Polyvinyl alcohol and chitosan composite hydrogel and preparation method thereof

Abstract

The invention relates to the technical field of biomedical materials, and discloses a polyvinyl alcohol and chitosan composite hydrogel and a preparation method thereof, the polyvinyl alcohol and chitosan composite hydrogel comprises the following components by weight: 6-10 parts of polyvinyl alcohol; 1 to 3 parts of chitosan; 0.5 to 1.5 parts of glutaraldehyde; 100 to 150 parts of acetic acid; 0.01 to 0.1 part of hydrochloric acid or sodium hydroxide; 0.01 to 1 part of sodium chloride; the hydrogel is of a three-dimensional network structure formed by a cross-linking reaction of polyvinyl alcohol and chitosan through glutaraldehyde, the concentration of glutaraldehyde is 0.25 mol / L, the volume of glutaraldehyde is 4 ml, and the cross-linking reaction of the hydrogel is carried out under the condition that the pH is 5.5-6.5. By constructing a glutaraldehyde difunctional bridging crosslinking mechanism and a dual-network synergistic structure and combining with a pH precise regulation and control and constant-temperature drying strategy, the mechanical property, rehydration property and structural stability of the hydrogel are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of biomedical materials, in particular to a polyvinyl alcohol and chitosan composite hydrogel and a preparation method thereof. Background Art

[0002] In the current research and development of functional hydrogel materials, polyvinyl alcohol (PVA) and chitosan (CS) have become the preferred material combination for preparing injectable, moldable, and responsive hydrogel systems due to their excellent biocompatibility, hydrogel properties, and film-forming properties. However, achieving an environmentally friendly and controllable preparation process while improving the stability of the gel structure and maintaining its elastic morphology remains a core technical concern in this field.

[0003] In existing technologies, PVA and CS are often combined to form hydrogel structures through freeze-thaw cycles, treatment with a single crosslinker, or physical doping. Typical methods include utilizing PVA's melt-and-condensation properties for physical crosslinking, or using crosslinkers such as glutaraldehyde to react with amino groups in CS to form a chemically bonded network. These methods can, to a certain extent, produce gel materials with formability and are suitable for some basic applications.

[0004] Although existing technologies have made certain progress in the development of hydrogel materials, there are still some shortcomings. For example, traditional cross-linking systems often fail to fully activate the bifunctional effects of glutaraldehyde, resulting in a lack of direct covalent connection between PVA and CS. The network is maintained only by hydrogen bonds or Schiff base bonds, and the overall resistance of the gel to mechanical deformation and swelling is limited. In addition, the lack of a systematic pH control strategy also makes the reactivity of the functional groups in a non-optimal state, which can easily lead to fluctuations in cross-linking efficiency and batch differences, affecting product consistency. In the drying process, although common vacuum or freeze-drying methods can remove moisture, they often cause the internal skeleton structure of the gel to collapse, which is not conducive to its structural recovery during storage and rehydration. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the present invention provides a polyvinyl alcohol and chitosan composite hydrogel and a preparation method thereof, which solves the problems of poor structural stability, low cross-linking efficiency, weak rehydration performance and easy structural damage caused by drying in traditional PVA-CS hydrogels.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: a polyvinyl alcohol and chitosan composite hydrogel, the composite hydrogel comprising the following components in parts by weight:

[0007] Polyvinyl alcohol: 6-10 parts;

[0008] Chitosan: 1-3 parts;

[0009] Glutaraldehyde: 0.5-1.5 parts;

[0010] In this system, the aldehyde groups at both ends of glutaraldehyde react with the –OH groups of PVA and the –NH2 of CS, respectively, through aldehyde-amine condensation (forming Schiff base bonds) and acetal reactions, thereby achieving a "bridging" covalent crosslinking of the two different polymers. PVA provides densely distributed –OH groups, providing reactive units for the acetal structure. The CS molecular chain, containing both –NH2 and –OH functional groups, can participate in the construction of Schiff base bonds and assist in the formation of acetal bonds. Glutaraldehyde acts as a "cross-node" in the system, connecting multiple chain segments to form a spatially staggered structure. This crosslinking process combines covalent strength with structural elasticity, allowing the gel to maintain its elastic rebound and spatial form even while retaining moisture.

[0011] Acetic acid: 100-150 parts;

[0012] Hydrochloric acid or sodium hydroxide: 0.01-0.1 parts;

[0013] Sodium chloride: 0.01-1 part;

[0014] The hydrogel is a three-dimensional network structure formed by cross-linking reaction of polyvinyl alcohol and chitosan through glutaraldehyde.

[0015] The present invention designs a common solvent system using acetic acid to simultaneously dissolve PVA and CS. Polyvinyl alcohol, as a hydrophilic synthetic polymer, has good water solubility and film-forming properties, while chitosan is a naturally derived alkaline polymer that can only be stably dissolved in an acidic environment.

[0016] Preferably, the glutaraldehyde concentration is 0.25 mol / L and the volume is 4 ml.

[0017] Preferably, the cross-linking reaction of the hydrogel is carried out at a pH of 5.5-6.5 to promote the cross-linking reaction of glutaraldehyde with PVA and chitosan.

[0018] Preferably, the degree of polymerization of the polyvinyl alcohol is 1750±50;

[0019] The acetic acid is a 2% aqueous solution, which exists as a solvent and is used to facilitate the dissolution of PVA and CS.

[0020] The present invention also provides a method for preparing a polyvinyl alcohol and chitosan composite hydrogel, which is used to prepare the above-mentioned polyvinyl alcohol and chitosan composite hydrogel, comprising the following steps:

[0021] S1, adding polyvinyl alcohol to an aqueous acetic acid solution, and fully dissolving it under heating and stirring conditions to form a polyvinyl alcohol solution;

[0022] S2, adding chitosan to the polyvinyl alcohol solution, and continuing stirring while maintaining heating to form a polyvinyl alcohol-chitosan mixed solution;

[0023] S3. Add hydrochloric acid or sodium hydroxide aqueous solution to the mixed solution as needed to adjust the pH value to a predetermined range;

[0024] S4, adding glutaraldehyde aqueous solution to the adjusted mixed solution, and performing a cross-linking reaction under constant temperature conditions to form a gel structure;

[0025] S5. Cutting into slices and drying to obtain a polyvinyl alcohol and chitosan composite hydrogel, the composite hydrogel is treated with sodium chloride for 24-48 hours.

[0026] Preferably, the step of preparing the polyvinyl alcohol solution comprises:

[0027] Add polyvinyl alcohol to acetic acid aqueous solution, heat to 85-90°C and stir for 30-60 minutes until a uniform and transparent polyvinyl alcohol solution is formed.

[0028] Preferably, the step of preparing the polyvinyl alcohol-chitosan mixed solution comprises:

[0029] Add chitosan to the polyvinyl alcohol solution and continue stirring at 60-70° C. for 60-90 minutes until the chitosan is completely dissolved and a homogeneous polyvinyl alcohol-chitosan mixed solution is formed;

[0030] The chitosan has a deacetylation degree of ≥85% and a viscosity average molecular weight of (1.0-2.0)×10 5 After the addition of Da water-soluble chitosan, the dissolution state was controlled by sampling and testing the pH and solubility every 5 minutes to ensure the formation of a uniform composite solution without flocs.

[0031] By using a weakly acidic acetic acid solution to directly dissolve the two polymers, and then achieving uniform dispersion through heating and stirring under appropriate temperature conditions, the intermolecular contact efficiency and the homogeneity of subsequent reactions were significantly improved. During this blending process, an intermolecular hydrogen bond network can be formed between the –OH groups on the PVA molecular chain and some –OH and –NH2 groups on the chitosan chain. This physical entanglement structure not only gives the solution a certain viscoelasticity and preformability, but also provides three-dimensional spatial structural support for subsequent chemical cross-linking reactions.

[0032] Preferably, the step of adjusting the pH value to a predetermined range comprises:

[0033] A hydrochloric acid aqueous solution or a sodium hydroxide aqueous solution is selectively added to the composite solution and stirred for 5-10 minutes to adjust the pH of the system to 5.5-6.5 to promote the subsequent cross-linking reaction.

[0034] The Schiff base reaction (aldehyde-amine condensation) will reduce its reactivity at too low a pH due to excessive protonation of the amino group. At too high a pH, glutaraldehyde is easily hydrolyzed into a hydrated structure, reducing the cross-linking efficiency. The pH value can be adjusted by adding hydrochloric acid or sodium hydroxide aqueous solution. Among them, hydrochloric acid (HCl) aqueous solution, as a strong acid, can quickly lower the pH value of the system, making the system acidic, effectively inhibiting the hydroxyl reaction side path caused by excessive alkalinity, while ensuring the active environment of the acetal reaction; sodium hydroxide (NaOH) aqueous solution can reversely increase the pH. Especially when the PVA-CS mixed solution is initially acidic, the introduction of an appropriate amount of NaOH can relieve the complete protonation state of the amino group, restore its nucleophilicity, and thus activate the Schiff base reaction pathway.

[0035] Preferably, the cross-linking reaction step includes:

[0036] The glutaraldehyde solution was added to the mixed solution after pH adjustment, and after sufficient stirring, the mixture was stirred and reacted in a constant temperature water bath of 55-60° C. for 50-70 minutes to form a gel structure.

[0037] Preferably, the steps of cutting and drying include:

[0038] After the reaction is completed, the gel structure is cut into 3-5 mm pieces and placed in a drying oven at 40-50° C. for drying to finally obtain PVA-CS dry gel.

[0039] The gel is cut into a specified shape (such as thin slices or small pieces) and dried under mild conditions to produce a dry gel. Unlike conventional freeze-drying or vacuum drying, the constant temperature drying method used retains the cross-linked three-dimensional skeleton structure within the gel, forming a stable porous network after removing free water. During this process, the covalent bond structure formed by cross-linking is not affected by mild drying, effectively maintaining the original spatial network configuration and physical continuity, giving the resulting dry gel significant advantages in water solubility, rehydration, and storage stability.

[0040] The present invention provides a polyvinyl alcohol and chitosan composite hydrogel and a preparation method thereof. It has the following beneficial effects:

[0041] 1. The present invention achieves bridging covalent crosslinking between polyvinyl alcohol and chitosan by introducing glutaraldehyde, and utilizes acetal reaction and Schiff base reaction to synergistically construct a three-dimensional crosslinked network, which not only enhances the overall mechanical strength of the hydrogel, but also effectively improves its structural integrity and hydrolysis resistance, and is particularly suitable for maintaining elasticity and morphology in high-humidity environments.

[0042] 2. Through the synergistic effects of physical entanglement and chemical crosslinking, this invention constructs a dual network at the molecular level, allowing polyvinyl alcohol and chitosan to form a well-dispersed system in solution. This structural foundation imparts excellent ductility, flexibility, and pre-forming capabilities to the gel, facilitating subsequent molding and functional expansion.

[0043] 3. By regulating the pH of the system under mild acidic conditions, this invention achieves precise control of the reactivity of functional groups during the cross-linking reaction, avoiding the problem of unstable cross-linking efficiency in conventional systems. By scientifically setting the pH window, this invention significantly improves the gel formation consistency and product batch stability of the hydrogel, enhancing the repeatability of its engineering applications.

[0044] 4. The present invention utilizes a low-temperature, constant-temperature drying method, effectively preserving the porous network structure formed by cross-linking. This allows the resulting dry gel to maintain structural stability during storage and rapidly recover its original gel properties upon rehydration. This property holds significant value for the application of functional hydrogels in fields such as biomedical materials and smart carriers. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 Schematic diagram of the preparation method of the present invention;

[0046] Figure 2 Schematic diagram of the swelling degree change curve of the hydrogel at different reaction temperatures of the present invention;

[0047] Figure 3 Schematic diagram of the swelling degree change curve of the hydrogel at different reaction times of the present invention;

[0048] Figure 4 Schematic diagram of the swelling degree change curve of the hydrogel at different cross-linking agent concentrations of the present invention;

[0049] Figure 5 Schematic diagram of the swelling degree change curve of the hydrogel when different cross-linking agent dosages are used in the present invention;

[0050] Figure 6 Schematic diagram of the swelling degree change curve of the hydrogel at different raw material mass ratios of the present invention;

[0051] Figure 7 Schematic diagram of the swelling degree change curve of the hydrogel of the present invention when it swells in deionized water at different temperatures;

[0052] Figure 8 Schematic diagram of the swelling degree change curve of the hydrogel of the present invention in aqueous solutions with different pH values;

[0053] Figure 9 Schematic diagram of the swelling degree change curve of the hydrogel of the present invention when it swells in salt solutions of different concentrations;

[0054] Figure 10 Schematic diagram of the FT-IR spectra of CS, PVA, and PVA-CS composite hydrogels of the present invention. DETAILED DESCRIPTION

[0055] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the specification of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0056] Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0057] By the attached Figure 1 Obtained composite hydrogel:

[0058] Before the experiment, the swelling degree of the composite hydrogel was measured:

[0059] Soak the sample at room temperature for 3-4 days, remove it, and use filter paper to absorb excess water from the sample surface. After the sample absorbs water and swells to a constant weight, weigh it and record the mass data. Use the following formula to calculate the swelling degree of the sample:

[0060]

[0061] Where SR is the saturated swelling degree of the hydrogel; m1 is the mass of the hydrogel when it has reached swelling equilibrium; and m2 is the mass of the hydrogel when it has not swelled:

[0062] Subsequent experimental results were characterized by Fourier transform infrared spectroscopy (FT-IR):

[0063] The prepared polyvinyl alcohol-chitosan composite hydrogel was dried, chopped and ground into powder by KBr tableting method, and tested by Fourier transform infrared spectrophotometer with a scanning range of 4000-500cm-1, 8 scans and a resolution of 4cm-1.

[0064] Test Example 1:

[0065] When using the controlled variable method to explore the effect of reaction polymerization temperature on the swelling degree of hydrogel.

[0066] Control the m(PVA):m(CS) ratio to 1:3, the reaction time to 1 hour, the glutaraldehyde concentration to 0.25 mol / L, and the crosslinker dosage to 4 ml. Change the reaction temperature. Record the data, calculate the corresponding gel equilibrium swelling ratio SR%, and draw a curve. The test results are shown in Figure 2 .

[0067] The test results show that the swelling curve of the PVA-CS composite hydrogel at different reaction temperatures shows an initial increase followed by a decrease. At 20°C, the swelling degree is 689%, indicating poor swelling performance. At 50°C, the swelling degree reaches 1136%, reaching the highest point of the swelling curve. As the temperature increases, the swelling degree gradually decreases, reaching 834% at 80°C, indicating optimal swelling performance at 50°C.

[0068] To explore the internal factors of the water absorption and swelling properties of hydrogels, we first noticed the key groups in the internal structure of its molecules, especially the amino and hydroxyl groups, which play an important role in the swelling properties because of their hydrophilicity.

[0069] At low temperatures, these hydrophilic groups tend to form hydrogen bonds with water molecules. This bonding breaks the hydrogen bonds and ester bonds within the hydrogel's molecules, weakening the intermolecular interactions and van der Waals forces. These factors work together to gradually enhance the hydrogel's water absorption and swelling properties. When the temperature continues to rise to a certain peak, the intermolecular hydrogen bonding begins to weaken. This change in turn promotes the increased hydrophobicity of the hydrophobic groups, ultimately leading to a gradual decrease in the hydrogel's water absorption and swelling properties.

[0070] Test Example 2:

[0071] When using the controlled variable method to explore the effect of reaction polymerization time on the swelling degree of hydrogel, control m(PVA):m(CS)=1:3, reaction temperature 50℃, glutaraldehyde concentration 0.25mol / L, crosslinker dosage 4ml. Change the reaction time, record the data, calculate the corresponding gel equilibrium swelling degree SR%, and draw a curve. The test results are shown in Figure 3 .

[0072] The test results show that the swelling curve of the PVA-CS composite hydrogel shows a rapid increase to a plateau before gradually decreasing at different reaction times. At 30 minutes, the swelling reached 910%, reaching its peak after 1 hour. The 1-2 hour curve plateaued, with the swelling level stabilizing at around 1136%. Extending the reaction time to 6 hours, the swelling reached 869%, indicating that the optimal swelling performance of the PVA-CS composite hydrogel prepared by chemical crosslinking is achieved when the reaction time is controlled between 1 and 2 hours.

[0073] This may be due to incomplete crosslinking in the initial stages of the reaction, i.e., 0.5 hours after the PVA and CS react, but as the crosslinking reaction between PVA and CS gradually intensifies over time, the degree of crosslinking between molecules deepens, causing the product's water absorption and swelling properties to show an upward trend. When crosslinking is more thorough, the hydrogel's swelling properties can reach their peak. However, as the reaction time increases, the excessive number of crosslinking points makes the product's internal network structure more dense. This overly tight structure restricts the free expansion of the three-dimensional network space during water absorption, preventing water molecules from entering the gel's pores, which in turn leads to a gradual decrease in the product's swelling properties.

[0074] Test Example 3:

[0075] When using the controlled variable method to explore the effect of the cross-linker concentration used in the reaction polymerization on the swelling degree of the hydrogel.

[0076] The ratio of m(PVA):m(CS) was controlled at 1:3, the reaction time was 1 hour, the reaction temperature was 50°C, the concentration of glutaraldehyde was varied (0.15 mol / L, 0.25 mol / L, 0.40 mol / L, 0.55 mol / L, 0.70 mol / L, 0.85 mol / L), and the amount of crosslinker was 4 ml. The data were recorded, the corresponding gel equilibrium swelling ratio SR% was calculated, and a curve was drawn. The test results are shown in Figure 2. Figure 4 .

[0077] Test results show that the swelling curve of the PVA-CS composite hydrogel at different crosslinker concentrations shows a rapid increase, a plateau, and then a gradual decrease. When the crosslinker concentration exceeds 0.40 mol / L, the rate of decrease accelerates significantly, eventually leveling off. Therefore, the hydrogel's swelling performance is optimal when the glutaraldehyde (GA) concentration is 0.25 mol / L.

[0078] When the concentration of the cross-linking agent is low, the CS molecular chains are not tightly connected due to insufficient intermolecular cross-linking points, making it impossible to form a stable gel-like structure with sufficient mechanical strength. In this case, the water absorption and swelling properties of the gel are often poor.

[0079] When the crosslinker concentration is too high, it leads to the formation of numerous crosslinking points between the CS molecular chains, significantly increasing the degree of crosslinking within the gel's internal structure. This high degree of crosslinking can lead to overly dense connections between the CS molecular chains, increasing the density of the gel's internal structure. While this can enhance the gel's mechanical strength to a certain extent, the excessive number of crosslinking points during water absorption makes it difficult for the CS molecular chains to fully expand. This results in a congested and restricted three-dimensional network within the gel, ultimately reducing its swelling properties.

[0080] Test Example 4:

[0081] When using the controlled variable method to explore the effect of the amount of cross-linker used in the reaction polymerization on the swelling degree of the hydrogel.

[0082] Control m(PVA):m(CS)=1:3, reaction temperature 50°C, glutaraldehyde concentration 0.25mol / L, and change the amount of crosslinker (2ml, 3ml, 4ml, 5ml, 6ml). Record the data, calculate the corresponding gel equilibrium swelling rate SR%, and draw a curve. The test results are shown in Figure 5 .

[0083] The test results show that the swelling degree of the hydrogel first rises, passes through a plateau, and then drops sharply as the amount of crosslinker increases. The swelling performance of the hydrogel reaches its peak when the amount of crosslinker is 4 ml.

[0084] The reasons for this may be: when the crosslinker dosage is too low, it is difficult to form a gel state, and a complete and stable hydrogel network structure has not yet formed, making it impossible to accommodate more water molecules into the hydrogel; when the crosslinker dosage is too high, on the one hand, the crosslinker is a prepared solution and the solvent is water molecules, which reduces the overall concentration of the hydrogel and prevents crosslinking. On the other hand, as the crosslinker dosage increases, the number of glutaraldehyde molecules contained in it increases, which increases the degree of crosslinking of the composite hydrogel, making its internal structure more compact and difficult to effectively accommodate more water molecules when absorbing water. This structural compactness restricts the free movement and diffusion of water molecules in the gel network, further leading to a significant decrease in swelling performance.

[0085] Test Example 5:

[0086] When using the controlled variable method to explore the effect of the mass ratio of raw materials used in reaction polymerization on the swelling degree of hydrogel.

[0087] Control the reaction time to be 1h, the reaction temperature to be 50℃, the concentration of glutaraldehyde to be 0.25mol / L, and the amount of crosslinker to be 4ml. Change the mass ratio of m(PVA):m(CS), record the data, calculate the corresponding gel equilibrium swelling degree SR%, and draw a curve. The test results are shown in Figure 6 .

[0088] The test results show that when the mass ratio of PVA to CS reaches 1:3, the swelling degree is the largest and the swelling performance is the best. The swelling degree of the hydrogel shows a trend of first increasing and then decreasing with the mass ratio of the two.

[0089] The reason may be: on the one hand, the side chains of polyvinyl alcohol (PVA) molecules are rich in -OH groups, which gives it excellent hydrophilicity. Therefore, when the number of polyvinyl alcohol (PVA) molecules increases, that is, its mass is appropriately improved, the hydrogel swelling degree increases. However, on the other hand, under the action of glutaraldehyde (GA) crosslinker, most of the -OH groups in PVA will form intermolecular hydrogen bonds with the -NH2 groups on chitosan (CS), constructing hydrogen bond cohesion entanglement points. The number of such cohesion entanglement points will increase rapidly with the increase of PVA content, which has an adverse effect on the swelling properties of the hydrogel and ultimately leads to a decrease in swelling degree.

[0090] Test Example 6:

[0091] Effect of swelling at different temperatures on the swelling properties of hydrogels.

[0092] The ratio of m(PVA):m(CS) was controlled at 1:3, the reaction time was 1 hour, the reaction temperature was 50°C, the concentration of glutaraldehyde was 0.25 mol / L, and the amount of crosslinker was 4 ml. The samples were placed in deionized water at different temperatures (15, 25, 35, 45, 55, 65, and 75°C). The data were recorded and the corresponding gel equilibrium swelling ratio (SR%) was calculated. A curve was drawn. The test results are shown in the table. Figure 7 .

[0093] The test results show that the swelling properties of the samples increase slowly at first and then decrease as the temperature of the deionized water increases. The swelling properties are optimal when the deionized water temperature reaches 25°C.

[0094] When the external deionized water temperature is low and not much different from room temperature, the hydrogel molecules easily hydrogen bond with the water molecules in the deionized water, resulting in the breaking of hydrogen bonds and ester bonds in the hydrogel molecules, which in turn weakens the intermolecular forces. Subsequently, the van der Waals forces also weaken, and finally the water absorption and swelling of the hydrogel are enhanced. When the external deionized water temperature continues to rise, on the one hand, the temperature increase will promote molecular movement, and the speed of water molecules will increase instantly. They will quickly enter the interior of the hydrogel molecules and occupy a larger space than originally required, thereby reducing the number of water molecules entering the hydrogel and causing a decrease in water absorption and swelling capacity. On the other hand, from the perspective of internal structure, the increase in temperature will weaken the effect of intermolecular hydrogen bonds, which will strengthen the hydrophobic effect of the hydrophobic groups, reduce the water absorption rate, and thus reduce the swelling rate.

[0095] Test Example 7:

[0096] Effect of swelling in different pH solutions on the swelling properties of hydrogels.

[0097] Control m(PVA):m(CS)=1:3, reaction time 1h, reaction temperature 50℃, glutaraldehyde concentration 0.25mol / L, crosslinker dosage 4ml. Take an appropriate amount of sample, divide it into 7 equal parts, and place it in solutions of different pH values (2, 4, 6, 8, 10, 12, 14). Record the data, calculate the corresponding gel equilibrium swelling rate SR%, and draw a curve. The test results are shown in Figure 8 .

[0098] The test results show that the swelling properties of the samples first increase and then decrease with the increase of the pH value of the solution. When the pH value is around 6, the swelling degree is the largest and the swelling performance is the best.

[0099] The reason for this may be that when the pH is less than 6, the small number of -OH groups in PVA undergo protonation, resulting in a positive charge. This repulsive interaction between the charges causes the PVA molecules to repel each other, which in turn causes the internal space of the molecules to expand, accommodating more water molecules. This expansion ultimately leads to a significant increase in the degree of swelling. However, in a strongly acidic environment, hydrogen in the solution reacts with the -OH groups in the PVA, so that the rate of increase in the degree of swelling is slower than the rate of decrease, resulting in a final decrease in the degree of swelling.

[0100] When the pH is > 6, the -OH groups in PVA undergo a deprotonation process. During the formation of the gel, not only do the CS molecules form hydrogen bonds with each other, but the -OH groups of PVA also form hydrogen bonds with CS molecules, thereby forming a physical interwoven network between CS and CS, and between CS and PVA molecules. As the PVA content increases, the structure of this interwoven network becomes denser, affecting the elasticity of the gel. Specifically, the molecular chains inside the gel will shrink within a small range, triggering further shrinkage of the entire molecular network. Therefore, under alkaline conditions, this shrinkage phenomenon will cause the swelling properties of the gel to gradually decrease.

[0101] Test Example 8:

[0102] Effect of swelling in salt solutions of different concentrations on the swelling properties of hydrogels.

[0103] Control m(PVA):m(CS)=1:3, reaction time 1h, reaction temperature 50℃, glutaraldehyde concentration 0.25mol / L, crosslinker dosage 4ml. Take an appropriate amount of sample, divide it into 6 equal parts, and place them in salt solutions of different concentrations (0mol / L, 0.01mol / L, 0.05mol / L, 0.10mol / L, 0.25mol / L, 0.50mol / L). Record the data, calculate the corresponding gel equilibrium swelling rate SR%, and draw a curve. The test results are shown in Figure 9 .

[0104] The test results show that as the concentration of the salt solution increases, the swelling rate decreases sharply. The swelling rate is highest when the salt solution concentration is 0 mol / L (i.e., deionized water), and the lowest is negative.

[0105] The reason may be that the hydrogel still contains a small amount of water. The external solution contains sodium chloride, a strong electrolyte. This electrolyte competes for water molecules in contact with the hydrogel, resulting in a smaller appearance and a sharp decrease in the degree of swelling. The reason for the negative value as the salt concentration increases is the salting-out effect of the hydrogel, which causes dehydration and shrinkage. This salting-out effect rapidly induces strong aggregation and crystallization among the hydrogel molecules. At the highest salt concentration, a layer of "hoar frost" forms on the surface of the hydrogel, and it becomes more solid. This suggests that in high-concentration salt solutions, the physical properties of the highly tough hydrogel, such as the dynamic ionic bond structure within the hydrogel, are affected, leading to embrittlement during swelling.

[0106] Test Example 9:

[0107] Analysis of infrared spectrum structure characterization, test results are shown in Figure 10 .

[0108] The infrared detection results showed that there might be changes and chemical interactions between PVA and CS.

[0109] As shown in the figure: the FTIR spectrum of CS is at 1658cm -1 、1599cm -1 、1086cm -1 The stretching vibration absorption bands of amide carbonyl (C=O), carbon-nitrogen single bond (CN) and carbon-oxygen bond (CO) are shown at 3460 cm -1 3355cm -1 The stretching vibration absorption peaks of the alcoholic hydroxyl (OH) and amide (NH) groups on the CS molecular chain are shown at 1086 cm -1 and 662cm -1 The peak at is the crystallization sensitive peak of CS.

[0110] The FTIR spectrum of PVA is at 3316 cm -1 、2930cm -1 、1081cm -1 The absorption peaks of OH stretching vibration, CH2 asymmetric stretching vibration and CO stretching vibration are shown at .

[0111] The FTIR spectrum of PVA-CS composite hydrogel is at 3408 cm -1 The contraction vibration frequency of the OH of -OH and the NH of -NH2 near the position becomes wider, and the PVA-CS composite hydrogel prepared by chemical cross-linking generates an imine (C=N) group.-1 、1088cm -1 This is confirmed by the characteristic absorption bands of amide carbonyl (C=O) and imine (C=N) at 662 cm -1 No obvious crystallization peak of CS was found, indicating that there was a strong hydrogen bond between PVA and CS, and the addition of PVA changed the crystallization state of CS.

[0112] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A polyvinyl alcohol and chitosan composite hydrogel, characterized in that: The composite hydrogel comprises the following components in parts by weight: Polyvinyl alcohol: 6-10 parts; Chitosan: 1-3 parts; Glutaraldehyde: 0.5-1.5 parts; Acetic acid: 100-150 parts; Hydrochloric acid or sodium hydroxide: 0.01-0.1 parts; Sodium chloride: 0.01-1 part; The hydrogel is a three-dimensional network structure formed by cross-linking reaction of polyvinyl alcohol and chitosan through glutaraldehyde.

2. A polyvinyl alcohol and chitosan composite hydrogel according to claim 1, characterized in that: The concentration of the glutaraldehyde is 0.25 mol / L, and the volume is 4 ml.

3. The polyvinyl alcohol and chitosan composite hydrogel according to claim 1, characterized in that: The cross-linking reaction of the hydrogel is carried out under the condition of pH 5.5-6.5 to promote the cross-linking reaction of glutaraldehyde with PVA and chitosan.

4. The polyvinyl alcohol and chitosan composite hydrogel according to claim 1, characterized in that: The degree of polymerization of the polyvinyl alcohol is 1750±50; The acetic acid is a 2% aqueous solution, which exists as a solvent and is used to facilitate the dissolution of PVA and CS.

5. A method for preparing a polyvinyl alcohol and chitosan composite hydrogel, characterized in that: The method for preparing a polyvinyl alcohol and chitosan composite hydrogel according to any one of claims 1 to 4 comprises the following steps: S1, adding polyvinyl alcohol to an aqueous acetic acid solution, and fully dissolving it under heating and stirring conditions to form a polyvinyl alcohol solution; S2, adding chitosan to the polyvinyl alcohol solution, and continuing stirring while maintaining heating to form a polyvinyl alcohol-chitosan mixed solution; S3. Add hydrochloric acid or sodium hydroxide aqueous solution to the mixed solution as needed to adjust the pH value to a predetermined range; S4, adding glutaraldehyde aqueous solution to the adjusted mixed solution, and performing a cross-linking reaction under constant temperature conditions to form a gel structure; S5. Cutting into slices and drying to obtain a polyvinyl alcohol and chitosan composite hydrogel, the composite hydrogel is treated with sodium chloride for 24-48 hours.

6. The method for preparing a polyvinyl alcohol and chitosan composite hydrogel according to claim 5, characterized in that: The steps of preparing the polyvinyl alcohol solution include: Add polyvinyl alcohol to acetic acid aqueous solution, heat to 85-90°C and stir for 30-60 minutes until a uniform and transparent polyvinyl alcohol solution is formed.

7. The method for preparing a polyvinyl alcohol and chitosan composite hydrogel according to claim 5, characterized in that: The steps of preparing the polyvinyl alcohol-chitosan mixed solution include: Add chitosan to the polyvinyl alcohol solution and continue stirring at 60-70° C. for 60-90 minutes until the chitosan is completely dissolved and a homogeneous polyvinyl alcohol-chitosan mixed solution is formed; The chitosan has a deacetylation degree of ≥85% and a viscosity average molecular weight of (1.0-2.0)×10 5 After the addition of Da water-soluble chitosan, the dissolution state was controlled by sampling and testing the pH and solubility every 5 minutes to ensure the formation of a uniform composite solution without flocs.

8. The method for preparing a polyvinyl alcohol and chitosan composite hydrogel according to claim 5, characterized in that: The step of adjusting the pH value to a predetermined range comprises: A hydrochloric acid aqueous solution or a sodium hydroxide aqueous solution is selectively added to the composite solution and stirred for 5-10 minutes to adjust the pH of the system to 5.5-6.5 to promote the subsequent cross-linking reaction.

9. The method for preparing a polyvinyl alcohol and chitosan composite hydrogel according to claim 5, characterized in that: The steps of the cross-linking reaction include: The glutaraldehyde solution was added to the mixed solution after pH adjustment, and after sufficient stirring, the mixture was stirred and reacted in a constant temperature water bath of 55-60° C. for 50-70 minutes to form a gel structure.

10. The method for preparing a polyvinyl alcohol and chitosan composite hydrogel according to claim 5, characterized in that: The steps of cutting and drying include: After the reaction is completed, the gel structure is cut into 3-5 mm pieces and placed in a drying oven at 40-50° C. for drying to finally obtain PVA-CS dry gel.

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