis [208] or the condensation effect of the chain at low temperature [355]. Similar to HPMC, other samples during the heating process There are also four stages in, and the reversible phenomenon occurs during the cooling process. However, it can be seen from the figure that G80 and A939 show a simplified process with no intersection between G’ and G”, and the curve of G80 does not even appear. The platform area at the rear.
For pure HPS, a higher degree of hydroxypropyl substitution can shift both the initial and final temperatures of gel formation, especially the initial temperature, which is 61 °C for G80, A939, and A1081, respectively. , 62 °C and 54 °C. In addition, for HPMC/HPS samples with the same compounding ratio, as the degree of substitution increases, the values of G′ and G″ both tend to decrease, which is consistent with the results of previous studies [357, 358]. As the degree of substitution increases, the texture of the gel becomes soft. Therefore, the hydroxypropylation breaks the ordered structure of native starch and improves its hydrophilicity [343].
For the HPMC/HPS compound samples, both G′ and G″ decreased with the increase of the HPS hydroxypropyl substitution degree, which was consistent with the results of pure HPS. Moreover, with the addition of HPMC, the substitution degree had a significant effect on G′ The effect with G” becomes less pronounced.
The viscoelastic curves of all HPMC/HPS composite samples showed the same trend, which corresponded to HPS at low temperature and HPMC at high temperature. In other words, at low temperature, HPS dominates the viscoelastic properties of the compounded system, while at high temperature HPMC determines the viscoelastic properties of the compounded system. This result is mainly attributable to HPMC. In particular, HPS is a cold gel, which changes from a gel state to a solution state when heated; on the contrary, HPMC is a hot gel, which gradually forms a gel with increasing temperature network structure. For the HPMC/HPS compound system, at low temperature, the gel properties of the compound system are mainly contributed by the HPS cold gel, and at high temperature, at warm temperatures, the gelation of HPMC dominates in the compound system.
Fig. 5-6 Storage modulus (G′), loss modulus (G″) and tan δ vs. temperature for HPS/HPMC blend solution with the different hydroypropyl substitution degree of HPS
The modulus of the HPMC/HPS composite system, as expected, is between the moduli of pure HPMC and pure HPS. Moreover, the complex system exhibits G′ > G″ in the entire temperature scanning range, which indicates that both HPMC and HPS can form intermolecular hydrogen bonds with water molecules, respectively, and can also form intermolecular hydrogen bonds with each other. In addition, On the loss factor curve, all complex systems have a tan δ peak at about 45 °C, indicating that the continuous phase transition has occurred in the complex system. This phase transition will be discussed in the next 5.3.6. continue the discussion.
5.3.4 Effect of temperature on compound viscosity
Understanding the effect of temperature on the rheological properties of materials is important because of the wide range of temperatures that may occur during processing and storage [359, 360]. In the range of 5 °C – 85 °C, the effect of temperature on the complex viscosity of HPMC/HPS compound solutions with different degrees of hydroxypropyl substitution HPS is shown in Figure 5-7. From Figure 5-7(a), it can be seen that the complex viscosity of pure HPS decreases significantly with the increase of temperature; the viscosity of pure HPMC decreases slightly from the initial to 45 °C with the increase of temperature. improve.
The viscosity curves of all compound samples showed similar trends with temperature, first decreasing with increasing temperature and then increasing with increasing temperature. In addition, the viscosity of the compounded samples is closer to that of HPS at low temperature and closer to that of HPMC at high temperature. This result is also related to the peculiar gelation behavior of both HPMC and HPS. The viscosity curve of the compounded sample showed a rapid transition at 45 °C, probably due to a phase transition in the HPMC/HPS compounded system. However, it is worth noting that the viscosity of the G80/HPMC 5:5 compound sample at high temperature is higher than that of pure HPMC, which is mainly due to the higher intrinsic viscosity of G80 at high temperature [361]. Under the same compounding ratio, the compound viscosity of the compounding system decreases with the increase of the HPS hydroxypropyl substitution degree. Therefore, the introduction of hydroxypropyl groups into starch molecules may lead to the breaking of intramolecular hydrogen bonds in starch molecules.
Fig. 5-7 Complex viscosity vs. temperature for HPS/HPMC blends with the different hydroypropyl substitution degree of HPS
The effect of temperature on the complex viscosity of the HPMC/HPS compound system conforms to the Arrhenius relationship within a certain temperature range, and the complex viscosity has an exponential relationship with temperature. The Arrhenius equation is as follows:
Among them, η* is the complex viscosity, Pa s;
A is a constant, Pa s;
T is the absolute temperature, K;
R is the gas constant, 8.3144 J·mol–1·K–1;
E is the activation energy, J·mol–1.
Fitted according to formula (5-3), the viscosity-temperature curve of the compound system can be divided into two parts according to the tan δ peak at 45 °C; the compound system at 5 °C – 45 °C and 45 °C – 85 ° The values of activation energy E and constant A obtained by fitting in the range of C are shown in Table 5-3. The calculated values of the activation energy E are between −174 kJ·mol−1 and 124 kJ·mol−1, and the values of the constant A are between 6.24×10−11 Pa·s and 1.99×1028 Pa·s. Within the fitting range, the fitted correlation coefficients were higher (R2 = 0.9071 –0.9892) except for the G80/HPMC sample. The G80/HPMC sample has a lower correlation coefficient (R2= 0.4435) in the temperature range of 45 °C – 85 °C, which may be due to the inherently higher hardness of G80 and its faster weight compared to other HPS Crystallization rate [362]. This property of G80 makes it more likely to form non-homogeneous compounds when compounded with HPMC.
In the temperature range of 5 °C – 45 °C, the E value of the HPMC/HPS composite sample is slightly lower than that of pure HPS, which may be due to the interaction between HPS and HPMC. Reduce the temperature dependence of viscosity. The E value of pure HPMC is higher than that of the other samples. The activation energies for all starch-containing samples were low positive values, indicating that at lower temperatures, the decrease in viscosity with temperature was less pronounced and the formulations exhibited a starch-like texture.
Table 5-3 Arrhenius equation parameters (E: activation energy; A: constant; R 2 : determination coefficient) from Eq.(1) for the HPS/HPMC blends with different degrees of hydroxypropylation of HPS
However, in the higher temperature range of 45 °C – 85 °C, the E value changed qualitatively between pure HPS and HPMC/HPS composite samples, and the E value of pure HPSs was 45.6 kJ·mol−1 – In the range of 124 kJ·mol−1, the E values of the complexes are in the range of -3.77 kJ·mol−1– -72.2 kJ·mol−1 . This change demonstrates the strong effect of HPMC on the activation energy of the complex system, as the E value of pure HPMC is -174 kJ mol−1. The E values of pure HPMC and the compounded system are negative, which indicates that at higher temperatures, the viscosity increases with increasing temperature, and the compound exhibits HPMC-like behavior texture.
The effects of HPMC and HPS on the complex viscosity of HPMC/HPS compound systems at high temperature and low temperature are consistent with the discussed viscoelastic properties.
5.3.5 Dynamic mechanical properties
Figures 5-8 show the frequency sweep curves at 5 °C of HPMC/HPS compound solutions of HPS with different degrees of hydroxypropyl substitution. It can be seen from the figure that pure HPS exhibits typical solid-like behavior (G′ > G″), while HPMC is liquid-like behavior (G′ < G″). All HPMC/HPS formulations exhibited solid-like behavior. For most of the samples, both G′ and G″ increase with increasing frequency, indicating that the solid-like behavior of the material is strong.
Pure HPMCs exhibit a clear frequency dependence that is difficult to see in pure HPS samples. As expected, the HPMC/HPS complex system exhibited a certain degree of frequency dependence. For all HPS-containing samples, n′ is always lower than n″, and G″ exhibits a stronger frequency dependence than G′, indicating that these samples are more elastic than viscous [352, 359, 363]. Therefore, the performance of the compounded samples is mainly determined by HPS, which is mainly because HPMC presents a lower viscosity solution state at low temperature.
Table 5-4 n′, n″, G0′ and G0″ for HPS/HPMC with different hydropropyl substitution degree of HPS at 5 °C as determined from Eqs. (5-1) and (5-2)
Fig. 5-8 Storage modulus (G′) and loss modulus (G″) vs. frequency for HPS/HPMC blends with the different hydroypropyl substitution degree of HPS at 5 °C
Pure HPMCs exhibit a clear frequency dependence that is difficult to see in pure HPS samples. As expected for the HPMC/HPS complex, the ligand system exhibited a certain degree of frequency dependence. For all HPS-containing samples, n′ is always lower than n″, and G″ exhibits a stronger frequency dependence than G′, indicating that these samples are more elastic than viscous [352, 359, 363]. Therefore, the performance of the compounded samples is mainly determined by HPS, which is mainly because HPMC presents a lower viscosity solution state at low temperature.
Figures 5-9 show the frequency sweep curves of HPMC/HPS compound solutions of HPS with different degrees of hydroxypropyl substitution at 85°C. As can be seen from the figure, all other HPS samples except A1081 exhibited typical solid-like behavior. For A1081, the values of G’ and G” are very close, and G’ is slightly smaller than G”, which indicates that A1081 behaves as a fluid.
This may be because A1081 is a cold gel and undergoes a gel-to-solution transition at high temperature. On the other hand, for samples with the same compounding ratio, the values of n′, n″, G0′ and G0″ (Table 5-5) all decreased with the increase of hydroxypropyl substitution degree, indicating that hydroxypropylation decreased the solid-like behavior of starch at high temperature (85°C). In particular, the n′ and n″ of G80 are close to 0, showing strong solid-like behavior; in contrast, the n′ and n″ values of A1081 are close to 1, showing strong fluid behavior. These n’ and n” values are consistent with the data for G’ and G”. In addition, as can be seen from Figures 5-9, the degree of hydroxypropyl substitution can significantly improve the frequency dependence of HPS at high temperature.
Fig. 5-9 Storage modulus (G′) and loss modulus (G″) vs. frequency for HPS/HPMC blends with the different hydroypropyl substitution degree of HPS at 85 °C
Figures 5-9 show that HPMC exhibits typical solid-like behavior (G′ > G″) at 85°C, which is mainly attributed to its thermogel properties. In addition, the G′ and G″ of HPMC vary with frequency The increase did not change much, indicating that it does not have a clear frequency dependence.
For the HPMC/HPS compound system, the values of n′ and n″ are both close to 0, and G0′ is significantly higher than G0 (Table″ 5-5), confirming its solid-like behavior. On the other hand, higher hydroxypropyl substitution can shift HPS from solid-like to liquid-like behavior, a phenomenon that does not occur in the compounded solutions. In addition, for the compound system added with HPMC, with the increase of frequency, both G’ and G” remained relatively stable, and the values of n’ and n” were close to those of HPMC. All these results suggest that HPMC dominates the viscoelasticity of the compounded system at high temperature of 85°C.
Table 5-5 n′, n″, G0′ and G0″ for HPS/HPMC with different hydropropyl substitution of HPS at 85 °C as determined from Eqs. (5-1) and (5-2)
5.3.6 Morphology of HPMC/HPS composite system
The phase transition of HPMC/HPS compound system was studied by iodine staining optical microscope. The HPMC/HPS compound system with a compound ratio of 5:5 was tested at 25 °C, 45 °C and 85 °C. The stained light microscope images below are shown in Figures 5-10. It can be seen from the figure that after dyeing with iodine, the HPS phase is dyed into a darker color, and the HPMC phase shows a lighter color because it cannot be dyed by iodine. Therefore, the two phases of HPMC/HPS can be are clearly distinguished. At higher temperatures, the area of dark regions (HPS phase) increases and the area of bright regions (HPMC phase) decreases. In particular, at 25 °C, HPMC (bright color) is the continuous phase in the HPMC/HPS composite system, and the small spherical HPS phase (dark color) is dispersed in the HPMC continuous phase. In contrast, at 85 °C, HPMC became a very small and irregularly shaped dispersed phase dispersed in the HPS continuous phase.
Fig. 5-8 Morphologies of dyed 1:1 HPMC/HPS blends at 25 °C, 45 °C and 85 °C
With the increase of temperature, there should be a transition point of the phase morphology of the continuous phase from HPMC to HPS in the HPMC/HPS compound system. In theory, it should occur when the viscosity of HPMC and HPS are the same or very similar. As can be seen from the 45 °C micrographs in Figures 5-10, the typical “sea-island” phase diagram does not appear, but a co-continuous phase is observed. This observation also confirms the fact that a phase transition of the continuous phase may have occurred at the tan δ peak in the dissipation factor-temperature curve discussed in 5.3.3.
It can also be seen from the figure that at low temperature (25 °C), some parts of the dark HPS dispersed phase show a certain degree of bright color, which may be because part of the HPMC phase exists in the HPS phase in the form of a dispersed phase. middle. Coincidentally, at high temperature (85 °C), some small dark particles are distributed in the bright-colored HPMC dispersed phase, and these small dark particles are the continuous phase HPS. These observations suggest that a certain degree of mesophase exists in the HPMC-HPS compound system, thus also indicating that HPMC has a certain compatibility with HPS.
5.3.7 Schematic diagram of phase transition of HPMC/HPS compound system
Based on the classical rheological behavior of polymer solutions and composite gel points [216, 232] and the comparison with the complexes discussed in the paper, a principle model for the structural transformation of HPMC/HPS complexes with temperature is proposed, as shown in Fig. 5-11.
Fig. 5-11 Schematic structures of the sol-gel transition of HPMC (a); HPS (b); and HPMC/HPS (c)
The gel behavior of HPMC and its related solution-gel transition mechanism have been studied a lot [159, 160, 207, 208]. One of the widely accepted ones is that the HPMC chains exist in solution in the form of aggregated bundles. These clusters are interconnected by wrapping some unsubstituted or sparingly soluble cellulose structures, and are connected to densely substituted regions by hydrophobic aggregation of methyl groups and hydroxyl groups. At low temperature, water molecules form cage-like structures outside methyl hydrophobic groups and water shell structures outside hydrophilic groups such as hydroxyl groups, preventing HPMC from forming interchain hydrogen bonds at low temperatures. As the temperature increases, HPMC absorbs energy and these water cage and water shell structures are broken, which is the kinetics of the solution-gel transition. The rupture of the water cage and water shell exposes the methyl and hydroxypropyl groups to the aqueous environment, resulting in a significant increase in free volume. At higher temperature, due to the hydrophobic association of hydrophobic groups and the hydrophilic association of hydrophilic groups, the three-dimensional network structure of the gel is finally formed, as shown in Figure 5-11(a).
After starch gelatinization, amylose dissolves from starch granules to form a hollow single helical structure, which is continuously wound and finally presents a state of random coils. This single-helix structure forms a hydrophobic cavity on the inside and a hydrophilic surface on the outside. This dense structure of starch endows it with better stability [230-232]. Therefore, HPS exists in the form of variable random coils with some stretched out helical segments in aqueous solution at high temperature. As the temperature decreases, the hydrogen bonds between HPS and water molecules are broken and bound water is lost. Finally, a three-dimensional network structure is formed due to the formation of hydrogen bonds between molecular chains, and a gel is formed, as shown in Figure 5-11(b).
Usually, when two components with very different viscosities are compounded, the high viscosity component tends to form a dispersed phase and is dispersed in the continuous phase of the low viscosity component. At low temperatures, the viscosity of HPMC is significantly lower than that of HPS. Therefore, HPMC forms a continuous phase surrounding the high-viscosity HPS gel phase. At the edges of the two phases, the hydroxyl groups on the HPMC chains lose part of the bound water and form intermolecular hydrogen bonds with the HPS molecular chains. During the heating process, the HPS molecular chains moved due to absorbing enough energy and formed hydrogen bonds with water molecules, resulting in the rupture of the gel structure. At the same time, the water-cage structure and water-shell structure on the HPMC chain were destroyed and gradually ruptured to expose hydrophilic groups and hydrophobic clusters. At high temperature, HPMC forms a gel network structure due to intermolecular hydrogen bonds and hydrophobic association, and thus becomes a high-viscosity dispersed phase dispersed in the HPS continuous phase of random coils, as shown in Figure 5-11(c). Therefore, HPS and HPMC dominated the rheological properties, gel properties and phase morphology of the composite gels at low and high temperatures, respectively.
The introduction of hydroxypropyl groups into starch molecules breaks its internal ordered intramolecular hydrogen bond structure, so that the gelatinized amylose molecules are in a swollen and stretched state, which increases the effective hydration volume of the molecules and inhibits the tendency of starch molecules to entangle randomly in aqueous solution [362]. Therefore, the bulky and hydrophilic properties of hydroxypropyl make the recombination of amylose molecular chains and the formation of cross-linking regions difficult [233]. Therefore, with the decrease of temperature, compared with native starch, HPS tends to form a looser and softer gel network structure.
With the increase of the hydroxypropyl substitution degree, there are more stretched helical fragments in the HPS solution, which can form more intermolecular hydrogen bonds with the HPMC molecular chain at the boundary of the two phases, thus forming a more uniform structure. In addition, hydroxypropylation reduces the viscosity of the starch, which reduces the viscosity difference between HPMC and HPS in the formulation. Therefore, the phase transition point in HPMC/HPS complex system shifts to low temperature with the increase of HPS hydroxypropyl substitution degree. This can be confirmed by the abrupt change in viscosity with temperature of the reconstituted samples in 5.3.4.
5.4 Chapter Summary
In this chapter, HPMC/HPS compound solutions with different HPS hydroxypropyl substitution degrees were prepared, and the effect of HPS hydroxypropyl substitution degree on the rheological properties and gel properties of the HPMC/HPS cold and hot gel compound system was investigated by rheometer. The phase distribution of HPMC/HPS cold and hot gel composite system was studied by iodine staining optical microscope analysis. The main findings are as follows:
- At room temperature, the viscosity and shear thinning of HPMC/HPS compound solution decreased with the increase of HPS hydroxypropyl substitution degree. This is mainly because the introduction of hydroxypropyl group into the starch molecule destroys its intramolecular hydrogen bond structure and improves the hydrophilicity of starch.
- At room temperature, the zero-shear viscosity h0, flow index n, and viscosity coefficient K of HPMC/HPS compound solutions are affected by both HPMC and hydroxypropylation. With the increase of HPMC content, the zero shear viscosity h0 decreases, the flow index n increases, and the viscosity coefficient K decreases; the zero shear viscosity h0, flow index n and viscosity coefficient K of pure HPS all increase with the hydroxyl With the increase of the degree of propyl substitution, it becomes smaller; but for the compound system, the zero shear viscosity h0 decreases with the increase of the degree of substitution, while the flow index n and the viscosity constant K increase with the increase of the degree of substitution.
- The shearing method with pre-shearing and the three-stage thixotropy can more accurately reflect the viscosity, flow properties and thixotropy of the compound solution.
- The linear viscoelastic region of the HPMC/HPS compound system narrows with the decrease of the hydroxypropyl substitution degree of HPS.
- In this cold-hot gel compound system, HPMC and HPS can form continuous phases at low and high temperatures, respectively. This phase structure change can significantly affect the complex viscosity, viscoelastic properties, frequency dependence and gel properties of the complex gel.
- As dispersed phases, HPMC and HPS can determine the rheological properties and gel properties of HPMC/HPS compound systems at high and low temperatures, respectively. The viscoelastic curves of the HPMC/HPS composite samples were consistent with HPS at low temperature and HPMC at high temperature.
- The different degree of chemical modification of starch structure also had a significant effect on the gel properties. The results show that the complex viscosity, storage modulus, and loss modulus all decrease with the increase of HPS hydroxypropyl substitution degree. Therefore, hydroxypropylation of native starch can disrupt its ordered structure and increase the hydrophilicity of starch, resulting in a soft gel texture.
- Hydroxypropylation can reduce the solid-like behavior of starch solutions at low temperature and the liquid-like behavior at high temperature. At low temperature, the values of n′ and n″ became larger with the increase of HPS hydroxypropyl substitution degree; at high temperature, n′ and n″ values became smaller with the increase of HPS hydroxypropyl substitution degree.
- The relationship between the microstructure, rheological properties and gel properties of HPMC/HPS composite system was established. Both the abrupt change in the viscosity curve of the compounded system and the tan δ peak in the loss factor curve appear at 45 °C, which is consistent with the co-continuous phase phenomenon observed in the micrograph (at 45 °C).
In summary, the HPMC/HPS cold-hot gel composite system exhibits special temperature-controlled phase morphology and properties. Through various chemical modifications of starch and cellulose, the HPMC/HPS cold and hot gel compound system can be used for the development and application of high-value smart materials.
Chapter 6 Effects of HPS Substitution Degree on Properties and System Compatibility of HPMC/HPS Composite Membranes
It can be seen from Chapter 5 that the change of the chemical structure of the components in the compound system determines the difference in the rheological properties, gel properties and other processing properties of the compound system. Overall performance has a significant impact.
This chapter focuses on the influence of the chemical structure of the components on the microstructure and macroscopic properties of the HPMC/HPS composite membrane. Combined with the influence of Chapter 5 on the rheological properties of the composite system, the rheological properties of the HPMC/HPS composite system are established- relationship between film properties.
6.1 Materials and Equipment
6.1.1 Main experimental materials
6.1.2 Main instruments and equipment
6.2 Experimental method
6.2.1 Preparation of HPMC/HPS composite membranes with different HPS hydroxypropyl substitution degrees
The total concentration of the compound solution is 8% (w/w), the HPMC/HPS compound ratio is 10:0, 5:5, 0:10, the plasticizer is 2.4% (w/w) polyethylene glycol, The edible composite film of HPMC/HPS was prepared by casting method. For the specific preparation method, see 3.2.1.
6.2.2 Microdomain structure of HPMC/HPS composite membranes with different HPS hydroxypropyl substitution degrees
6.2.2.1 The principle of microstructure analysis of synchrotron radiation small-angle X-ray scattering
Small Angel X-ray Scattering (SAXS) refers to the scattering phenomenon caused by the X-ray beam irradiating the sample under test within a small angle close to the X-ray beam. Based on the nanoscale electron density difference between the scatterer and the surrounding medium, small-angle X-ray scattering is commonly used in the study of solid, colloidal, and liquid polymer materials in the nanoscale range. Compared with wide-angle X-ray diffraction technology, SAXS can obtain structural information on a larger scale, which can be used to analyze the conformation of polymer molecular chains, long-period structures, and the phase structure and phase distribution of polymer complex systems. Synchrotron X-ray light source is a new type of high-performance light source, which has the advantages of high purity, high polarization, narrow pulse, high brightness, and high collimation, so it can obtain the nanoscale structural information of materials more quickly and accurately. Analyzing the SAXS spectrum of the measured substance can qualitatively obtain the uniformity of electron cloud density, the uniformity of single-phase electron cloud density (positive deviation from Porod or Debye’s theorem), and the clarity of two-phase interface (negative deviation from Porod or Debye’s theorem). ), scatterer self-similarity (whether it has fractal features), scatterer dispersity (monodispersity or polydispersity determined by Guinier) and other information, and the scatterer fractal dimension, gyration radius, and average layer of repeating units can also be quantitatively obtained. Thickness, average size, scatterer volume fraction, specific surface area and other parameters.
6.2.2.2 Test method
At the Australian Synchrotron Radiation Center (Clayton, Victoria, Australia), the world’s advanced third-generation synchrotron radiation source (flux 1013 photons/s, wavelength 1.47 Å) was used to determine the micro-domain structure and other related information of the composite film. The two-dimensional scattering pattern of the test sample was collected by the Pilatus 1M detector (169 × 172 μm area, 172 × 172 μm pixel size), and the measured sample was in the range of 0.015 < q < 0.15 Å−1 (q is the scattering vector) The inner one-dimensional small-angle X-ray scattering curve is obtained from the two-dimensional scattering pattern by ScatterBrain software, and the scattering vector q and the scattering angle 2 are converted by the formula i / , where is the X-ray wavelength. All data were pre-normalized before data analysis.
6.2.3 Thermogravimetric analysis of HPMC/HPS composite membranes with different degrees of HPS hydroxypropyl substitution
6.2.3.1 Principle of thermogravimetric analysis
Same as 3.2.5.1
6.2.3.2 Test method
See 3.2.5.2
6.2.4 Tensile properties of HPMC/HPS composite films with different degrees of HPS hydroxypropyl substitution
6.2.4.1 Principle of tensile property analysis
Same as 3.2.6.1
6.2.4.2 Test method
See 3.2.6.2
Using ISO37 standard, it is cut into dumbbell-shaped splines, with a total length of 35mm, a distance between the marking lines of 12mm, and a width of 2mm. All test specimens were equilibrated at 75% humidity for more than 3 d.
6.2.5 Oxygen permeability of HPMC/HPS composite membranes with different degrees of HPS hydroxypropyl substitution
6.2.5.1 Principle of oxygen permeability analysis
Same as 3.2.7.1
6.2.5.2 Test method
See 3.2.7.2
6.3 Results and Discussion
6.3.1 Crystal structure analysis of HPMC/HPS composite films with different degrees of HPS hydroxypropyl substitution
Figure 6-1 shows the small angle X-ray scattering spectra of HPMC/HPS composite films with different degrees of HPS hydroxypropyl substitution. It can be seen from the figure that in the relatively large-scale range of q > 0.3 Å (2θ > 40), obvious characteristic peaks appear in all membrane samples. From the X-ray scattering pattern of the pure component film (Fig. 6-1a), pure HPMC has a strong X-ray scattering characteristic peak at 0.569 Å, indicating that HPMC has an X-ray scattering peak in the wide-angle region of 7.70 (2θ > 50). Crystal characteristic peaks, indicating that HPMC has a certain crystalline structure here. Both pure A939 and A1081 starch film samples exhibited a distinct X-ray scattering peak at 0.397 Å, indicating that HPS has a crystalline characteristic peak in the wide-angle region of 5.30, which corresponds to the B-type crystalline peak of starch. It can be clearly seen from the figure that A939 with low hydroxypropyl substitution has a larger peak area than A1081 with high substitution. This is mainly because the introduction of hydroxypropyl group into the starch molecular chain breaks the original ordered structure of starch molecules, increases the difficulty of rearrangement and cross-linking between starch molecular chains, and reduces the degree of starch recrystallization. With the increase of the substitution degree of hydroxypropyl group, the inhibitory effect of hydroxypropyl group on starch recrystallization is more obvious.
It can be seen from the small-angle X-ray scattering spectra of the composite samples (Fig. 6-1b) that the HPMC-HPS composite films all showed obvious characteristic peaks at 0.569 Å and 0.397 Å, corresponding to the 7.70 HPMC crystal characteristic peaks, respectively. The peak area of HPS crystallization of HPMC/A939 composite film is significantly larger than that of HPMC/A1081 composite film. The rearrangement is suppressed, which is consistent with the variation of the HPS crystallization peak area with the degree of hydroxypropyl substitution in pure component films. The crystalline peak area corresponding to HPMC at 7.70 for the composite membranes with different degrees of HPS hydroxypropyl substitution did not change much. Compared with the spectrum of pure component samples (Fig. 5-1a), the areas of HPMC crystallization peaks and HPS crystallization peaks of the composite samples decreased, which indicated that through the combination of the two, both HPMC and HPS could be effective for the other group. The recrystallization phenomenon of the film separation material plays a certain inhibitory role.
Fig. 6-1 SAXS spectra of HPMC/HPS blend films with various hydroxypropyl substitution degree of HPS
In conclusion, the increase of HPS hydroxypropyl substitution degree and the compounding of the two components can inhibit the recrystallization phenomenon of HPMC/HPS composite membrane to a certain extent. The increase of the hydroxypropyl substitution degree of HPS mainly inhibited the recrystallization of HPS in the composite membrane, while the two-component compound played a certain inhibitory role in the recrystallization of HPS and HPMC in the composite membrane.
6.3.2 Self-similar fractal structure analysis of HPMC/HPS composite membranes with different HPS hydroxypropyl substitution degrees
The average chain length (R) of polysaccharide molecules such as starch molecules and cellulose molecules is in the range of 1000-1500 nm, and q is in the range of 0.01-0.1 Å-1, with qR >> 1. According to the Porod formula, the polysaccharide film samples can be seen The relationship between the small-angle X-ray scattering intensity and the scattering angle is:
Among this, I(q) am the small-angle X-ray scattering intensity;
q is the scattering angle;
α is the Porod slope.
The Porod slope α is related to the fractal structure. If α < 3, it indicates that the material structure is relatively loose, the surface of the scatterer is smooth, and it is a mass fractal, and its fractal dimension D = α; if 3 < α <4, it indicates that the material structure is dense and the scatterer is the surface is rough, which is a surface fractal, and its fractal dimension D = 6 – α.
Figure 6-2 shows the lnI(q)-lnq plots of HPMC/HPS composite membranes with different degrees of HPS hydroxypropyl substitution. It can be seen from the figure that all samples present a self-similar fractal structure within a certain range, and the Porod slope α is less than 3, indicating that the composite film presents mass fractal, and the surface of the composite film is relatively smooth. The mass fractal dimensions of HPMC/HPS composite membranes with different degrees of HPS hydroxypropyl substitution are shown in Table 6-1.
Table 6-1 shows the fractal dimension of HPMC/HPS composite membranes with different degrees of HPS hydroxypropyl substitution. It can be seen from the table that for pure HPS samples, the fractal dimension of A939 substituted with low hydroxypropyl is much higher than that of A1081 substituted with high hydroxypropyl, which indicates that with the increase of the degree of hydroxypropyl substitution, in the membrane The density of the self-similar structure is significantly reduced. This is because the introduction of hydroxypropyl groups on the starch molecular chain significantly hinders the mutual bonding of HPS segments, resulting in a decrease in the density of the self-similar structure in the film. Hydrophilic hydroxypropyl groups can form intermolecular hydrogen bonds with water molecules, reducing the interaction between molecular segments; larger hydroxypropyl groups limit the recombination and cross-linking between starch molecular segments, so with the increasing degree of hydroxypropyl substitution, HPS forms a more loose self-similar structure.
For the HPMC/A939 compound system, the fractal dimension of HPS is higher than that of HPMC, which is because the starch recrystallizes, and a more ordered structure is formed between the molecular chains, which leads to the self-similar structure in the membrane. High density. The fractal dimension of the compound sample is lower than that of the two pure components, because through compounding, the mutual binding of the molecular segments of the two components is hindered by each other, resulting in the density of self-similar structures decreases. In contrast, in the HPMC/A1081 compound system, the fractal dimension of HPS is much lower than that of HPMC. This is because the introduction of hydroxypropyl groups in starch molecules significantly inhibits the recrystallization of starch. The self-similar structure in the wood is more-loose. At the same time, the fractal dimension of the HPMC/A1081 compound sample is higher than that of pure HPS, which is also significantly different from the HPMC/A939 compound system. Self-similar structure, the chain-like HPMC molecules can enter the cavity of its loose structure, thereby improving the density of the self-similar structure of HPS, which also indicates that HPS with high hydroxypropyl substitution can form a more uniform complex after compounding with HPMC. ingredients. From the data of rheological properties, it can be seen that hydroxypropylation can reduce the viscosity of starch, so during the compounding process, the viscosity difference between the two components in the compounding system is reduced, which is more conducive to the formation of a homogeneous compound.
Fig. 6-2 lnI(q)-lnq patterns and its fit curves for HPMC/HPS blend films with various hydroxypropyl substitution degree of HPS
Table 6-1 Fractal structure parameters of HPS/HPMC blend films with various hydroxypropyl substitution degree of HPS
For the composite membranes with the same compounding ratio, the fractal dimension also decreases with the increase of the substitution degree of hydroxypropyl group. The introduction of hydroxypropyl into the HPS molecule can reduce the mutual bonding of polymer segments in the compound system, thereby reducing the density of the composite membrane; HPS with high hydroxypropyl substitution has better compatibility with HPMC, Easier to form uniform and dense compound. Therefore, the density of the self-similar structure in the composite membrane decreases with the increase of the substitution degree of HPS, which is the result of the joint influence of the substitution degree of HPS hydroxypropyl and the compatibility of the two components in the composite system.
6.3.3 Thermal stability analysis of HPMC/HPS composite films with different HPS hydroxypropyl substitution degrees
Thermogravimetric analyzer was used to test the thermal stability of HPMC/HPS edible composite films with different degrees of hydroxypropyl substitution. Figure 6-3 shows the thermogravimetric curve (TGA) and its weight loss rate curve (DTG) of the composite films with different degrees of hydroxypropyl substitution HPS. It can be seen from the TGA curve in Figure 6-3(a) that the composite membrane samples with different HPS hydroxypropyl substitution degrees. There are two obvious thermogravimetric change stages with the increase of temperature. First, there is a small weight loss stage at 30~180 °C, which is mainly caused by the volatilization of the water adsorbed by the polysaccharide macromolecule. There is a large weight loss phase at 300~450 °C, which is the real thermal degradation phase, mainly caused by the thermal degradation of HPMC and HPS. It can also be seen from the figure that the weight loss curves of HPS with different degrees of hydroxypropyl substitution are similar and significantly different from those of HPMC. Between the two types of weight loss curves for pure HPMC and pure HPS samples.
From the DTG curves in Figure 6-3(b), it can be seen that the thermal degradation temperatures of pure HPS with different degrees of hydroxypropyl substitution are very close, and the thermal degradation peak temperatures of A939 and A081 samples are 310 °C and 305 °C, respectively The thermal degradation peak temperature of pure HPMC sample is significantly higher than that of HPS, and its peak temperature is 365 °C; HPMC/HPS composite film has two thermal degradation peaks on the DTG curve, corresponding to the thermal degradation of HPS and HPMC, respectively. Characteristic peaks, which indicate that there is a certain degree of phase separation in the composite system with a composite ratio of 5:5, which is consistent with the thermal degradation results of the composite film with a composite ratio of 5:5 in Chapter 3. The thermal degradation peak temperatures of HPMC/A939 composite film samples were 302 °C and 363 °C, respectively; the thermal degradation peak temperatures of HPMC/A1081 composite film samples were 306 °C and 363 °C, respectively. The peak temperatures of the composite film samples were shifted to lower temperatures than the pure component samples, which indicated that the thermal stability of the composite samples was reduced. For the samples with the same compounding ratio, the thermal degradation peak temperature decreased with the increase of the hydroxypropyl substitution degree, indicating that the thermal stability of the composite film decreased with the increase of the hydroxypropyl substitution degree. This is because the introduction of hydroxypropyl groups into starch molecules reduces the interaction between molecular segments and inhibits the orderly rearrangement of molecules. It is consistent with the results that the density of self-similar structures decreases with the increase of the degree of hydroxypropyl substitution.
Fig. 6-3 TGA curves (a) and their derivative (DTG) curves (b) of HPMC/HPS blend films with various hydroxypropyl substitution degree of HPS
6.3.4 Mechanical properties analysis of HPMC/HPS composite membranes with different HPS hydroxypropyl substitution degrees
Fig. 6-5 Tensile properties of HPMC/HPS films with various hydroxypropyl substitution degree of HPS
The tensile properties of HPMC/HPS composite films with different HPS hydroxypropyl substitution degrees were tested by mechanical property analyzer at 25 °C and 75% relative humidity. Figures 6-5 show the elastic modulus (a), elongation at break (b) and tensile strength (c) of composite films with different degrees of HPS hydroxypropyl substitution. It can be seen from the figure that for the HPMC/A1081 compound system, with the increase of HPS content, the elastic modulus and tensile strength of the composite film gradually decreased, and the elongation at break increased significantly, which was consistent with 3.3.5 medium and high humidity. The results of the composite membranes with different compounding ratios were consistent.
For pure HPS membranes, both the elastic modulus and tensile strength increased with decreasing HPS hydroxypropyl substitution degree, suggesting that hydroxypropylation reduces the stiffness of the composite membrane and improves its flexibility. This is mainly because with the increase of hydroxypropyl substitution degree, the hydrophilicity of HPS increases, and the membrane structure becomes more-loose, which is consistent with the result that the fractal dimension decreases with the increase of substitution degree in the small angle X-ray scattering test. However, the elongation at break decreases with the decrease of the substitution degree of HPS hydroxypropyl group, which is mainly because the introduction of hydroxypropyl group into the starch molecule can inhibit the recrystallization of starch. The results are consistent with the increase and decrease.
For the HPMC/HPS composite membrane with the same compound ratio, the elastic modulus of the membrane material increases with the decrease of the HPS hydroxypropyl substitution degree, and the tensile strength and elongation at break both decrease with the decrease of the substitution degree. It is worth noting that the mechanical properties of the composite membranes vary completely with the compounding ratio with the different degrees of HPS hydroxypropyl substitution. This is mainly because the mechanical properties of the composite membrane are not only affected by the HPS substitution degree on the membrane structure, but also by the compatibility between the components in the compound system. The viscosity of HPS decreases with the increase of the hydroxypropyl substitution degree, it is more favorable to form a uniform compound by compounding.
6.3.5 Oxygen permeability analysis of HPMC/HPS composite membranes with different HPS hydroxypropyl substitution degrees
Oxidation caused by oxygen is the initial stage in many ways of causing food spoilage, so edible composite films with certain oxygen barrier properties can improve food quality and prolong food shelf life [108, 364]. Therefore, the oxygen transmission rates of HPMC/HPS composite membranes with different HPS hydroxypropyl substitution degrees were measured, and the results are shown in Figure 5-6. It can be seen from the figure that the oxygen permeability of all pure HPS membranes is much lower than that of pure HPMC membranes, indicating that HPS membranes have better oxygen barrier properties than HPMC membranes, which is consistent with the previous results. For pure HPS membranes with different degrees of hydroxypropyl substitution, the oxygen transmission rate increases with the increase of the degree of substitution, which indicates that the area where oxygen permeates in the membrane material increases. This is consistent with the microstructure analysis of small angle X-ray scattering that the structure of the membrane becomes looser with the increase of the degree of hydroxypropyl substitution, so the permeation channel of oxygen in the membrane becomes larger, and the oxygen in the membrane permeates As the area increases, the oxygen transmission rate also increases gradually.
Fig. 6-6 Oxygen permeability of HPS/HPMC films with various hydroxypropyl substitution degree of HPS
For the composite membranes with different HPS hydroxypropyl substitution degrees, the oxygen transmission rate decreases with the increase of hydroxypropyl substitution degree. This is mainly because in the 5:5 compounding system, HPS exists in the form of dispersed phase in the low-viscosity HPMC continuous phase, and the viscosity of HPS decreases with the increase of hydroxypropyl substitution degree. The smaller the viscosity difference, the more conducive to the formation of a homogeneous compound, the more tortuous the oxygen permeation channel in the membrane material, and the smaller the oxygen transmission rate.
6.4 Chapter Summary
In this chapter, HPMC/HPS edible composite films were prepared by casting HPS and HPMC with different degrees of hydroxypropyl substitution, and adding polyethylene glycol as a plasticizer. The effect of different HPS hydroxypropyl substitution degrees on the crystal structure and microdomain structure of the composite membrane was studied by synchrotron radiation small-angle X-ray scattering technology. The effects of different HPS hydroxypropyl substitution degrees on the thermal stability, mechanical properties and oxygen permeability of composite membranes and their laws were studied by thermogravimetric analyzer, mechanical property tester and oxygen permeability tester. The main findings are as follows:
- For the HPMC/HPS composite membrane with the same compounding ratio, with the increase of hydroxypropyl substitution degree, the crystallization peak area corresponding to HPS at 5.30 decreases, while the crystallization peak area corresponding to HPMC at 7.70 does not change much, indicating that the hydroxypropylation of starch can inhibit the recrystallization of starch in the composite film.
- Compared with the pure component membranes of HPMC and HPS, the crystallization peak areas of HPS (5.30) and HPMC (7.70) of the composite membranes are reduced, which indicates that through the combination of the two, both HPMC and HPS can be effective in the composite membranes. The recrystallization of another component plays a certain inhibitory role.
- All HPMC/HPS composite membranes showed self-similar mass fractal structure. For composite membranes with the same compound ratio, the density of the membrane material decreased significantly with the increase of hydroxypropyl substitution degree; low HPS hydroxypropyl substitution The density of the composite membrane material is significantly lower than that of the two-pure component material, while the density of the composite membrane material with high HPS hydroxypropyl substitution degree is higher than that of the pure HPS membrane, which is mainly because the density of the composite membrane material is affected at the same time. The effect of HPS hydroxypropylation on the reduction of polymer segment binding and the compatibility between the two components of the compound system.
- Hydroxypropylation of HPS can reduce the thermal stability of HPMC/HPS composite films, and the thermal degradation peak temperature of composite films shifts to the low temperature region with the increase of hydroxypropyl substitution degree, which is because the hydroxypropyl group in starch molecules. The introduction reduces the interaction between molecular segments and inhibits the orderly rearrangement of molecules.
- The elastic modulus and tensile strength of pure HPS membrane decreased with the increase of HPS hydroxypropyl substitution degree, while the elongation at break increased. This is mainly because the hydroxypropylation inhibits the recrystallization of starch and makes the composite film form a looser structure.
- The elastic modulus of HPMC/HPS composite film decreased with the increase of HPS hydroxypropyl substitution degree, but the tensile strength and elongation at break increased, because the mechanical properties of the composite film were not affected by the HPS hydroxypropyl substitution degree. In addition to the influence of, it is also affected by the compatibility of the two components of the compound system.
- The oxygen permeability of pure HPS increases with the increase of hydroxypropyl substitution degree, because hydroxypropylation reduces the density of HPS amorphous region and increases the area of oxygen permeation in the membrane; HPMC/HPS composite membrane The oxygen permeability decreases with the increase of the hydroxypropyl substitution degree, which is mainly because the hyperhydroxypropylated HPS has better compatibility with HPMC, which leads to the increased tortuosity of the oxygen permeation channel in the composite membrane. Reduced oxygen permeability.
The above experimental results show that the macroscopic properties such as mechanical properties, thermal stability and oxygen permeability of HPMC/HPS composite membranes are closely related to their internal crystalline structure and amorphous region structure, which are not only affected by the HPS hydroxypropyl substitution, but also by the complex. Influence of two-component compatibility of ligand systems.
Conclusion and Outlook
- Conclusion
In this paper, the thermal gel HPMC and the cold gel HPS are compounded, and the HPMC/HPS cold and hot reverse gel compound system is constructed. The solution concentration, compounding ratio and shearing effect on the compound system are systematically studied the influence of rheological properties such as viscosity, flow index and thixotropy, combined with the mechanical properties, dynamic thermomechanical properties, oxygen permeability, light transmission properties and thermal stability of composite films prepared by casting method. Comprehensive properties, and iodine wine dyeing the compatibility, phase transition and phase morphology of the composite system were studied by optical microscopy, and the relationship between the microstructure and macroscopic properties of HPMC/HPS was established. In order to control the properties of the composites by controlling the phase structure and compatibility of the HPMC/HPS composite system according to the relationship between the macroscopic properties and the micromorphological structure of the HPMC/HPS composite system. By studying the effects of chemically modified HPS with different degrees on the rheological properties, gel properties, microstructure and macroscopic properties of membranes, the relationship between the microstructure and macroscopic properties of the HPMC/HPS cold and hot inverse gel system was further investigated. The relationship between the two, and a physical model was established to clarify the gelation mechanism and its influencing factors and laws of the cold and hot gel in the compound system. Relevant studies have drawn the following conclusions.
- Changing the compounding ratio of HPMC/HPS compound system can significantly improve the rheological properties such as viscosity, fluidity and thixotropy of HPMC at low temperature. The relationship between the rheological properties and the microstructure of the compound system was further studied. The specific results are as follows:
(1) At low temperature, the compound system is a continuous phase-dispersed phase “sea-island” structure, and the continuous phase transition occurs at 4:6 with the decrease of the HPMC/HPS compound ratio. When the compounding ratio is high (more HPMC content), HPMC with low viscosity is the continuous phase, and HPS is the dispersed phase. For the HPMC/HPS compound system, when the low-viscosity component is the continuous phase and the high-viscosity component is the continuous phase, the contribution of the continuous phase viscosity to the viscosity of the compound system is significantly different. When the low-viscosity HPMC is the continuous phase, the viscosity of the compound system mainly reflects the contribution of the continuous-phase viscosity; when the high-viscosity HPS is the continuous phase, the HPMC as the dispersed phase will reduce the viscosity of the high-viscosity HPS. effect. With the increase of HPS content and solution concentration in the compound system, the viscosity and shear thinning phenomenon of the compound system gradually increased, the fluidity decreased, and the solid-like behavior of the compound system was enhanced. The viscosity and thixotropy of HPMC are balanced by the formulation with HPS.
(2) For a 5:5 compounding system, HPMC and HPS can form continuous phases at low and high temperatures, respectively. This phase structure change can significantly affect the complex viscosity, viscoelastic properties, frequency dependence and gel properties of the complex gel. As dispersed phases, HPMC and HPS can determine the rheological properties and gel properties of HPMC/HPS compound systems at high and low temperatures, respectively. The viscoelastic curves of the HPMC/HPS composite samples were consistent with HPS at low temperature and HPMC at high temperature.
(3) The relationship between the microstructure, rheological properties and gel properties of HPMC/HPS composite system was established. Both the abrupt change in the viscosity curve of the compounded system and the tan delta peak in the loss factor curve appear at 45 °C, which is consistent with the co-continuous phase phenomenon observed in the micrograph (at 45 °C).
- By studying the microstructure and mechanical properties, dynamic thermomechanical properties, light transmittance, oxygen permeability and thermal stability of the composite membranes prepared under different compounding ratios and solution concentrations, combined with iodine dyeing optical microscopy technology, research The phase morphology, phase transition and compatibility of the complexes were investigated, and the relationship between the microstructure and the macroscopic properties of the complexes was established. The specific results are as follows:
(1) There is no obvious two-phase interface in the SEM images of the composite films with different compounding ratios. Most of the composite films have only one glass transition point in the DMA results, and most of the composite films have only one thermal degradation peak in the DTG curve. These together indicate that HPMC has a certain compatibility with HPS.
(2) Relative humidity has a significant effect on the mechanical properties of HPMC/HPS composite films, and the degree of its effect increases with the increase of HPS content. At lower relative humidity, both the elastic modulus and tensile strength of the composite films increased with the increase of HPS content, and the elongation at break of the composite films was significantly lower than that of the pure component films. With the increase of relative humidity, the elastic modulus and tensile strength of the composite film decreased, and the elongation at break increased significantly, and the relationship between the mechanical properties of the composite film and the compounding ratio showed a completely opposite change pattern under different relative humidity. The mechanical properties of composite membranes with different compounding ratios show an intersection under different relative humidity conditions, which provides the possibility to optimize product performance according to different application requirements.
(3) The relationship between the microstructure, phase transition, transparency and mechanical properties of the HPMC/HPS composite system was established. a. The lowest point of transparency of the compound system is consistent with the phase transition point of HPMC from the continuous phase to the dispersed phase and the minimum point of the decrease of tensile modulus. b. The Young’s modulus and elongation at break decrease with the increase of solution concentration, which is causally related to the morphological change of HPMC from continuous phase to dispersed phase in the compound system.
(4) The addition of HPS increases the tortuosity of the oxygen permeation channel in the composite membrane, significantly reduces the oxygen permeability of the membrane, and improves the oxygen barrier performance of the HPMC membrane.
- The effect of HPS chemical modification on the rheological properties of the composite system and the comprehensive properties of the composite membrane such as crystal structure, amorphous region structure, mechanical properties, oxygen permeability and thermal stability were studied. The specific results are as follows:
(1) The hydroxypropylation of HPS can reduce the viscosity of the compound system at low temperature, improve the fluidity of the compound solution, and reduce the phenomenon of shear thinning; the hydroxypropylation of HPS can narrow the linear viscoelastic region of the compound system , reduce the phase transition temperature of the HPMC/HPS compound system, and improve the solid-like behavior of the compound system at low temperature and the fluidity at high temperature.
(2) The hydroxypropylation of HPS and the improvement of the compatibility of the two components can significantly inhibit the recrystallization of starch in the membrane, and promote the formation of a looser self-similar structure in the composite membrane. The introduction of bulky hydroxypropyl groups on the starch molecular chain limits the mutual binding and orderly rearrangement of HPS molecular segments, resulting in the formation of a more-loose self-similar structure of HPS. For the complex system, the increase of the degree of hydroxypropyl substitution allows the chain-like HPMC molecules to enter the loose cavity region of HPS, which improves the compatibility of the complex system and improves the density of the self-similar structure of HPS. The compatibility of the compound system increases with the increase of the substitution degree of hydroxypropyl group, which is consistent with the results of rheological properties.
(3) The macroscopic properties such as mechanical properties, thermal stability and oxygen permeability of HPMC/HPS composite membrane are closely related to its internal crystalline structure and amorphous region structure. The combined effect of the two effects of the compatibility of the two components.
- By studying the effects of solution concentration, temperature and chemical modification of HPS on the rheological properties of the compound system, the gelation mechanism of the HPMC/HPS cold-heat inverse gel compound system was discussed. The specific results are as follows:
(1) There is a critical concentration (8%) in the compound system, below the critical concentration, HPMC and HPS exist in independent molecular chains and phase regions; when the critical concentration is reached, the HPS phase is formed in the solution as a condensate. The gel center is a microgel structure connected by the intertwining of HPMC molecular chains; above the critical concentration, the intertwining is more complex and the interaction is stronger, and the solution exhibits a behavior similar to that of a polymer melt.
(2) The complex system has a transition point of continuous phase with the change of temperature, which is related to the gel behavior of HPMC and HPS in the complex system. At low temperatures, the viscosity of HPMC is significantly lower than that of HPS, so HPMC forms a continuous phase surrounding the high-viscosity HPS gel phase. At the edges of the two phases, the hydroxyl groups on the HPMC chain lose part of their binding water and form intermolecular hydrogen bonds with the HPS molecular chain. During the heating process, the HPS molecular chains moved due to absorbing enough energy and formed hydrogen bonds with water molecules, resulting in the rupture of the gel structure. At the same time, the water-cage and water-shell structures on the HPMC chains were destroyed, and gradually ruptured to expose hydrophilic groups and hydrophobic clusters. At high temperature, HPMC forms a gel network structure due to intermolecular hydrogen bonds and hydrophobic association, and thus becomes a high-viscosity dispersed phase dispersed in the HPS continuous phase of random coils.
(3) With the increase of the hydroxypropyl substitution degree of HPS, the compatibility of the HPMC/HPS compound system improves, and the phase transition temperature in the compound system moves to low temperature. With the increase of the hydroxypropyl substitution degree, there are more stretched helical fragments in the HPS solution, which can form more intermolecular hydrogen bonds with the HPMC molecular chain at the boundary of the two phases, thus forming a more uniform structure. Hydroxypropylation reduces the viscosity of starch, so that the viscosity difference between HPMC and HPS in the compound is narrowed, which is conducive to the formation of a more homogeneous compound, and the minimum value of the viscosity difference between the two components moves to the low temperature region.
2. Innovation points
1. Design and construct the HPMC/HPS cold and hot reversed-phase gel compound system, and systematically study the unique rheological properties of this system, especially the concentration of compound solution, compound ratio, temperature and chemical modification of components. The influence laws of the rheological properties, gel properties and compatibility of the compound system were further studied, and the phase morphology and phase transition of the compound system were further studied combined with the observation of the iodine dyeing optical microscope, and the micro-morphological structure of the compound system was established- Rheological properties-gel properties relationship. For the first time, the Arrhenius model was used to fit the gel formation law of the cold and hot reversed-phase composite gels in different temperature ranges.
2. The phase distribution, phase transition and compatibility of HPMC/HPS composite system were observed by iodine dyeing optical microscope analysis technology, and the transparency-mechanical properties were established by combining the optical properties and mechanical properties of composite films. The relationship between microstructure and macroscopic properties such as properties-phase morphology and concentration-mechanical properties-phase morphology. It is the first time to directly observe the change law of the phase morphology of this compound system with compounding ratio, temperature and concentration, especially the conditions of phase transition and the effect of phase transition on the properties of the compound system.
3. The crystalline structure and amorphous structure of composite membranes with different HPS hydroxypropyl substitution degrees were studied by SAXS, and the gelation mechanism and influence of composite gels were discussed in combination with rheological results and macroscopic properties such as oxygen permeability of composite membranes. Factors and laws, it was found for the first time that the viscosity of the composite system is related to the density of the self-similar structure in the composite membrane, and directly determines the macroscopic properties such as oxygen permeability and mechanical properties of the composite membrane, and establishes rheological properties-microstructure-membrane relationship between material properties.
3. Outlook
In recent years, the development of safe and edible food packaging materials using renewable natural polymers as raw materials has become a research hotspot in the field of food packaging. In this paper, natural polysaccharide is used as the main raw material. By compounding HPMC and HPS, the cost of raw materials is reduced, the processing performance of HPMC at low temperature is improved, and the oxygen barrier performance of the composite membrane is improved. Through the combination of rheological analysis, iodine dyeing optical microscope analysis and composite film microstructure and comprehensive performance analysis, the phase morphology, phase transition, phase separation and compatibility of the cold-hot reversed-phase gel composite system were studied. The relationship between the microstructure and macroscopic properties of the composite system was established. According to the relationship between the macroscopic properties and the micromorphological structure of the HPMC/HPS composite system, the phase structure and compatibility of the composite system can be controlled to control the composite material. The research in this paper has important guiding significance for the actual production process; the formation mechanism, influencing factors and laws of cold and hot inverse composite gels are discussed, which is a similar composite system of cold and hot inverse gels. The research of this paper provides a theoretical model to provide theoretical guidance for the development and application of special temperature-controlled smart materials. The research results of this paper have good theoretical value. The research of this paper involves the intersection of food, material, gel and compounding and other disciplines. Due to the limitation of time and research methods, the research of this topic still has many unfinished points, which can be deepened and improved from the following aspects. expand:
Theoretical aspects:
- To explore the effects of different chain branch ratios, molecular weights and varieties of HPS on the rheological properties, membrane properties, phase morphology, and compatibility of the compound system, and to explore the law of its influence on the gel formation mechanism of the compound system.
- Investigate the effects of HPMC hydroxypropyl substitution degree, methoxyl substitution degree, molecular weight and source on the rheological properties, gel properties, membrane properties and system compatibility of the compound system, and analyze the effect of HPMC chemical modification on compound condensation. Influence rule of gel formation mechanism.
- The influence of salt, pH, plasticizer, cross-linking agent, antibacterial agent and other compound systems on rheological properties, gel properties, membrane structure and properties and their laws were studied.
Application:
- Optimize the formula for the packaging application of seasoning packets, vegetable packets and solid soups, and study the preservation effect of seasonings, vegetables and soups during the storage period, the mechanical properties of materials, and the changes in product performance when subjected to external forces, and Water solubility and hygienic index of the material. It can also be applied to granulated foods such as coffee and milk tea, as well as edible packaging of cakes, cheeses, desserts and other foods.
- Optimize the formula design for the application of botanical medicinal plant capsules, further study the processing conditions and the optimal selection of auxiliary agents, and prepare hollow capsule products. Physical and chemical indicators such as friability, disintegration time, heavy metal content, and microbial content were tested.
- For the fresh-keeping application of fruits and vegetables, meat products, etc., according to the different processing methods of spraying, dipping, and painting, select the appropriate formula, and study the rotten fruit rate, moisture loss, nutrient consumption, hardness of vegetables after packaging during the storage period, gloss and flavor and other indicators; the color, pH, TVB-N value, thiobarbituric acid and number of microorganisms of meat products after packaging.
Post time: Oct-17-2022