Crystalline-amorphous double-network PVA hydrogel as well as preparation method and application thereof

Abstract

The invention relates to crystalline-amorphous double-network PVA (polyvinyl alcohol) hydrogel. The double networks of the hydrogel are composed of crystalline polyvinyl alcohol and amorphous polyvinyl alcohol. The preparation method comprises the following steps: (1) adding sulfuric acid and triaminophenylboronic acid into a 5-15% polyvinyl alcohol aqueous solution, and uniformly stirring; (2) sealing and storing the mixed solution at 0-10 DEG C for more than 12 hours, and fully cross-linking; and (3) performing freeze-thaw cycle treatment on the mixed solution obtained in the step (2), then soaking the gel in dilute sulphuric acid for more than 24 hours, and finally soaking the gel in deionized water of which the volume is 10 times of that of the gel for 24 hours so as to remove residual sulphuric acid and unreacted monomers in the gel, thereby obtaining the crystal-amorphous double-network PVA gel. According to the method, the crystallinity of the polyvinyl alcohol gel is improved on the premise that the water content is slightly reduced. The highly-compact nanoscale small-pore amorphous PVA network inhibits water loss of the gel in salting-out salt, so that the ionic conductivity of the gel in salting-out salt sodium sulfate is improved by 408.1%.

Description

Technical Field

[0001] The invention belongs to the field of hydrogels and relates to a crystalline-amorphous double-network PVA hydrogel and a preparation method thereof. Background Art

[0002] By encapsulating a large amount of water through a three-dimensional cross-linked polymer network, hydrogels combine the deformability of solids and the mass transfer capacity of liquids, and have unique structures and properties, thus showing unparalleled application potential in many fields such as bionic engineering, implantable electronic devices, energy storage components, and intelligent robots. In particular, when used as an ion transport carrier for energy storage components, due to the high water content of the gel, the gel electrolyte constructed based on hydrogels can often obtain ultra-high ion transport efficiency close to that of pure liquid electrolytes, and can inherit the deformability of the gel, thereby conveniently constructing flexible energy storage components and suppressing the leakage problem of energy storage components constructed based on liquid electrolytes. This material has received widespread attention.

[0003] The monomer of polyvinyl alcohol (PVA) consists of two carbon atoms and one hydroxyl group. The extremely simple chemical structure makes PVA gel stably compatible with most common aqueous electrolyte systems, making it one of the most widely used gel electrolytes. However, the most common freeze-thaw polyvinyl alcohol gel relies on a small amount of hydrogen bonds and intermolecular van der Waals forces to build a gel network, and its mechanical properties are extremely poor. The polyhydroxy structure causes polyvinyl alcohol gel to lose water rapidly in neutral salt electrolytes where salting-out effect is prevalent, resulting in extremely low ion transfer efficiency under salting-out salts, which greatly limits its application in actual production.

[0004] The mechanical properties of the gel can be improved by introducing a second cross-linking network or increasing the crystallinity. The introduction of a second cross-linking network can effectively increase the cross-linking density without significantly reducing the water content of the gel, thereby improving the mechanical properties. However, the introduction of a new phase will weaken the electrolyte compatibility, which is the biggest advantage of the polyvinyl alcohol system. The crystallization method based on inducing hydrogen bonds between polyvinyl alcohol molecules to form a large number of high-strength polyvinyl alcohol nanocrystalline regions can significantly enhance the mechanical strength of the gel without weakening the electrolyte compatibility of the polyvinyl alcohol system, but it is often accompanied by a sharp decrease in water content, resulting in obstruction of ion transport within the gel. Summary of the invention

[0005] The purpose of the present invention is to provide a crystalline-amorphous double network PVA hydrogel and a preparation method thereof, so as to overcome the above-mentioned shortcomings. The present invention organically combines the double network strategy with the crystallization strategy, so as to improve the mechanical performance problems faced by the polyvinyl alcohol gel without reducing the mechanical properties based on the water content and electrolyte compatibility of the polyvinyl alcohol gel, and improves the ion transport problem of the polyvinyl alcohol gel under neutral salting-out salt by constraining water through the amorphous network.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows.

[0007] A crystalline-amorphous double-network PVA hydrogel, wherein the double network of the hydrogel is composed of crystalline polyvinyl alcohol and amorphous polyvinyl alcohol.

[0008] Preferably, the crystallinity of the crystalline polyvinyl alcohol is 30% to 40%.

[0009] Preferably, the pore size of the amorphous polyvinyl alcohol is 50-200 nm.

[0010] A method for preparing a crystalline-amorphous double network PVA hydrogel, characterized in that it comprises the following steps: (1) Add sulfuric acid and triaminophenylboric acid to a 5%-15% polyvinyl alcohol aqueous solution and stir evenly; (2) The mixed solution is sealed and stored at 0-10°C for more than 12 hours to achieve full cross-linking; (3) The mixed solution obtained in step (2) is subjected to a freeze-thaw cycle treatment, and then the gel is immersed in dilute sulfuric acid for more than 24 hours. Finally, the gel is immersed in deionized water with a volume 10 times that of the gel for 24 hours to remove the residual sulfuric acid and unreacted monomers in the gel, thereby obtaining a crystalline-amorphous double network PVA gel.

[0011] Preferably, in step (1), the mass ratio of polyvinyl alcohol solution, sulfuric acid and triaminophenylboric acid is: polyvinyl alcohol solution, sulfuric acid and triaminophenylboric acid = 20: (2-40): (1-2).

[0012] Preferably, in the freeze-thaw cycle treatment in step (3), the freezing temperature is -20°C, the freezing time is 16 hours, the natural thawing time is 4 hours, and the number of cycles is 1 to 5 times.

[0013] Preferably, in the sulfuric acid soaking treatment in step (3), the concentration of the dilute sulfuric acid used is 5wt%~30wt%, and the soaking time is 3~5 days.

[0014] Preferably, in the sulfuric acid soaking treatment in step (3), the concentration of the dilute sulfuric acid used is 5wt%~30wt%, and the soaking time is 3 days.

[0015] An application of a crystalline-amorphous double network PVA hydrogel, characterized in that the crystalline-amorphous double network PVA gel is used in ion conductive materials.

[0016] The beneficial effects of the invention are as follows: Compared with the prior art, the present invention has the following advantages: (1) The present invention utilizes a combination of chemical cross-linking, freeze-thaw cycles and acid leaching crystallization to achieve an increase in the crystallinity of polyvinyl alcohol gel while only slightly reducing the water content. (2) The double network structure of crystalline PVA and amorphous PVA proposed in the present invention increases the ultimate elongation of the gel from 233% to 498% and the ultimate fracture stress from 68kPa to 550kPa without reducing the electrolyte compatibility of the gel, thereby achieving a significant enhancement in mechanical properties. (3) The highly dense nanoscale pore amorphous PVA network inhibits the water loss of the gel in the salting-out salt, thereby increasing the ionic conductivity of the gel in the salting-out salt sodium sulfate by 408.1%. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The figure is a comparison chart of the crystallinity of the examples and comparative examples calculated by differential scanning calorimetry.

[0018] Figure 2 This is a schematic diagram of the dual network structure of Example 1.

[0019] Figure 3 This is an overall scanning electron microscope image of the crystalline network and the amorphous network of Example 1.

[0020] Figure 4 This is a scanning electron microscope image of the amorphous network of Example 1.

[0021] Figure 5 The following are Fourier infrared spectra of Example 1, Example 2 and Example 3 and polyvinyl alcohol molecules.

[0022] Figure 6 The water content comparison chart of Example 1, Example 1, Example 2 and Example 3 calculated by thermogravimetric method is shown in FIG.

[0023] Figure 7 Schematic diagram of ionic conductivity of Example 1 in various common electrolytes.

[0024] Figure 8 The figure is a comparison chart of the mechanical properties between comparative example 1 and example 1.

[0025] Fig. 9 The figure is a comparison chart of the water content and ion conductivity of Example 1 and Example 1 in a 14.2wt% salting-out salt sodium sulfate aqueous solution. DETAILED DESCRIPTION

[0026] The specific implementation modes of the present invention are described below in conjunction with embodiments to facilitate a better understanding of the present invention. Example

[0027] The method for preparing the crystalline-amorphous double network PVA hydrogel in this embodiment is characterized by comprising the following steps: (1) Mix 20 mL of 10 wt % polyvinyl alcohol solution with 8 mL of 35 wt % sulfuric acid solution; (2) adding 0.13 g of triaminophenylboronic acid to the mixed solution and stirring uniformly to obtain a mixed solution; (3) placing the mixture at 5°C for 12 h, then freezing it at -20°C for 16 h, and then leaving it at room temperature for 4 h to complete gelation; (4) The obtained gel was immersed in a 10 wt % sulfuric acid solution for 3 days, and then the gel was taken out and immersed in deionized water with a volume 10 times that of the gel for 24 hours to remove the residual sulfuric acid and unreacted monomers in the gel, thereby obtaining a sample. Example

[0028] The method for preparing the crystalline-amorphous double network PVA hydrogel in this embodiment is characterized by comprising the following steps: (1) Mix 20 mL of 10 wt % polyvinyl alcohol solution with 8 mL of 35 wt % sulfuric acid solution.

[0029] (2) Add 0.13 g of triaminophenylboric acid to the mixed solution and stir evenly to obtain a mixed solution.

[0030] (3) The mixture was placed at 5°C for 12 h, then frozen at -20°C for 16 h, and then allowed to stand at room temperature for 4 h to complete gelation.

[0031] (4) The obtained gel was immersed in a 10 wt % sulfuric acid solution for 4 days, and then the gel was taken out and immersed in deionized water with a volume 10 times that of the gel for 24 hours to remove the residual sulfuric acid and unreacted monomers in the gel, thereby obtaining a sample. Example

[0032] The method for preparing the crystalline-amorphous double network PVA hydrogel in this embodiment is characterized by comprising the following steps: (1) Mix 20 mL of 10 wt % polyvinyl alcohol solution with 8 mL of 35 wt % sulfuric acid solution.

[0033] (2) Add 0.2 g of triaminophenylboric acid to the mixed solution and stir evenly to obtain a mixed solution.

[0034] (3) The mixture was placed at 5°C for 12 h, then frozen at -20°C for 16 h, and then allowed to stand at room temperature for 4 h to complete gelation.

[0035] (4) The obtained gel was immersed in a 10 wt % sulfuric acid solution for 3 days, and then the gel was taken out and immersed in deionized water with a volume 10 times that of the gel for 24 hours to remove the residual sulfuric acid and unreacted monomers in the gel, thereby obtaining a sample.

[0036] Comparative Example 1 The preparation method of PVA hydrogel in this comparative example is as follows: 20 mL of 10 wt % polyvinyl alcohol solution is poured into a mold, the mold is frozen at -20°C for 16 h, and then thawed at room temperature for 4 h. After repeating the above process three times, the polyvinyl alcohol solution completes gelation.

[0037] Comparative Example 2 The preparation method of PVA hydrogel in this comparative example comprises the following steps: (1) Mix 20 mL of 10 wt % polyvinyl alcohol solution with 8 mL of 35 wt % sulfuric acid solution.

[0038] (2) The mixture was placed at 5°C for 12 h, then frozen at -20°C for 16 h, and then allowed to stand at room temperature for 4 h to complete gelation.

[0039] (3) The obtained gel was immersed in a 10 wt % sulfuric acid solution for 3 days, and then the gel was taken out and immersed in deionized water with a volume 10 times that of the gel for 24 hours to remove the residual sulfuric acid and unreacted monomers in the gel, thereby obtaining a sample.

[0040] Comparative Example 3 The preparation method of PVA hydrogel in this comparative example comprises the following steps: (1) 20 mL of 10 wt % polyvinyl alcohol solution and 8 mL of 35 wt % sulfuric acid solution were mixed. 0.13 g of triaminophenylboric acid was added to the mixed solution and stirred to obtain a mixed solution.

[0041] (2) The mixed solution was placed at 5°C for 12 hours to complete gelation.

[0042] (3) The obtained gel was immersed in a 10 wt % sulfuric acid solution for 3 days, and then the gel was taken out and immersed in deionized water with a volume 10 times that of the gel for 24 hours to remove the residual sulfuric acid and unreacted monomers in the gel, thereby obtaining a sample.

[0043] Comparative Example 4 The preparation method of PVA hydrogel in this comparative example comprises the following steps: (1) Mix 20 mL of 10 wt % polyvinyl alcohol solution with 8 mL of 35 wt % sulfuric acid solution.

[0044] (2) Add 0.13 g of triaminophenylboric acid to the mixed solution and stir evenly to obtain a mixed solution.

[0045] (3) The mixture was placed at 5°C for 12 h, then frozen at -20°C for 16 h, and then allowed to stand at room temperature for 4 h to complete gelation.

[0046] (4) The gel was immersed in deionized water with a volume 10 times that of the gel for 24 h to remove the residual sulfuric acid and unreacted monomers in the gel to obtain a sample.

[0047] Figure 1 The crystallinity of Examples 1 to 4 is compared with that of Examples 1 to 3. Compared with the crystallinity of only 6% of the PVA gel (Comparative Example 1) obtained by the most widely used traditional freeze-thaw method, the crystallinity of the embodiment is increased to more than 35%, proving that the combination of chemical crosslinking, freeze-thaw cycle and acid leaching crystallization achieves a higher crystallinity, and the crystallinity of the gel can be regulated by regulating the acid leaching time, regulating the amount of crosslinking agent added, etc.; as the acid leaching time increases, the overall crystallinity gradually decreases, which corresponds to the generation of an amorphous network under the re-crosslinking action of sulfuric acid. The preparation processes of Comparative Example 2, Comparative Example 3 and Comparative Example 4 respectively remove the chemical crosslinking step, ice crystal extrusion step and acid leaching crystallization step in the crystallization preparation process. The crystallinity of the above three comparative examples is 3%, 3% and 13% respectively, proving that in the crystallization process, the three steps of chemical crosslinking, ice crystal extrusion and acid leaching crystallization are indispensable.

[0048] Figure 2 Schematic diagram of the dual network structure of Example 1. The main network of Example 1 is crystallized with a larger pore size, and the secondary network is partially filled in the pores of the crystallized network with a smaller pore size.

[0049] Figure 3 This is the overall scanning electron microscope image of the crystalline network and the amorphous network in Example 1, showing that a large number of dense micro-network structures are filled in the macroporous network channels, the small pores and macroporous networks correspond to the amorphous network and the crystalline network respectively, and the pore size of the crystalline network is mainly distributed between 300nm and 500nm. The crystalline network plays a very important role in improving the mechanical strength of the gel.

[0050] Figure 4 This is a scanning electron microscope image of the secondary amorphous network of Example 1. The amorphous network presents a very dense pore structure with a pore size of about 50nm to 200nm. The dense amorphous network plays an important role in enhancing the ductility of the gel and resisting salting out and water loss.

[0051] Figure 5The Fourier infrared spectra of Examples 1 to 3. The infrared diffraction peaks of the examples are completely consistent with those of pure PVA, and some diffraction peaks are slightly offset, which proves that the examples are completely composed of PVA without any additional chemical components. The absence of additional chemical components makes the examples have the electrolyte compatibility that is completely consistent with that of traditional PVA gel.

[0052] Figure 6 The water content of Example 1 is compared with that of Examples 1 to 3. The water content of Example 1 is 86.7%, the water content of Example 2 is 88.1%, and the water content of Example 3 is 89.1%. Compared with the 90% water content of the most widely used standard PVA gel (Comparative Example 1), the water content of the examples is only slightly reduced, which proves that the method of combining chemical crosslinking, freeze-thaw cycle and acid leaching crystallization can achieve the improvement of crystallinity under the condition of only slightly reducing the water content of the gel, ensuring that the ion transmission efficiency of the examples is not affected by the reduction of water content caused by crystallization.

[0053] Figure 7 The ionic conductivity of Example 1 under various common electrolyte environments is shown. Based on the structural characteristics of polyvinyl alcohol, Example 1 can achieve stable compatibility and application with all common aqueous electrolytes, and can also be compatible with some organic electrolytes and ionic liquid electrolytes.

[0054] Figure 8 To compare the mechanical properties of Example 1 with Example 2. The crystalline network is very beneficial to the improvement of gel strength, while the amorphous network is very beneficial to the improvement of gel ductility. Therefore, the dual network combined with the two increases the ultimate elongation of the gel from 233% to 498%, and the ultimate fracture stress from 68kPa to 550kPa.

[0055] Fig. 9 To compare the water content and ion conductivity of Example 1 and Example 1 in the salting-out sodium sulfate aqueous solution, the concentration of the sodium sulfate solution is 14.2wt%. Example 1 can maintain a high water content of 52.28% in sodium sulfate, which is about 154.2% higher than 20.57% of Comparative Example 1. Corresponding to the increase in water content, the ion conductivity of Example 1 in sodium sulfate is 33.84 mS / cm, which is 408.1% higher than 6.66 mS / cm of Comparative Example 1. Sodium sulfate has a strong salting-out effect, which can dehydrate the gel by hydrating water molecules, and the dense amorphous pores of Example 1 can restrain water from dehydration and thus ensure ion transmission efficiency.

[0056] The above is a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the principle of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A crystalline-amorphous double network PVA hydrogel, characterized in that: The double network of the hydrogel is composed of crystalline polyvinyl alcohol and amorphous polyvinyl alcohol.

2. The crystalline-amorphous double network PVA hydrogel according to claim 1, characterized in that: The crystallinity of the crystalline polyvinyl alcohol is 30% to 40%.

3. The crystalline-amorphous double network PVA hydrogel according to claim 1, characterized in that: The double network consists of a macroporous crystalline PVA network and an amorphous PVA network grown inside the pores of the crystalline PVA.

4. The crystalline-amorphous double network PVA hydrogel according to claim 1, characterized in that: The pore size of the amorphous polyvinyl alcohol is 50-200 nm.

5. A method for preparing a crystalline-amorphous double network PVA hydrogel, characterized in that: The following steps are involved: (1) Add sulfuric acid and triaminophenylboric acid to a 5%-15% polyvinyl alcohol aqueous solution and stir evenly; (2) The mixed solution is sealed and stored at 0-10°C for more than 12 hours to achieve full cross-linking; (3) The mixed solution obtained in step (2) is subjected to a freeze-thaw cycle treatment, and then the gel is immersed in dilute sulfuric acid for more than 24 hours. Finally, the gel is immersed in deionized water with a volume 10 times that of the gel for 24 hours to remove the residual sulfuric acid and unreacted monomers in the gel, thereby obtaining a crystalline-amorphous double network PVA gel.

6. The method for preparing the crystalline-amorphous double network PVA hydrogel according to claim 4, characterized in that: In the step (1), the mass ratio of polyvinyl alcohol, sulfuric acid and triaminophenylboric acid is: polyvinyl alcohol solution, sulfuric acid, triaminophenylboric acid = 20: (2-40): (1-2).

7. The method for preparing the crystalline-amorphous double network PVA hydrogel according to claim 4, characterized in that: In the freeze-thaw cycle treatment in step (3), the freezing temperature is -20°C, the freezing time is 16 hours, the natural thawing time is 4 hours, and the number of cycles is 1 to 5 times.

8. The method for preparing the crystalline-amorphous double network PVA hydrogel according to claim 4, characterized in that: In the sulfuric acid soaking treatment in step (3), the concentration of the dilute sulfuric acid used is 5wt%~30wt%, and the soaking time is 3~5 days.

9. The method for preparing the crystalline-amorphous double network PVA hydrogel according to claim 7, characterized in that: In the sulfuric acid soaking treatment in step (3), the concentration of the dilute sulfuric acid used is 5wt%~30wt%, and the soaking time is 3 days.

10. An application of a crystalline-amorphous double network PVA hydrogel, characterized in that: Application of the crystalline-amorphous double network PVA gel in ion conductive materials.

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