A high-performance composite hydrogel of starch / ionic liquid / polyvinyl alcohol, its preparation method and application

Abstract

The present invention relates to a high-performance composite hydrogel of starch / ionic liquid / polyvinyl alcohol, its preparation method and application. An ionic liquid is compounded with ethylene glycol and water as a ternary eutectic solvent system, and polyvinyl alcohol and starch are used as the composite skeleton of the hydrogel, and a high-performance composite hydrogel is obtained through physical cross-linking by freeze-thaw cycles. In the present invention, polyvinyl alcohol and starch are compounded as the supporting materials of the double-network hydrogel, and the ternary eutectic solvent system is used to improve the internal material compatibility of the double-network hydrogel, regulate the force-electricity performance of the hydrogel, endow the hydrogel with additional properties such as high moisture retention and anti-freezing, expand its application range, and form flexible electronic devices such as sensors for human motion health monitoring.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and particularly relates to a high-performance composite hydrogel of starch / ionic liquid / polyvinyl alcohol, and a preparation method and application thereof. Background Art

[0002] In recent years, the concept of portable monitoring of sports health has become increasingly popular among people. The concept of smart terminals can be seen everywhere in people's daily lives. Flexible and intelligent wearable devices have received extensive attention and research from scholars due to their characteristics such as being miniature, soft, and wearable. Among them, flexible piezoresistive strain sensors are a type of sensor device that converts external signals (such as physiological / exercise signals) into electrical signals for output, and have potential applications in the fields of human motion health monitoring, soft robots, etc.

[0003] Hydrogels are formed by physical cross-linking or chemical cross-linking of small solvent molecules and polymer molecular networks, etc., and are a type of three-dimensional network structure material with extremely strong hydrophilicity. They simultaneously possess the characteristics of solids and liquids, and their properties are similar to those of biological soft tissues, having good flexibility and biocompatibility. Since their appearance in 1894, they have received the attention of researchers. Conductive hydrogels combine the soft characteristics of hydrogels and the electrochemical characteristics of conductive polymers. Due to the good flexibility, high stretchability, and biocompatibility of flexible hydrogels, they are considered to be very promising materials in flexible strain sensors.

[0004] Starch is a renewable polymer with a wide range of sources, low cost, and degradability. It is formed by alternating crystalline regions and amorphous regions. The intermolecular and intramolecular hydrogen bond interactions in starch itself are too strong to be soluble in traditional solvents such as water. Pure starch hydrogels not only have too strong hygroscopic swelling performance, resulting in poor mechanical properties, but also show brittle characteristics at low temperatures. Polyvinyl alcohol is a water-soluble polymer material that is easy to process and biodegradable, but has a high cost and a limited degradation rate. Although the composite hydrogel formed by starch and polyvinyl alcohol can improve the disadvantages of single components to a certain extent, its semi-crystalline structure limits the compatibility between the two, and the crystalline structure hinders the electrical conductivity. Although plasticizers such as glycerol and ethylene glycol in alcohol solvents have been proven to be helpful in improving the compatibility between starch and polyvinyl alcohol, the compatibility effect is still limited, and the formed hydrogel has poor electrical conductivity, greatly limiting its application range. Therefore, how to further effectively improve the compatibility of starch / polyvinyl alcohol hydrogels to improve their mechanical properties and electrical conductivity while still being able to make a precise linear response to a wide range of strain signals is the key issue for starch / polyvinyl alcohol hydrogel strain sensors. In addition, the problem of limited use of hydrogels in low-temperature environments needs to be solved. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel, its preparation method and application. This composite hydrogel has high sensing sensitivity characteristics in a wide strain range in flexible sensing applications.

[0006] The present invention provides a starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel, which uses an ionic liquid compounded with ethylene glycol and water as a ternary eutectic solvent system, uses polyvinyl alcohol and starch as the hydrogel composite skeleton, and is obtained by physical cross-linking through freeze-thaw cycles.

[0007] Preferably, the ionic liquid is 1-allyl-3-methylimidazolium chloride ([AMim]Cl). In the present invention, the ionic liquid not only increases the conductivity of the hydrogel system, but also improves the compatibility between polymers in the system by compounding with the mixed solvent of ethylene glycol and water, increases the cross-linking effect in the system, greatly improves the mechanical properties of the hydrogel and endows additional properties such as moisture retention and frost resistance.

[0008] Preferably, the starch is corn starch.

[0009] The present invention also provides a preparation method of a starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel, which includes the following steps:

[0010] (1) Prepare an aqueous starch solution with a mass concentration of 1-10 wt%, and perform preliminary pregelatinization under the condition of oil bath heating at 80-90 °C;

[0011] (2) Add an ionic liquid and ethylene glycol to the aqueous starch solution, and stir and mix at room temperature for 5-15 min;

[0012] (3) Add polyvinyl alcohol PVA particles to the above solution according to a mass concentration of 10-15 wt%, stir and swell at room temperature for 20-40 min, and then heat and stir at 90-95 °C for 2-3 h to fully dissolve and mix the materials to obtain a hydrogel precursor solution;

[0013] (4) Pour the hydrogel precursor solution into a mold, perform freeze-thaw cycles in a refrigerator, and finally obtain a starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel.

[0014] The mass ratio of the ionic liquid to ethylene glycol in step (2) is 1:9-9:1.

[0015] The mass ratio of the starch to (ionic liquid and ethylene glycol) in step (2) is 0.2:10-2:10.

[0016] The freeze-thaw cycle in step (4) is to perform multiple freeze-thaw cycles in environments of -20 °C and 20 °C.

[0017] In step (2), the stirring speed at room temperature is 300 r / min; in step (3), the stirring speed during heating is 400 r / min.

[0018] The present invention also provides an application of a starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel in a flexible wearable multifunctional sensor.

[0019] The present invention uses an ionic liquid compounded with ethylene glycol and water as a ternary eutectic solvent system. The imidazolium-based ionic liquid serves as a hydrogen bond acceptor, ethylene glycol serves as the first hydrogen bond donor, and deionized water acts as the second hydrogen bond donor while reducing the viscosity of the system, greatly improving the compatibility of two semi-crystalline polymers, polyvinyl alcohol and starch, enhancing the cross-linking effect in the hydrogel system, thereby improving the mechanical properties of the polyvinyl alcohol / starch hydrogel. The ionic liquid plays multiple roles in the hydrogel system. While endowing the hydrogel with excellent electrical conductivity, the compounded ternary eutectic solvent system enables the hydrogel to have additional properties such as low-temperature antifreeze and high moisture retention against drying. The prepared ionic hydrogel has high sensing sensitivity characteristics in a wide strain range in flexible sensing applications.

[0020] The double-network structure in the double-network hydrogel of the present invention refers to a starch macromolecular network and a PVA physical network: the first layer is the starch macromolecular network. However, the mechanical properties of the starch single-layer network hydrogel material are poor and difficult to meet the requirements of industrial production. By introducing the second layer of PVA physical network for hydrogen bond association and using the synergistic effect of multiple strong and weak hydrogen bonds to enhance the cross-linking mode of the gel network structure, a starch / polyvinyl alcohol double-network hydrogel is used as the matrix support material.

[0021] Beneficial effects

[0022] 1. The preparation method of the present invention is simple and environmentally friendly, green and pollution-free to the environment, and the material sources are abundant;

[0023] 2. The imidazolium-based ionic liquid compounded with ethylene glycol and water as a ternary eutectic solvent system in the hydrogel system prepared by the present invention enables starch and polyvinyl alcohol to have high compatibility (the crystallinity of the hydrogel system is as low as 9.89%), greatly improving the mechanical properties of the starch / polyvinyl alcohol-based hydrogel (the elongation at break is as high as 1250.29%, and the tensile strength increases to 1324.43 kPa).

[0024] 3. The introduction of the imidazolium-based ionic liquid in the present invention plays multiple roles. It not only enhances the mechanical properties of the starch / polyvinyl alcohol-based hydrogel, but also endows the hydrogel with excellent additional properties such as electrical conductivity, low-temperature antifreeze, and high moisture retention against swelling (the weight loss rate is about 17%) in cooperation with ethylene glycol, greatly broadening its application scope.

[0025] 4. The hydrogel prepared by the present invention has the sensing performance with an ultra-wide strain range, and can stably detect human joint movement signals within a wide strain range, having potential applications in human health monitoring. Description of the Drawings

[0026] Figure 1 The tensile curves of starch / polyvinyl alcohol hydrogels without and with ethylene glycol added.

[0027] Figure 2 The tensile curves of high-performance composite starch / ionic liquid / polyvinyl alcohol hydrogels with different mass ratios of ionic liquid / ethylene glycol.

[0028] Figure 3 The X-ray diffraction patterns and crystallinity of high-performance composite starch / ionic liquid / polyvinyl alcohol hydrogels.

[0029] Figure 4 The comparative chart of the crystallinity of high-performance composite starch / ionic liquid / polyvinyl alcohol hydrogels.

[0030] Figure 5 The water loss curve of high-performance composite starch / ionic liquid / polyvinyl alcohol hydrogels.

[0031] Figure 6 The freezing diagram of high-performance composite starch / ionic liquid / polyvinyl alcohol hydrogels.

[0032] Figure 7 The strain sensing curve of high-performance composite starch / ionic liquid / polyvinyl alcohol hydrogels.

[0033] Figure 8 The signal diagram of high-performance composite starch / ionic liquid / polyvinyl alcohol hydrogels for detecting human finger joint movement. Detailed Embodiments

[0034] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0035] Comparative Example 1

[0036] Weigh 0.25 g of corn starch and disperse it in 5 g of deionized water. Conduct preliminary pregelatinization under the condition of heating in an 80 °C oil bath. Then add 0.5 g of PVA particles, stir and swell at room temperature for 30 min. After swelling, heat and stir at 95 °C for 3 h to obtain a hydrogel precursor solution. Pour the mixed solution into a polytetrafluoroethylene mold, freeze it at -20 °C in the refrigerator for 16 h, thaw it at room temperature for 8 h, and cycle the freezing-thawing process three times to finally obtain a starch / polyvinyl alcohol (SP) hydrogel.

[0037] Comparative Example 2

[0038] Weigh 0.5275 g of starch and disperse it in 5 g of deionized water. Conduct preliminary pregelatinization under the condition of heating in an 80 °C oil bath. Then add 5.55 g of ethylene glycol, stir and mix at room temperature for 15 min, and then add 1.055 g of PVA particles. Stir and swell at room temperature for 30 min. After swelling, heat and stir at 95 °C for 3 h to obtain a hydrogel precursor solution. Pour the mixed solution into a polytetrafluoroethylene mold, freeze it at -20 °C in the refrigerator for 16 h, thaw it at room temperature for 8 h, and cycle the freezing-thawing process three times to finally obtain a starch / polyvinyl alcohol (SEP or SA0E 10 P) composite hydrogel.

[0039] Example 1

[0040] Weigh 0.5275 g of starch and disperse it in 5 g of deionized water. Conduct preliminary pregelatinization under the condition of heating in an 80 °C oil bath. Then add 0.555 g of ionic liquid and 4.995 g of ethylene glycol, stir and mix at room temperature for 15 min, and then add 1.055 g of PVA particles. Stir and swell at room temperature for 30 min. After swelling, heat and stir at 95 °C for 3 h to obtain a hydrogel precursor solution. Pour the mixed solution into a polytetrafluoroethylene mold, freeze it at -20 °C in the refrigerator for 16 h, thaw it at room temperature for 8 h, and cycle the freezing-thawing process three times to finally obtain a high-performance starch / ionic liquid / polyvinyl alcohol (SA1E9P) composite hydrogel.

[0041] Example 2

[0042] Weigh 0.5275 g of starch and disperse it in 5 g of deionized water. Conduct preliminary pregelatinization under the condition of heating in an 80 °C oil bath. Then add 1.11 g of ionic liquid and 4.44 g of ethylene glycol, stir and mix at room temperature for 15 min, and then add 1.055 g of PVA particles. Stir and swell at room temperature for 30 min. After swelling, heat and stir at 95 °C for 3 h to obtain a hydrogel precursor solution. Pour the mixed solution into a polytetrafluoroethylene mold, freeze it at -20 °C in the refrigerator for 16 h, thaw it at room temperature for 8 h, and cycle the freezing-thawing process three times to finally obtain a high-performance starch / ionic liquid / polyvinyl alcohol (SA2E8P / SAEP) composite hydrogel.

[0043] Example 3

[0044] Weigh 0.5275 g of starch and disperse it in 5 g of deionized water. Conduct preliminary pregelatinization under the condition of heating in an 80 °C oil bath. Then add 1.665 g of ionic liquid and 3.885 g of ethylene glycol. After stirring and mixing at room temperature for 15 min, add 1.055 g of PVA particles. Stir and swell at room temperature for 30 min. After swelling, heat and stir at 95 °C for 3 h to obtain a hydrogel precursor solution. Pour the mixed solution into a polytetrafluoroethylene mold, freeze it at -20 °C in the refrigerator for 16 h, thaw it at room temperature for 8 h, and cycle the freezing-thawing process three times to finally obtain a high-performance composite hydrogel of starch / ionic liquid / polyvinyl alcohol (SA3E7P).

[0045] Example 4

[0046] Weigh 0.5275 g of starch and disperse it in 5 g of deionized water. Conduct preliminary pregelatinization under the condition of heating in an 80 °C oil bath. Then add 2.22 g of ionic liquid and 3.33 g of ethylene glycol. After stirring and mixing at room temperature for 15 min, add 1.055 g of PVA particles. Stir and swell at room temperature for 30 min. After swelling, heat and stir at 95 °C for 3 h to obtain a hydrogel precursor solution. Pour the mixed solution into a polytetrafluoroethylene mold, freeze it at -20 °C in the refrigerator for 16 h, thaw it at room temperature for 8 h, and cycle the freezing-thawing process three times to finally obtain a high-performance composite hydrogel of starch / ionic liquid / polyvinyl alcohol (SA4E6P).

[0047] Example 5

[0048] Weigh 0.5275 g of starch and disperse it in 5 g of deionized water. Conduct preliminary pregelatinization under the condition of heating in an 80 °C oil bath. Then add 2.775 g of ionic liquid and 2.775 g of ethylene glycol. After stirring and mixing at room temperature for 15 min, add 1.055 g of PVA particles. Stir and swell at room temperature for 30 min. After swelling, heat and stir at 95 °C for 3 h to obtain a hydrogel precursor solution. Pour the mixed solution into a polytetrafluoroethylene mold, freeze it at -20 °C in the refrigerator for 16 h, thaw it at room temperature for 8 h, and cycle the freezing-thawing process three times to finally obtain a high-performance composite hydrogel of starch / ionic liquid / polyvinyl alcohol (SA5E5P).

[0049] Performance analysis of the high-performance composite hydrogel of starch / ionic liquid / polyvinyl alcohol:

[0050] (1) Fix the standard dumbbell-shaped hydrogel sample on a universal tensile testing machine, control the tensile rate at 100 mm / min, and detect its mechanical properties. As Figure 1Shown are the tensile stress-strain curves of the hydrogels of Comparative Example 1 and Comparative Example 2. The addition of ethylene glycol significantly enhances the mechanical properties of the hydrogel. The elongation at break and tensile strength of the SEP hydrogel increase from 411.13% and 289.18 kPa of the SP hydrogel to 613.24% and 654.63 kPa, respectively. This is mainly due to the plasticizer effect of ethylene glycol.

[0051] (2) Figure 2 The figure shows the tensile curves of starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogels with different ethylene glycol / ionic liquid contents. As the content of the ionic liquid increases, the elongation at break and tensile strength of the starch / ionic liquid / polyvinyl alcohol hydrogel also increase. This may be because the interaction between the systems is enhanced, and the crosslinking density and network structure of the hydrogel gradually reach the optimum. The hydrogel of Example 2 has excellent mechanical properties, with an elongation at break as high as 1250.29% and a tensile strength also increasing to 1324.43 kPa. However, as the ionic liquid is further increased, the mechanical properties of the starch / ionic liquid / polyvinyl alcohol hydrogel show a downward trend, which may be due to excessive crosslinking in the hydrogel system.

[0052] (3)X-ray diffraction was used to study the crystallinity of the starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel. As Figure 3 shown, it can be found that ethylene glycol can, to a certain extent, destroy the semi-crystalline structure of polyvinyl alcohol and the inherent A-type crystal structure of starch. After adding the ionic liquid, the crystalline structure of starch / polyvinyl alcohol is further significantly destroyed, showing weaker diffraction peaks and a diffuse peak shape. As Figure 4 shown, starch and polyvinyl alcohol themselves have a relatively high crystallinity. Since both are semi-crystalline substances, their compatibility is limited. The addition of ethylene glycol reduces the crystallinity of the system, while the ionic liquid further destroys the crystalline structure of starch and polyvinyl alcohol, increasing their compatibility. The crystallinity of the starch / ionic liquid / polyvinyl alcohol hydrogel system is as low as 9.89%. This may be because the imidazolium ions form new crosslinking interactions with the molecular chains of starch and polyvinyl alcohol, inhibiting the retrogradation of starch and the crystallization of starch / polyvinyl alcohol.

[0053] (4) The starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel in Example 2 and the starch / polyvinyl alcohol hydrogel in Comparative Example 1 without ionic liquid / ethylene glycol solvent were blotted with filter paper to remove the residual moisture on the surface and then weighed to obtain the initial mass W0. They were placed under constant temperature and humidity conditions for 20 days, and weighed at the same time every day. The mass after water loss on the i-th day was Wi. The water loss rate of the hydrogel was (W0 - Wi) / W0 × 100%. As Figure 5As shown, at the same storage time, the starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel of the ionic liquid / ethylene glycol / water ternary eutectic solvent loses less water and still retains about 83% of its weight after 20 days. In addition, the freezing temperatures of the hydrogels containing the ionic liquid / ethylene glycol / water ternary eutectic solvent and those without the ionic liquid / ethylene glycol solvent were measured by DSC. As Figure 6 shown, the freezing temperature of Example 2 can be as low as -128.9 °C, proving that it can still exhibit excellent antifreeze behavior at low temperatures.

[0054] (5) Fix the hydrogel in Example 2 on a universal tensile testing machine with a tensile speed of 100 mm / min. Use a copper sheet as the electrode and connect it to the CHI660E electrochemical workstation of Shanghai Chenhua. Record the resistance change of the hydrogel generated by the tensile strain, and calculate the sensitivity of the strain sensor GF = (R - R0 / R0) / ε (i.e., the curve slope). As Figure 7 shown, the starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel in Example 2 exhibits ultra-wide-range strain sensitivity. The strain sensitivities at 0-55%, 55%-250%, 250%-650%, and 650%-1000% are 0.99, 2.19, 2.49, and 3.28 respectively, and the resistance signal changes smoothly with high linearity.

[0055] (6) Apply the strain sensing patch assembled from the hydrogel obtained in Example 2 to the human finger joint for motion signal detection. Figure 8 As shown, it can be found that the relative resistance change rate shows obvious differences at different finger bending angles, indicating that the hydrogel can effectively serve as a flexible wearable strain sensor device for application in the field of sports health monitoring.

Claims

1. A starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel, characterized in that: It is obtained by using an ionic liquid to compound ethylene glycol and water as a ternary deep eutectic solvent system, with polyvinyl alcohol and starch as the hydrogel composite skeleton, and physically crosslinking through freeze-thaw cycles; wherein, the ionic liquid is 1-allyl-3-methylimidazolium chloride; the starch is corn starch.

2. A method for preparing the starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel as described in claim 1, comprising the following steps: (1) Prepare an aqueous starch solution with a mass concentration of 1-10 wt%, and conduct preliminary pregelatinization under the condition of oil bath heating at 80-90 °C; (2) Add an ionic liquid and ethylene glycol to the aqueous starch solution, and stir and mix at room temperature for 5-15 min; (3) Add polyvinyl alcohol PVA particles to the above solution at a mass concentration of 10-15 wt%, stir and swell at room temperature for 20-40 min, and then heat and stir at 90-95 °C for 2-3 h to fully dissolve and mix the materials to obtain a hydrogel precursor solution; (4) Pour the hydrogel precursor solution into a mold, conduct freeze-thaw cycles in a refrigerator, and finally obtain the starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel.

3. The preparation method according to claim 2, characterized in that: The mass ratio of the ionic liquid to ethylene glycol in step (2) is 1:9 - 9:

1.

4. The preparation method according to claim 2, characterized in that: The mass ratio of the starch to (ionic liquid and ethylene glycol) in step (2) is 0.2:10 - 2:

10.

5. The preparation method according to claim 2, characterized in that: The freeze-thaw cycle in step (4) is to conduct multiple freeze-thaw cycles in environments of -20 °C and 20 °C.

6. The preparation method according to claim 2, characterized in that: The stirring speed at room temperature in step (2) is 300 r / min; the heating and stirring speed in step (3) is 400 r / min.

7. An application of the starch / ionic liquid / polyvinyl alcohol high-performance composite hydrogel as described in claim 1 in a flexible wearable multifunctional sensor.

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