Hydrogel with highly adjustable mechanical properties based on ionic crosslinking and preparation method thereof

Abstract

The invention relates to the field of hydrogel materials, and discloses hydrogel with highly adjustable mechanical properties based on ionic crosslinking and a preparation method of the hydrogel. Through free radical copolymerization of acrylic acid and acrylamide, and electrostatic interaction between polycations of polyallylamine and polyanions of polyacrylic acid, a cross-linked network is constructed. By introducing and adjusting the dosage of acrylamide, the ionic crosslinking density and hydrogen-bond interaction in the network are effectively regulated and controlled, so that the large-range adjustability of mechanical properties is realized. According to the invention, a dehydration-re-soaking process is provided, so that the electrostatic interaction between molecular chains is enhanced, a gel network is more compact, and the mechanical strength of the gel network is improved. The preparation method is simple, low in cost and high in safety, has good thermal stability, and has wide application prospects in the fields of biomedicine, flexible electronics and the like.

Description

Technical Field

[0001] This invention relates to the field of polymer materials, and in particular to a method for preparing an ion-crosslinked hydrogel with highly tunable mechanical properties. Background Technology

[0002] Hydrogels are functional materials constructed from three-dimensional network hydrophilic polymers. Their structural characteristics enable them to retain large amounts of water without dissolving. These materials, with their high water content, excellent biocompatibility, and mechanical tunability, demonstrate enormous application potential in interdisciplinary fields such as tissue engineering, flexible sensors, and pollution control. However, traditional physical / chemical cross-linked hydrogel systems often suffer from drawbacks such as weak mechanical properties, limited application temperatures, sluggish environmental response, and low functional integration, which restrict their development in mid-to-high-end applications. Therefore, multi-linked systems, as an effective means to improve hydrogel performance, have been extensively studied in recent years. Among them, ionic cross-linked hydrogels, with their dynamically tunable ionic bonding, have become a cutting-edge direction in the research and development of novel aqueous materials.

[0003] Currently, research on ion-crosslinked hydrogels has made some progress. For example, Chinese patent CN108250468A combines acrylamide and sodium alginate in Ca... 2+ The gel is cured under UV light and then immersed in Fe. 3+ The solution is used to increase its stiffness by 10-100 times, and then Fe is added to vitamin C. 3+ Reduced to Fe 2+ A sodium alginate-acrylamide / Ca²⁺ tough hydrogel with adjustable elastic modulus, controllable deformation, and good biocompatibility was obtained, suitable for intelligent deformation devices and tissue engineering. Chinese patent CN112426981B grafted small polyhydrazide molecules onto sodium hyaluronate and sodium alginate to obtain a hydrazide-modified polymer; then, an aqueous solution of this polymer was reacted with Ca²⁺... 2+ Cu 2+ When metal ion solutions are mixed at room temperature, they instantly coordinate and crosslink to form a gel, creating a hydrazide-metal ion crosslinked hydrogel with adjustable gel time, adjustable mechanical strength, self-healing properties, and antibacterial properties, suitable for drug delivery, tumor models, and heavy metal wastewater treatment. Chinese patent CN103446897B uses sodium alginate as a matrix and a water-soluble pore-forming agent to create pores, first through Ca... 2+ / Ba 2+ / Al 3+ Plasma and carboxyl ions crosslink to form a first network, which is then covalently reinforced with chemical crosslinking agents such as glutaraldehyde to form a second network. Finally, secondary ionic crosslinking is used for shaping, resulting in a hydrogel membrane that is resistant to high pressure, has adjustable pore size, low swelling, and is completely biodegradable. Chinese patent CN110818921A uses methacrylic anhydride-modified gelling gum and glycidyl methacrylate-modified collagen in Ca...2+ In the bath, ions crosslink to form a first network, and UV-initiated double-bond free radical polymerization forms a second network, resulting in a double-crosslinked hydrogel with short curing time, uniform internal pore distribution, and good biocompatibility, providing a good three-dimensional support environment for the survival and proliferation of stem cells.

[0004] However, most of the system formulations reported in existing reports are quite complex. For example, in the aforementioned Chinese patent CN103446897B, after scraping a film with an aqueous solution of sodium alginate and a porogen, the film is first immersed in a metal salt solution to form an ionicly crosslinked hydrogel film, then transferred to a chemical crosslinking agent solution for covalent crosslinking, and finally washed with water to remove the porogen to obtain a high-strength hydrogel sheet film. This complex double-crosslinking system is not only prone to embrittlement of the gel film due to over-crosslinking, but also prone to potential toxicity of the gel due to residual porogen. Such performance defects make the hydrogel unstable in interdisciplinary fields such as tissue engineering, flexible sensors, and pollution control, making it difficult to achieve efficient utilization. Therefore, finding a simple and quick way to prepare the gel, endow it with excellent mechanical properties, and simultaneously provide a high degree of performance tunability through simple and effective methods would help improve the application value of hydrogels and enable them to meet the performance requirements of different scenarios to the greatest extent. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: to develop a hydrogel based on ion crosslinking, which improves the mechanical properties of the gel (tensile strength up to 4435 kPa) and makes the properties adjustable (tensile strength and elongation at break can be adjusted between 92-4435 kPa and 172-452%, respectively).

[0006] The technical solution adopted in this invention is: to achieve copolymerization of acrylic acid and acrylamide using a free radical polymerization method, utilizing the polycationic (-NH3) of polyallylamine. + ) and the carboxylate ions (-COO) on the polyacrylic acid molecular chain - The construction of an elastic network is achieved through electrostatic interactions between the two bonds, and the density of ionic crosslinking is reduced by polyacrylamide, thereby replacing ionic bonds with hydrogen bonds and achieving highly tunable mechanical properties.

[0007] The main reactants used in this invention include: polyallylamine, acrylic acid, potassium persulfate, and acrylamide.

[0008] Its raw material composition, in parts by weight, mainly includes: 65-85 parts by weight of polyallylamine Acrylic acid 73 - 145 parts by weight Acrylamide 0-72 parts by weight 900-1100 parts by weight of deionized water Potassium persulfate 1-3 parts by weight The molecular formula of polyallylamine is (C3H7N). n It is a pale yellow viscous liquid or solid powder, soluble in water, and is obtained by reducing polyallylamine hydrochloride after removing chloride ions; Acrylic acid has the molecular formula C3H4O2. It is a colorless to slightly yellow transparent liquid with strong irritation. It is infinitely miscible with water, ethanol and ether. Potassium persulfate, with the molecular formula K2S2O8, is a colorless or white crystalline powder, odorless, and readily soluble in water. Acrylamide, with the molecular formula C3H5NO, is a white to off-white crystalline powder or flakes that is readily soluble in water, methanol, ethanol, and acetone. The amount of acrylamide added is 10%-50% of the molar amount of acrylic acid. Preferably, the amount of acrylamide added is 20% of the molar amount of acrylic acid.

[0009] This invention provides a hydrogel with tunable mechanical properties based on ion crosslinking and its preparation method, comprising the following steps: Step 1: Raw Material Processing: As mentioned earlier, polyallylamine (PA) is derived from the reduction of polyallylamine hydrochloride (PAH). The specific method is as follows: Accurately weigh 1000 parts by weight of polyallylamine hydrochloride (PAH) powder, completely dissolve it in 1500 parts by weight of deionized water to form a transparent solution, and then add 1500 parts by weight of triethylamine (TEA) dropwise to the solution to remove the -NH3 groups from the PAH. + The product is reduced to -NH₂ to obtain polyallylamine, simultaneously generating triethylamine hydrochloride. The resulting solution is added dropwise to ethanol. At this point, the polyallylamine product precipitates due to decreased solubility, while most of the triethylamine hydrochloride and residual triethylamine and other soluble impurities remain in the liquid phase. Subsequent washing with ethanol further removes residual TEA, triethylamine hydrochloride, and other soluble impurities. After washing, the precipitate is separated and dried under vacuum at 60 °C for 2 hours to obtain the product. In this product, most chloride ions are removed. However, it should be noted that this step does not yield pure PA; the PA molecules in the product still contain a small amount of chloride ions. However, the presence of this small amount of chloride ions does not significantly affect the subsequent preparation of the hydrogel.

[0010] Step 2: Dissolve the polyallylamine product obtained in Step 1 in a measured amount of deionized water to prepare a solution.

[0011] Step 3: Place the polyallylamine solution prepared in Step 2 on a magnetic stirrer, and add acrylic acid solution dropwise while stirring. After 4-5 minutes, add acrylamide powder while stirring. After 4-5 minutes, add potassium persulfate while stirring to initiate the polymerization reaction.

[0012] Step 4: Transfer the prepared solution into a polytetrafluoroethylene mold, seal it, and place it in a 60℃ constant temperature oven for thermally initiated polymerization. After polymerization, equilibrate and cool the gel sample at 25℃ for 2 hours to obtain the hydrogel sample.

[0013] Step 5: Gel Dehydration and Immersion Treatment. The prepared hydrogel sample is dehydrated at room temperature for 3-4 days to strengthen ionic and hydrogen bonds, forming a denser network structure. The dried gel is then completely immersed in deionized water, with the water changed every 8 hours to remove residual monomers, salts, and small molecule byproducts. The gel that has reached swelling equilibrium is the final sample.

[0014] In this system, the primary step is to remove chloride ions from polyallylamine hydrochloride (PAH) to prepare the key component, polyallylamine (PA). If the chloride ions in polyallylamine hydrochloride are not removed, Cl... - It can bind to amino groups, occupying some positively charged sites and shielding some of the electrostatic interactions between polymer chains, thus weakening the electrostatic interaction between polycations and polyanions. This reduces the crosslinking density, leading to insufficient gel strength or failure to form.

[0015] In this system, potassium persulfate (KPS) exhibits strong oxidizing properties. The primary amine group (-NH2) of polyallylamine after chloride removal has reducing properties, forming a redox system with KPS to generate free radicals, which initiate the polymerization of acrylic acid and acrylamide to form polyacrylic acid and polyacrylamide. The -NH3 group of polyallylamine dissolved in water... + Acrylamide, through electrostatic interactions with the -COO⁻ group of polyacrylic acid, constructs an elastic network, causing cross-linking between polymer chains to form a three-dimensional network. Acrylamide introduces amide bonds into the molecular chain via copolymerization, simultaneously reducing the density of carboxylate ions, thus regulating the strength of electrostatic interactions and network structure, and consequently modulating the mechanical properties of the gel.

[0016] In this invention, the key step in enhancing the gel's properties is a dehydration-re-soaking process. During dehydration, as water molecules escape, the molecular network shrinks, and the distance between molecular chains gradually decreases. At this point, the electrostatic interactions, which were initially weakened by the shielding effect of water molecules, are strengthened. During re-soaking, the enhanced electrostatic interaction increases the cohesive force of the network, and the tightly bound polyanions and cations reduce the intrusive ability of water molecules. At this point, the plasticizing effect of water molecules is greatly reduced, thus achieving a significant improvement in the mechanical strength of the hydrogel.

[0017] Compared with the prior art, the present invention achieves the following beneficial effects:

[0018] (1) This invention prepares ion-crosslinked hydrogels via free radical polymerization, which is simple and safe. Furthermore, all reagents used are conventional monomers and initiators, exhibiting low toxicity and minimal risk. In addition, the reaction conditions are mild, requiring no complex equipment or harsh environments, resulting in low overall cost. Therefore, this method not only improves the efficiency of hydrogel preparation but also provides an economical and safe guarantee for large-scale production and application.

[0019] (2) The ionically crosslinked hydrogel prepared by free radical polymerization in this invention has highly adjustable mechanical properties, with tensile strength adjustable between 92 and 4435 kPa. The polycationic (-NH3) in the polyallylamine of this invention... + ) and polyanionic (-COO) of polyacrylic acid - Through electrostatic interactions, polymer chains cross-link to form a stable network structure. The addition and copolymerization of acrylamide can adjust the density of electrostatic interactions, thereby forming a hydrogel with tunable mechanical properties.

[0020] (3) The hydrogel network structure in this invention exhibits high stability. The cations (-NH3) on the polyallylamine molecule... + The anions (-COO⁻) on polyacrylic acid molecules and the polyacrylic acid molecules form a three-dimensional gel network through electrostatic interactions. This network is not easily destroyed by increasing temperature and retains its 10... 4 The Pa storage modulus is due to the stable network structure formed by dense ionic bonds through electrostatic interactions. This property endows the material with long-term service capability under intermediate temperature conditions, extending its application to cutting-edge fields such as implantable flexible electronics, artificial ligaments, and wearable sensors. Attached Figure Description

[0021] Figure 1 These are the infrared spectra of PAH and PA.

[0022] Figure 2 These are temperature scan diagrams from rheological tests in Examples 1, 2, 5, and 6.

[0023] Figure 3 These are frequency scan diagrams of rheological tests in Examples 1, 2, 5, and 6.

[0024] Figure 4 This is a moisture content diagram for the AM-AAx% system.

[0025] Figure 5 This is a stress-strain diagram of the AM-AAx% system after gel formation.

[0026] Figure 6This is a stress-strain diagram of the AM-AAx% system hydrogel after it has been dehydrated to a dry state at room temperature and then immersed in water to reach swelling equilibrium. Detailed Implementation

[0027] This invention is not limited to the following specific embodiments. Those skilled in the art can implement this invention using other specific embodiments based on the content disclosed herein. Any simple changes or modifications made to the design structure and concept of this invention fall within the protection scope of this invention.

[0028] The following performance tests were performed on the sodium hydrogels prepared in the following examples using the following methods: 1. Infrared testing PAH powder and the processed PA powder were separately mixed with potassium bromide and ground into powders, then compressed into tablets using a tablet press to prepare test discs. The tablets were baked under an infrared lamp, and the samples were characterized using Fourier transform infrared spectroscopy to analyze the composition and changes in the gel microstructure. The wavenumber scanning range was 4000 cm⁻¹. -1 - 400 cm -1 .

[0029] 2. Rheological property testing The synthesized hydrogel sheets were punched using a standard stainless steel disc mold to obtain circular samples with uniform geometric parameters. The samples were then precisely positioned between the parallel plate clamps of the rheometer (plate spacing 1.0 mm), and the temperature control system was set to a constant temperature of 25°C.

[0030] Oscillation frequency scanning: After cleaning the rheometer platform with a brush and scraper, use tools such as scrapers and tweezers to take samples and place them on the rheometer platform. Adjust the gap to 1000.0 µm and clean up any overflowing samples. Set the rotational rheometer (TA-Discovery: HR-2) to oscillation frequency testing mode, with a temperature of 25℃, strain of 0.5%, frequency range of 0.01-100 Hz, and a testing time of approximately 15 minutes.

[0031] Oscillating Temperature Scan: Sample preparation steps and precautions are the same as for oscillating frequency scan. After cleaning the rheometer platform with a brush and scraper, use tools such as a scraper and tweezers to place the sample on the rheometer platform, adjust the gap to 1000.0 µm, and clean up any overflowing sample. Adjust the rotational rheometer (TA-Discovery: HR-2) test mode to oscillating temperature test, with a strain of 1%, a frequency of 1 Hz, a temperature range of 15-70℃, and a temperature gradient of 3 ℃ / min. During the test, care should be taken to prevent the rheometer temperature from rising too quickly. After each set of sample tests, wait for the rheometer to cool down to 15℃ before starting the next set of tests.

[0032] 3. Moisture content test The water content is obtained by measuring the mass difference of hydrogels with different solid contents before and after water loss, where W0 represents the original mass value of the gel sample before water loss, W... t This represents the mass value of the gel after dehydration. To ensure the accuracy of the test results, each sample was tested multiple times, and the average value was taken. The hydrogel was dried in a vacuum drying oven at 80°C until the gel mass reached a constant. The formula is:

[0033] 4. Mechanical property testing The strain and stress values ​​of hydrogels with different solid contents were measured under tension of 100 mm / min.

[0034] In the following embodiments, unless otherwise specified, the mass parts and percentages of the components involved are all mass parts and mass percentages.

[0035] The present invention will be further described in detail below with reference to the embodiments: Example 1

[0036] Solution preparation: Dissolve 75 parts by mass of polyallylamine powder in 1000 parts by mass of deionized water. Place the prepared polyallylamine solution on a magnetic stirrer and add 145 parts by mass of acrylic acid solution dropwise while stirring. After 4-5 minutes, add 2 parts by mass of potassium persulfate while stirring. Seal the container and place it in a 60℃ constant temperature oven for thermally initiated polymerization. Cool the gel sample at 25℃ for 2 hours to obtain the polyallylamine / acrylic acid hydrogel.

[0037] Performance test results The polyallylamine / acrylic hydrogel prepared in Example 1 had a water content of 60.34% after gel formation, a tensile strength of 159 kPa, and an elongation at break of 417%. The prepared hydrogel sample was dehydrated at room temperature, and then the dried gel was completely immersed in deionized water to reach swelling equilibrium. The water content of the gel after dehydration and re-immersion was 31.67%, the tensile strength was 4435 kPa, and the elongation at break was 45%. Example 2

[0038] Solution preparation: Dissolve 75 parts by mass of polyallylamine powder in 1000 parts by mass of deionized water. Place the prepared polyallylamine solution on a magnetic stirrer and add 131 parts by mass of acrylic acid solution dropwise while stirring. After 4 to 5 minutes, add 14 parts by mass of acrylamide powder while stirring. After 4 to 5 minutes, add 2 parts by mass of potassium persulfate while stirring. Seal the container and place it in a 60°C constant temperature oven for thermally initiated polymerization. Equilibrate and cool the gel sample at 25°C for 2 hours to obtain the polyallylamine / acrylic acid / acrylamide hydrogel.

[0039] Performance test results The polyallylamine / acrylic hydrogel prepared in Example 2 had a water content of 49.71% after gel formation, a tensile strength of 175 kPa, and an elongation at break of 350%. The prepared hydrogel sample was dehydrated at room temperature, and then the dried gel was completely immersed in deionized water to reach swelling equilibrium. The water content after dehydration and re-immersion was 32.92%, the tensile strength was 1320 kPa, and the elongation at break was 241%. Example 3

[0040] Solution preparation: Dissolve 75 parts by mass of polyallylamine powder in 1000 parts by mass of deionized water. Place the prepared polyallylamine solution on a magnetic stirrer and add 116 parts by mass of acrylic acid solution dropwise while stirring. After 4 to 5 minutes, add 29 parts by mass of acrylamide powder while stirring. After 4 to 5 minutes, add 2 parts by mass of potassium persulfate while stirring. Seal the container and place it in a 60°C constant temperature oven for thermally initiated polymerization. Equilibrate and cool the gel sample at 25°C for 2 hours to obtain the polyallylamine / acrylic acid / acrylamide hydrogel.

[0041] Performance test results The polyallylamine / acrylic hydrogel prepared in Example 3 had a water content of 53.24% after gel formation, a tensile strength of 92 kPa, and an elongation at break of 291%. The prepared hydrogel sample was dehydrated at room temperature, and then the dried gel was completely immersed in deionized water to reach swelling equilibrium. The water content after dehydration and re-immersion was 34.64%, the tensile strength was 756 kPa, and the elongation at break was 347%. Example 4

[0042] Solution preparation: Dissolve 75 parts by mass of polyallylamine powder in 1000 parts by mass of deionized water. Place the prepared polyallylamine solution on a magnetic stirrer and add 102 parts by mass of acrylic acid solution dropwise while stirring. After 4 to 5 minutes, add 43 parts by mass of acrylamide powder while stirring. After 4 to 5 minutes, add 2 parts by mass of potassium persulfate while stirring. Seal the container and place it in a 60°C constant temperature oven for thermally initiated polymerization. Equilibrate and cool the gel sample at 25°C for 2 hours to obtain the polyallylamine / acrylic acid / acrylamide hydrogel.

[0043] Performance test results The polyallylamine / acrylic hydrogel prepared in Example 4 had a water content of 56.67% after gel formation, a tensile strength of 97 kPa, and an elongation at break of 297%. The prepared hydrogel sample was dehydrated at room temperature, and then the dried gel was completely immersed in deionized water to reach swelling equilibrium. The water content after dehydration and re-immersion was 35.06%, the tensile strength was 2143 kPa, and the elongation at break was 240%. Example 5

[0044] Solution preparation: Dissolve 75 parts by mass of polyallylamine powder in 1000 parts by mass of deionized water. Place the prepared polyallylamine solution on a magnetic stirrer and add 87 parts by mass of acrylic acid solution dropwise while stirring. After 4 to 5 minutes, add 57 parts by mass of acrylamide powder while stirring. After 4 to 5 minutes, add 2 parts by mass of potassium persulfate while stirring. Seal the container and place it in a 60°C constant temperature oven for thermally initiated polymerization. Equilibrate and cool the gel sample at 25°C for 2 hours to obtain the polyallylamine / acrylic acid / acrylamide hydrogel.

[0045] Performance test results The polyallylamine / acrylic hydrogel prepared in Example 5 had a water content of 62.69% after gel formation, a tensile strength of 295 kPa, and an elongation at break of 172%. The prepared hydrogel sample was dehydrated at room temperature, and then the dried gel was completely immersed in deionized water to reach swelling equilibrium. The water content after dehydration and re-immersion was 43.43%, the tensile strength was 1632 kPa, and the elongation at break was 364%. Example 6

[0046] Solution preparation: Dissolve 75 parts by mass of polyallylamine powder in 1000 parts by mass of deionized water. Place the prepared polyallylamine solution on a magnetic stirrer and add 73 parts by mass of acrylic acid solution dropwise while stirring. After 4 to 5 minutes, add 72 parts by mass of acrylamide powder while stirring. After 4 to 5 minutes, add 2 parts by mass of potassium persulfate while stirring. Seal the container and place it in a 60°C constant temperature oven for thermally initiated polymerization. Cool the gel sample at 25°C for 2 hours to obtain the polyallylamine / acrylic acid / acrylamide hydrogel.

[0047] Performance test results The polyallylamine / acrylic hydrogel prepared in Example 6 had a water content of 65.43% after gel formation, a tensile strength of 147 kPa, and an elongation at break of 131%. The prepared hydrogel sample was dehydrated at room temperature, and then the dried gel was completely immersed in deionized water to reach swelling equilibrium. The water content after dehydration and re-immersion was 49.97%, the tensile strength was 1074 kPa, and the elongation at break was 452%.

[0048] The total mass fraction of polyallylamine, acrylic acid, and acrylamide in the gel system was approximately 220 parts by mass. The total monomer concentration was controlled at 18% (w / w) to ensure polymerization efficiency. When acrylamide (AM) was added to the system, the amount of acrylamide was 10%, 20%, 30%, 40%, and 50% of the molar percentage of acrylic acid in AM-AA-0%. The resulting AM-AA hydrogels were labeled as AM-AA0%, AM-AA10%, AM-AA20%, AM-AA30%, AM-AA40%, and AM-AA50%, respectively. The specific mass fractions are shown in Table 1 below.

[0049] Table 1 AM-AAx% Hydrogel Formulation

[0050] Table 2 Performance parameters of AM-AAx% hydrogel

[0051] Figure 1 shows the infrared spectra of polyallylamine hydrochloride (PAH) and polyallylamine. The spectra indicate that PAH has a high infrared intensity at 1630 cm⁻¹. -1 With 1550 cm -1 Presented at –NH3 + Asymmetric and symmetric bending vibrations with two peaks, at 2800-3000 cm⁻¹ -1 A broad and strong N–H stretching absorption is formed; after dechlorination, the 1550 cm⁻¹ peak is significantly weakened, and the 1630 cm⁻¹ peak is reduced. -1 Peak intensity decreased, 3000 cm -1 The absorption band in the region shrinks and narrows. This proves that most of the -NH3... + It loses its positive charge and is successfully converted into -NH2, meaning that PAH has been essentially reduced to PA.

[0052] Figure 2 is Example 1 ( Figure 2 a) Example 2 ( Figure 2 b) Example 5 Figure 2 c) and Example 6 ( Figure 2 d) Rheological temperature scan. As shown in Figure 2, the storage modulus (G') of AM-AA0%, AM-AA10%, AM-AA40%, and AM-AA50% did not decrease significantly with increasing temperature, indicating that the gel has good thermal stability in the range of 15–70 °C. This is because the dense ionic bonds form a tightly cross-linked network through electrostatic interactions, making them less susceptible to damage at higher temperatures.

[0053] However, at the same temperature point, for example, 21℃, when the percentage of AM replacing AA is 0%, 10%, 40%, and 50%, the storage modulus G' is 28126 Pa, 57275 Pa, 8230 Pa, and 5027.41 Pa, respectively, showing an overall trend of first increasing and then sharply decreasing. This is because AM introduces hydrogen bonds through amide bonds (-CONH2), providing new hydrogen bond crosslinking points. Although this enhances the physical crosslinking between molecules, causing G' to increase temporarily, as the proportion of AM replacing AA increases to 40% and 50%, the number of amide bonds increases, the carboxylate ion density is diluted, and the electrostatic interaction between polyallylamine and polyacrylic acid is weakened. The number of ionic crosslinking points constituting the gel network decreases sharply, so G' drops sharply. At the same time, although the number of hydrogen bonds increases, the strength of hydrogen bonds is much lower than that of ionic bonds forming electrostatic interactions, so the overall network strength decreases, leading to a decrease in G'.

[0054] Figure 3 shows the rheological frequency scan diagrams for Examples 1, 2, 5, and 6, respectively. Figure 3 (a), (b), (c), and (d) are shown in Figure 3. It can be seen that the storage modulus (G') of AM-AA0% and AM-AA10% gradually increases with increasing frequency, and the rate of increase is approximately the same. This is because the gel system mainly achieves cross-linking through non-covalent interactions. As the frequency increases, the relaxation of the gel network cannot keep up with the vibration rate. The ionic bonds and hydrogen bonds in the network do not have enough time to undergo relaxation behaviors such as breakage and recombination, and thus cannot dissipate energy in time, resulting in an increase in the storage modulus G'. In the low-frequency region (0.1-10 Hz), G' is always higher than G'', indicating that the gel network maintains the characteristics of an elastic gel in this range. This is because under low-frequency deformation, the ionic bonds and hydrogen bonds in the network have sufficient time to relax and undergo reversible breakage and recombination. However, in the high-frequency region (10-100 Hz), the ionic bonds and hydrogen bonds in the network do not have enough time to undergo relaxation behaviors such as breakage and recombination, and thus cannot dissipate energy in time, resulting in an increase in the storage modulus G'. When the frequency is high enough, the entire network cannot adapt to rapid deformation, and G'' exceeds G', indicating that the gel network structure is damaged and exhibits viscous characteristics.

[0055] In the testing range of 0.1–100 Hz, the storage modulus (G') of AM-AA40% and AM-AA50% increases slightly with increasing frequency, and the curve is almost flat. This is because a large number of hydrogen bonds with short relaxation times are introduced into the gel network. These hydrogen bonds can break and recombine extremely rapidly, and their reconstruction rate is much faster than the rate of external deformation. Therefore, the network can always keep up with the frequency change, and energy is dissipated in time. At this time, G' increases slightly, and it always shows that G' is higher than G'', and the gel network remains an elastic network that is not destroyed.

[0056] Figure 4 is a moisture content scan of Examples 1-6. As shown in the figure, the moisture content of the entire gel system after gel formation is between 50% and 60%. The moisture content of the gel system after dehydration to a dry state at room temperature is about 10%. The moisture content of the gel system after dehydration to a dry state at room temperature and then soaking in water to reach swelling equilibrium is between 30% and 40%.

[0057] When the gel is untreated, the network segments are in a stretched state, and water molecules and -COO - -NH3 + The hydrophilic groups such as -CONH2 are present in large quantities in the network pores, resulting in a high water content. After dehydration, the water content of the gel drops sharply to 10%. At this point, the alkyl backbone will move closer to each other due to the loss of water, and the hydration layer between charges will be destroyed, the water molecule shielding effect will disappear, and the polycations and anions can be tightly bound together, enhancing the electrostatic interaction and making the network more compact. After dehydration and re-soaking, the water content rises back to 30-40%. At this point, the enhanced electrostatic interaction between polymer chains due to dehydration forms a strong and dense ionic cross-linked network. This network has an extremely high cross-linking density, making it difficult for water molecules to enter when re-soaked, so the water content only rises to 30-40%.

[0058] Figure 5 shows the stress-strain diagrams after gel formation in Examples 1 and 3-5. As can be seen from the figure, the AM-AA0% system exhibits the highest elongation at break at 417%, with a tensile strength of 159 kPa. With increasing acrylamide (AM) content, the tensile strength of the AM-AA20% and AM-AA30% systems decreases to approximately 95 kPa, and the elongation at break decreases to approximately 290%. Further increasing the AM content to 40%, the AM-AA40% system reaches its maximum tensile strength of 295 kPa, but at this point, the elongation at break is relatively low at only 172%.

[0059] In the AM-AA0% system without the introduction of AM, the crosslinking network is a polycationic (-NH3) of polyallylamine. + Electrostatic interactions between AM and the polyanion (-COO⁻) of polyacrylic acid form a strong, dense cross-linked network. This network, acting as a strong non-covalent bond, requires significant stress to resist deformation under external forces, resulting in high tensile strength. Under external force, the sacrificial breaking and rebonding of ionic bonds effectively dissipates energy, preventing crack propagation and allowing the network to undergo substantial deformation without breaking, thus achieving an elongation at break of 417%. In the AM-AA20% and AM-AA30% systems, the addition of AM introduces amide bonds, diluting the carboxylate (-COO⁻) group that forms strong ionic bonds. -The reduced crosslinking density significantly weakens the electrostatic interaction of the network. Simultaneously, the few remaining hydrogen bonds cannot effectively replace the lost ionic bonds, leading to a decrease in the overall strength and toughness of the network, thus reducing stress and strain. In the AM-AA 40% system, the crosslinking density further decreases, and the network becomes looser. At this point, electrostatic crosslinking points are very few, and the network is mainly maintained by hydrogen bonds. When the AM content is sufficiently high, a large number of amide groups are no longer isolated; they form dense hydrogen bond clusters, thereby forming a dense crosslinked network, significantly improving the tensile strength of the material to 295 kPa.

[0060] Figure 6 shows the stress-strain diagrams of Examples 1-4 and Example 6 after dehydration to a dry state at room temperature, followed by immersion in water to reach swelling equilibrium. Figure 6 It can be seen that the AM-AA0% system exhibits the highest tensile strength after dehydration and re-soaking, reaching 4435 kPa, with an elongation at break of 45%. The AM-AA10% and AM-AA20% systems have tensile strengths of 1320 and 756 kPa, respectively, with elongations at break of 241% and 347%. The AM-AA30% system, after dehydration and re-soaking, has a tensile strength of 2143 kPa and an elongation at break of 240%. The AM-AA40% system has a tensile strength of 1632 kPa and an elongation at break of 364%, while the AM-AA50% system has the highest elongation at break of 452%, with a tensile strength around 1074 kPa. A comparison of Figure 6 and Figure 5 shows that after dehydration and re-soaking, the tensile strength of the gel changes from 92-295 kPa to 756-4435 kPa. This is because during dehydration, the polycationic -NH3 molecules, originally shielded by water molecules, are removed. + With polyanionic-COO - They are tightly bound together, forming extremely strong and dense ionic bonds. Therefore, after dehydration and rehydration, water molecules cannot completely penetrate and disrupt the dense and stable gel network, resulting in an order-of-magnitude increase in its tensile strength.

[0061] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A hydrogel with tunable mechanical properties based on ionic crosslinking, characterized in that, The hydrogel comprises a three-dimensional network structure prepared by free radical polymerization of polyallylamine, acrylic acid, acrylamide, and initiator raw materials. The network structure is ionically crosslinked through electrostatic interactions between the cationic groups on polyallylamine and the anionic carboxyl groups on the polyacrylic acid segments polymerized from acrylic acid.

2. The ion-crosslinked hydrogel with tunable mechanical properties according to claim 1, characterized in that, The hydrogel is prepared from the following raw materials in parts by weight: 65-85 parts by weight of polyallylamine; 73-145 parts by weight of acrylic acid; 0-72 parts by weight of acrylamide; 900-1100 parts by weight of deionized water; and 1-3 parts by weight of initiator.

3. The ion-crosslinked hydrogel with tunable mechanical properties according to claim 1, characterized in that, The initiator is potassium persulfate.

4. The ion-crosslinked hydrogel with tunable mechanical properties according to claim 1, characterized in that, The amount of acrylamide added is 10%-50% of the molar amount of acrylic acid.

5. The hydrogel with tunable mechanical properties based on ion crosslinking according to claim 4, characterized in that, The amount of acrylamide added is 20% of the molar amount of acrylic acid.

6. A method for preparing a hydrogel according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Dissolve polyallylamine, acrylic acid, and acrylamide in deionized water, add an initiator, and mix well to obtain a precursor solution; (2) The precursor solution is subjected to thermally initiated polymerization, and the initial hydrogel is obtained after cooling; (3) The initial hydrogel is dehydrated and then immersed in deionized water until swelling equilibrium is reached to obtain a hydrogel with enhanced mechanical properties.

7. The method for preparing the hydrogel according to claim 6, characterized in that, The thermally initiated polymerization in step (2) is carried out at 60°C.

8. The method for preparing the hydrogel according to claim 6, characterized in that, The dehydration process in step (3) is carried out at room temperature for 3-4 days, after which the dried gel is completely immersed in deionized water.

9. The method for preparing the hydrogel according to claim 6, characterized in that, The method further includes a step of treating polyallylamine hydrochloride to prepare polyallylamine before step (1). The treatment step includes: dissolving polyallylamine hydrochloride in water, adding triethylamine for treatment, and then obtaining the polyallylamine product by ethanol precipitation, washing and vacuum drying.

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