WO2023004930A1 - Zwitterionic anti-freezing organic hydrogel, preparation method therefor, and use thereof - Google Patents

Abstract

A zwitterionic anti-freezing organic hydrogel, a preparation method therefor, and the use thereof. A zwitterionic Poly-EG organic hydrogel is obtained by randomly copolymerizing SBMA and HEMA in a mixed solution of ethylene glycol (EG) and water. Due to the excellent anti-freezing property of the ethylene glycol solution, the hydrogel exhibits a good freezing resistance. In addition, after inorganic salt LiCl is introduced into the system, this not only imbues the hydrogel with electrical conductivity, but also further improves the anti-freezing property of the hydrogel. The interaction of organic molecules and the polymer network also improves the mechanical stability of the hydrogel, such that the hydrogel has good mechanical properties. In addition, due to the interaction between amphoteric groups and intramolecular groups, the hydrogel further exhibits a good adhesion and a good volatility resistance. With a good electrical conductivity, the organic hydrogel has certain application prospects in both strain sensors and supercapacitors, and due to the excellent anti-freezing property, such uses can also extend to below zero degrees centigrade.

Description

A zwitterionic antifreeze organic hydrogel and its preparation method and application technical field

The invention relates to the technical field of supercapacitors, in particular to a zwitterionic antifreeze organic hydrogel and a preparation method and application thereof.

Background technique

With the development of science and technology, people's demand for life and production tools is more and more extensive. Flexible electronics technology has attracted more and more attention because of its incomparable application advantages in the fields of electrodes, flexible energy storage devices, sensors, and wearable devices. The key point of preparing flexible electronic materials is to combine the excellent mechanical flexibility and electrical conductivity of materials. Hydrogels have excellent flexibility and tunable mechanical properties, and their dispersed water can serve as a medium for ion migration, which has great development potential in the intended integration.

Hydrogels with conductive properties need to be stable when used as flexible electronics, however, conventional hydrogels freeze at freezing point stability, which not only limits their ion-conducting ability, but also makes them rigid and fragile. thereby losing its mechanical properties. In addition, even at room temperature, water volatilization will inevitably occur in hydrogel materials. The loss of water will directly cause the hydrogel to dry and harden, which will seriously weaken its mechanical properties, such as flexibility and ductility, and limit the long-term use of hydrogel materials. Therefore, in order to realize practical applications, flexible wearable electronic devices must be able to work under different climatic conditions.

Chinese patent document CN112768255A (application number 202011419388.0) discloses a LiCl-bonded poly(SBMA-HEA) antifreeze zwitterionic hydrogel electrolyte, in the presence of LiCl salt, through random copolymerization of zwitterionic monomers SBMA and HEA, In the whole system, the anion and cation groups on the zwitterionic chain are conducive to the dissociation of lithium metal salts and provide channels for ion migration. The high concentration of LiCl greatly reduces the freezing point of the hydrogel polymer, making the electrolyte have a good Antifreeze performance. However, this hydrogel is still a non-organic hydrogel, which fails to solve the problem of poor mechanical properties of organic hydrogels. Therefore, in order to broaden the application range of hydrogel electrolytes, further improvement of hydrogel electrolytes is still needed.

Contents of the invention

The purpose of the present invention is to overcome above-mentioned deficiencies in the prior art, provide a kind of zwitterionic antifreeze organic hydrogel and its preparation method, application, mix ethylene glycol with water as the solvent of organic hydrogel, make hydrogel It has good frost resistance, and the interaction between organic molecules and the polymer network also improves the mechanical stability of the hydrogel, so that the hydrogel has good mechanical properties; at the same time, the conductivity of the hydrogel is further enhanced by using the inorganic salt LiCl resistance, antifreeze.

To achieve the above object, the present invention adopts the following technical solutions:

The invention provides a method for preparing an organic hydrogel electrolyte. In a mixed solution of ethylene glycol (EG) and water, SBMA (methacryloyl ethyl sulfobetaine) and HEMA (hydrophilic monomer formazan) hydroxyethyl acrylate) copolymerization to obtain zwitterionic polySH-EG organic hydrogel electrolyte, the specific steps are as follows:

1) Dissolve SBMA and HEMA monomers in aqueous ethylene glycol solution, place the solution in an ice bath and stir, then add LiCl and mix well;

2) adding an initiator to the solution prepared in step 1), stirring evenly in an ice bath, and ultrasonically removing air bubbles to obtain a precursor solution;

3) inject the precursor solution prepared in step 2) into a mold, and polymerize in a sealed environment to obtain an organic hydrogel electrolyte. The organic hydrogel electrolyte is abbreviated as polySH-EGx-y, where x is the concentration of ethylene glycol, y is the molar concentration of LiCl, and x and y are both positive integers.

The zwitterionic antifreeze organic hydrogel electrolyte prepared by the above preparation method has good antifreeze performance due to the mixture of ethylene glycol and water as a solvent, and the introduction of inorganic salt LiCl in the hydrogel, the hydrogel not only has Conductivity, while further improving the antifreeze of the hydrogel; in addition, due to the interaction between amphiphilic groups and molecules, the hydrogel also exhibits good adhesion and anti-volatility.

Preferably, the temperature of the ice bath in step 1) is 0°C to 5°C.

Preferably, the concentration of the aqueous ethylene glycol solution in step 1) is 20%-60% by volume; more preferably, the concentration of the aqueous ethylene glycol solution is 30%-40% by volume.

Preferably, in step 1), the molar ratio of SBMA to HEMA is SBMA:HEMA=1:0.5˜1:4. In step 1), the total mass of SBMA and HEMA is the total mass of the monomers, and the total mass of the monomers is dropped into the aqueous ethylene glycol solution according to the ratio of mass concentration of 1g/2ml.

Preferably, in step 1), the solution is stirred in an ice bath for 0.2h-0.8h.

Preferably, in step 1), the dissolved LiCl added has a concentration of 1M-5M.

Preferably, in step 2), the amount of the initiator added is equivalent to 0.5wt%-2wt% of the total mass of the monomers. Initiators are persulfides or oxides. More preferably, the initiator is ammonium persulfate (APS).

Preferably, in the step 2), the ultrasonic time for ultrasonically removing air bubbles is 5 min-15 min.

Preferably, the polymerization condition in the sealed environment in the step 3) is to polymerize in a sealed environment at 35°C-40°C for 10h-15h.

The zwitterionic antifreeze organic hydrogel electrolyte prepared by the above method has a tensile stress range of 6.5 kPa to 23.0 kPa at 25° C. and 30% RH. The tensile strain range of the organic hydrogel electrolyte is 400%-840% at 25° C. and 30% RH.

The electrical conductivity of the organic hydrogel electrolyte at 25°C and 30% RH is 7.9mS·cm ^-1 to 42.0mS·cm ^-1 . (RH is relative humidity, Relative Humidity).

When the temperature is changed in the range of -20-25 °C, the organic hydrogel electrolyte shows different resistance changes, and its resistance at -20 °C is 9.8 times that at 25 °C. And when the temperature is kept constant, its resistance also remains stable. The temperature sensitivity of the organic hydrogel electrolyte makes it show different resistances at different temperatures, which can be used as a material for temperature sensors or for temperature measurement. The present invention also provides the application of the organic hydrogel electrolyte for temperature sensor or temperature measurement.

When the hydrogel electrolyte is stretched to 400% at room temperature, the resistance of the organic hydrogel electrolyte increases by 20 times compared with the unstretched state; when the stretching length remains constant, the resistance of the organic hydrogel electrolyte also increases. has remained constant. And when the stretching length is kept constant, the electrical resistance of the hydrogel also remains stable. When stretching 100%, the resistance change rate at 25°C is 0.254, the resistance change rate at -10°C is 0.100, and the resistance change rate at -25°C is 0.065, indicating that as the temperature decreases, the hydrogel electrolyte is The sensitivity to stretching at low temperatures is also reduced.

The organic hydrogel electrolyte has good strain recovery performance, and the dissipated energy of the hydrogel can still be maintained at a high level after 15-30 stretching cycles. The organic hydrogel electrolyte also exhibited excellent adhesion, as shown in Figure 5(a), when cotton cloth was used as the substrate, the hydrogel exhibited a high adhesion of 500N·m ^-1 , when using The hydrogel also exhibited a high adhesion of 200N·m ^-1 to aluminum flakes. Even for PTFE with low surface energy, the hydrogel still exhibits an adhesion of 20N·m ^-1 . Figure 5(b) demonstrates the adhesion of polySH-EG40-4M hydrogels to different solid objects.

The present invention also provides the application of the above-mentioned hydrogel electrolyte for human motion detection, strain response elements or in supercapacitors.

A supercapacitor is characterized in that the supercapacitor is an electric double layer capacitor, and an organic hydrogel electrolyte is sandwiched between two AC electrodes to form a sandwich structure.

The assembly steps of the supercapacitor are as follows:

1) Preparation of activated carbon (AC) electrode:

Disperse activated carbon, conductive carbon black and PVDF in NMP at a mass ratio of 8:1:1 to prepare a uniform dispersion slurry, coat the slurry on carbon cloth and dry it in a vacuum oven at 180°C for 24 hours. After drying, an AC electrode is obtained;

2) Assemble the supercapacitor:

Take two pieces of AC electrodes with the same loading area, cover them with the organic hydrogel electrolyte on both sides to form a sandwich structure to prepare a supercapacitor; then add 1 drop to 4 drops of the hydrogel electrolyte to the electrodes on both sides of the supercapacitor before The bulk solution was used to wet the electrodes; the total thickness of the prepared supercapacitor was 1 mm–1.5 mm, and the thickness of the organic hydrogel electrolyte was half of the thickness of the supercapacitor.

The internal resistance of the supercapacitor is 8.2Ω at 25°C, ^31.1Ω at -20°C, and 6.6Ω at 60°C; The specific capacity is 49.6F·g ^-1 , the mass specific capacity at 60°C is 53.6F·g ^-1 , and the mass specific capacity at -20°C is 24.2F·g ^-1 .

A stress-strain recording device comprising an organic hydrogel electrolyte.

The beneficial effects of the present invention are:

The invention overcomes the defect of poor electrical conductivity of the organic hydrogel, and the interaction between the organic molecule and the polymer network also improves the mechanical stability of the hydrogel, so that the hydrogel has good mechanical properties. Due to the formation of stable molecular clusters between ethylene glycol and water molecules, which compete with the hydrogen bonds in water, the saturated vapor pressure of the solvent in the entire system decreases, and the amphoteric groups in the polymer structure also have a certain hydration ability, making a part of water become "Structural water", at the same time, as a strong water and functional salt, LiCl interacts with water to prevent the volatilization of water molecules, making the hydrogel have excellent water retention.

The present invention uses ethylene glycol aqueous solution as a solvent, and uses zwitterionic monomer methacryloyl ethyl sulfobetaine (SBMA) and hydrophilic monomer hydroxyethyl methacrylate (HEMA), one pot random Copolymerization to prepare organohydrogels, and the one-pot preparation provides convenience for the large-scale preparation of organohydrogels. In the whole system, the anionic and cationic groups on the zwitterionic chain facilitate the dissociation of metal lithium salts and provide channels for ion migration.

The organic hydrogel electrolyte provided by the present invention has a stress of 6.5kPa to 23.0kPa and a strain of 400% to 840% at room temperature (see Figure 1a), and the conductivity of the organic hydrogel electrolyte at room temperature is 7.9mS·cm ^-1 ~42.0 mS·cm ^-1 (see Figure 1b). When the organic hydrogel electrolyte is stretched to 400%, the resistance increases by nearly 20 times compared with the original length state (see Figure 6). Organic hydrogel has high sensitivity to temperature and tensile rate, and the test values under the same conditions are basically the same, so it is used in the test element.

When the concentration of ethylene glycol in the organic hydrogel is 40%, and the concentration of LiCl is 4M, the organic hydrogel not only has excellent electrical conductivity (7.9mS·cm ^-1 ~42.0mS·cm ^-1 ), stretchability (The tensile stress range is 6.5kPa～23.0kPa, the tensile strain range is 400%～840%) and fatigue resistance (after completing 30 cycles, it is found that the dissipation energy of the hydrogel can still be maintained at a high level ), also has excellent adhesion (when applied to low surface energy PTFE, the hydrogel still exhibits an adhesion of 20N·m ^-1 ), these properties enable organohydrogels to be used to record stress-strain Behavior, that is, motion detection applicable to the human body.

Description of drawings

Figure 1: (a) Comparison of mechanical properties of polySH hydrogels with different EG contents; (b) Conductivity of hydrogels with different EG contents under the addition of 2M LiCl;

Figure 2: (a) Physical photos of polySH hydrogels with different EG contents at different temperatures; (b) Water retention rate of polySH hydrogels with different EG contents at room temperature;

Figure 3: (a) Electrical conductivity of polySH-EG40 hydrogels with different LiCl contents; (b) Mechanical stretching of polySH-EG40 hydrogels with different LiCl contents;

Figure 4: 30 stretching cycles of polySH-EG40-4M hydrogel;

Figure 5: (a) Adhesion of polySH-EG40-4M hydrogel on different substrates; (b) Physical photos of polySH-EG40-4M hydrogel adhered to different substrates;

Figure 6: Electrical resistance response of polySH-EG40-4M hydrogel stretched at different ratios;

Figure 7: Electrical resistance strain map of polySH-EG40-4M hydrogel recording elbow joint motion:;

Figure 8: Electrical resistance strain map of polySH-EG40-4M hydrogel recording knuckle movement:

Figure 9: Electrical resistance strain map of polySH-EG40-4M hydrogel recording throat activity:

Figure 10: (a) photo of polySH-EG40-4M hydrogel connected to LED bulbs as a conductor at different temperatures; (b) resistance change of polySH-EG40-4M hydrogel at different temperatures;

Figure 11: Resistance changes of polySH-EG40-4M hydrogels subjected to 100% stretching cycles at different temperatures;

Figure 12: CV curve of polySH-EG40-4M hydrogel-based supercapacitor at 25 °C;

Figure 13: CV curve of polySH-EG40-4M hydrogel-based supercapacitor at 60 °C;

Figure 14: CV curve of polySH-EG40-4M hydrogel-based supercapacitor at -20°C;

Figure 15: CV curves of polySH-EG40-4M hydrogel-based supercapacitor at different temperatures (50mV s ^-1 scan rate):

Figure 16: GCD curves of polySH-EG40-4M hydrogel-based supercapacitors at different temperatures (0.2A g ^-1 current density):

Figure 17: Impedance EIS curves of polySH-EG40-4M hydrogel-based supercapacitors at different temperatures:

Figure 18: Capacity retention of polySH-EG40-4M hydrogel-based supercapacitors at different temperatures and different current densities.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, but the present invention is not limited thereto.

Lithium chloride (LiCl), ethyl methacryloylsulfobetaine (SBMA), ammonium persulfate (APS), and hydroxyethyl methacrylate (HEMA) were purchased from Aladdin Reagent Co., Ltd. Polyvinylidene fluoride (PVDF) and N-methylpyrrolidone (NMP) were purchased from McLean Reagent Co., Ltd. Ethylene glycol (EG) was purchased from Sinopharm Group. Carbon cloth was purchased from Taiwan Carbon Energy Co., Ltd. Activated carbon was purchased from Kuraray Corporation, Japan. Carbon black was purchased from Alfa Aisha.

Glossary:

SBMA: ethyl methacryloyl sulfobetaine; HEMA: hydroxyethyl methacrylate; EG: ethylene glycol; polySH-EG: poly(SBMA-HEMA) electrolyte in aqueous ethylene glycol solvent; PVDF: Polyvinylidene fluoride; NMP: N-methylpyrrolidone; AC: activated carbon; AIBA: azobisisobutylamidine hydrochloride; PVDF: polyvinylidene fluoride; APS: ammonium persulfate.

Electrochemical test

Ionic conductivity was measured by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation (CHI 660E). First, the polySH-EG hydrogel was loaded into the CR927 battery case, and then the electrolyte was stabilized at different temperatures for 5 h, and then EIS test was performed. Each sample was measured three times to minimize errors. The ionic conductivity (σ,mS·cm ^-1 ) is obtained by the following formula:

Among them, R is the resistance (Ω), S is the contact area (cm ² ) between the electrolyte and the battery case, and L is the thickness (cm) of the tested hydrogel battery case.

The electrochemical properties of supercapacitors, such as cyclic voltammetry curve (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge curve (GCD), were measured on a CHI 660E electrochemical workstation with a two-electrode system. The supercapacitors were placed at different temperatures for 5 h before the electrochemical test. The mass specific capacity Csp(F·g ^-1 ) of a monolithic electrode is calculated by GCD discharge time, and the calculation formula is:

Where I is the applied current (mA), Δt is the discharge time (s), m _device is the total mass of the capacitor electrode (mg), and ΔV represents the discharge voltage (V).

Mechanical property test

The mechanical test was carried out with universal mechanical testing equipment (Jinan Hengsi Shengda Instrument Co., Ltd.). The tensile sample is a cylinder with a diameter of 5 mm and a length of 40 mm, and the strain rate is 50 mm·min ^-1 . The tensile cycle test is to stretch the sample to 400% strain at a speed of 50mm·min ^-1 , and then continue for 30 times, and the sample recovers for 5min after each stretch.

T-peel experiments were performed on a universal mechanical testing machine. First, a hydrogel film with a thickness of about 1.2 mm was sandwiched between two different substrates, so that the hydrogel and the substrate were in complete contact. When stretching, one side of the peeled sample was fixed on the fixed chuck of the stretching machine, and the other side was stretched and peeled on the movable chuck at a tensile rate of 20 mm·min ^-1 .

Strain sensing performance testing of polySH-EG hydrogels

The strain-sensing response of the polySH-EG hydrogel was tested using the ohm-time mode of the Universal Digital Source Meter (Tektronix, hereinafter referred to as the Universal Source Meter). First, connect the hydrogel into the test circuit of the instrument, and then measure the electrical resistance response of the hydrogel under different tensile strains. When measuring the strain at low temperature, the hydrogel was first stabilized in a low temperature environment (-30 °C) for 5 h, and then measured in a low temperature environment. For the signal detection of human activities, a hydrogel film is used, which is pasted on different parts and then the resistance response under different actions is recorded.

Example 1

Preparation of polySH-EG zwitterionic antifreeze organic hydrogel electrolyte

Zwitterionic polySH-EG organohydrogels were obtained by random copolymerization of SBMA and HEMA in a mixed solution of ethylene glycol and water. Firstly, SBMA and HEMA monomers (total mass is 2g, molar ratio is SBMA:HEMA=1:2) are dissolved in 4ml of ethylene glycol aqueous solution with a volume concentration of 40%, and the solution is placed in an ice bath and stirred for 0.5h After adding LiCl to make it dissolve, the molar concentration of LiCl obtained was 4 mol/L. After mixing evenly, 0.02 g of initiator APS (equivalent to 1 wt % of the total monomer mass) was added to the solution. After stirring evenly in an ice bath and ultrasonicating for 10 minutes to remove air bubbles, the precursor solution was obtained. Then the precursor solution was injected into the mold, sealed and placed in a 37°C environment for polymerization for 12 hours. The hydrogel obtained by polymerization is abbreviated as polySH- EG40-4, wherein 40 is the volume concentration of ethylene glycol, 40%, and 4 is the molar concentration of LiCl, 4mol/L.

Example 2

Others are the same as in Example 1, except that the concentration of LiCl in the organic hydrogel changes. Zwitterionic polySH-EG organohydrogels were obtained by random copolymerization of SBMA and HEMA in a mixed solution of ethylene glycol and water. Firstly, SBMA and HEMA monomers (total mass is 2g, molar ratio is SBMA:HEMA=1:2) are dissolved in 4ml of ethylene glycol aqueous solution with a volume concentration of 40%, and the solution is placed in an ice bath and stirred for 0.5h After adding LiC to dissolve it, the molar concentration of LiCl was (1, 2, 3, 5) mol/L. After mixing evenly, 0.02 g of initiator APS (equivalent to 1 wt% of the total monomer mass) was added to the solution. After stirring evenly in an ice bath and ultrasonicating for 10 minutes to remove air bubbles, the precursor solution was obtained. Then the precursor solution was injected into the mold, sealed and placed in a 37°C environment for polymerization for 12 hours. The hydrogel obtained by polymerization is abbreviated as polySH- EG40-y, where 40 is the volume concentration of ethylene glycol (40%), and y is the molar concentration of LiCl (1, 2, 3, 5) mol/L.

Different amounts of LiCl were dissolved into the solvent to prepare organic hydrogels with different LiCl concentrations (1, 2, 3, 5) mol/L.

Example 3

Others are the same as in Example 1, except that the concentration of ethylene glycol in the organic hydrogel changes. Zwitterionic polySH-EG organohydrogels were obtained by random copolymerization of SBMA and HEMA in a mixed solution of ethylene glycol and water. Firstly, SBMA and HEMA monomers (total mass is 2g, molar ratio is SBMA:HEMA=1:2) are dissolved in 4ml of ethylene glycol aqueous solution whose volume concentration is (0, 20, 30, 60)%. After stirring in an ice bath for 0.5 h, LiC was added to dissolve it to obtain a LiCl molar concentration of 4 mol/L. After mixing evenly, 0.02 g of initiator APS (equivalent to 1 wt % of the total monomer mass) was added to the solution. After stirring evenly in an ice bath and ultrasonicating for 10 minutes to remove air bubbles, the precursor solution was obtained. Then the precursor solution was injected into the mold, sealed and placed in a 37°C environment for polymerization for 12 hours. The hydrogel obtained by polymerization is abbreviated as polySH- EGx-4, wherein x is the volume concentration (0, 20, 30, 60)% of ethylene glycol, and 4 is the molar concentration (4mol/L) of LiCl.

Add different volume concentrations of ethylene glycol to the ethylene glycol aqueous solution to prepare organic hydrogels with different ethylene glycol concentrations (0, 20, 30, 60)%.

Example 4

The organic hydrogel electrolyte prepared in Example 1 was made into a 1.2 mm thick strip film, which was tightly pasted on the elbow joint, finger joint and throat of the arm respectively, and the resistance change was recorded with a multimeter.

Example 5

Assembly of organic hydrogel electrolyte-based supercapacitors

Preparation of activated carbon (AC) electrode: Activated carbon, conductive carbon black and PVDF (mass ratio 8:1:1) are dispersed in NMP to prepare a uniform dispersion slurry, and the slurry is coated on carbon cloth and placed in a vacuum at 180°C Dry in an oven for 24 hours to obtain an AC electrode;

Assembling the supercapacitor: Take two pieces of AC electrodes with the same loading area, and cover them on both sides of the organic hydrogel electrolyte to form a sandwich structure to prepare a supercapacitor. Subsequently, 2 drops of the organic hydrogel electrolyte precursor solution were dropped on the electrodes on both sides of the supercapacitor to wet the electrodes. The total thickness of the prepared supercapacitor is about 1.2 mm, and the thickness of the electrolyte is about 0.6 mm. The prepared capacitors were sealed before electrochemical tests to prevent moisture evaporation.

Result analysis

Because the interaction of organic molecules with the polymer network is slightly higher than that of water molecules, the introduction of organic molecules into the hydrogel system will not only affect the freezing of water, but also introduce more non-covalent interaction. These interactions, as sacrificial bonds, can effectively dissipate the external energy during deformation and improve the mechanical stability of hydrogels to a certain extent. Therefore, the mechanical properties of organohydrogels are slightly better than those of pure hydrogels. In the experimental preparation process, we did not use a cross-linking agent, but used the interaction between ethylene glycol and polar groups to regulate the mechanical properties of the hydrogel.

As shown in Fig. 1(a), the mechanical properties of hydrogels with different contents of EG were compared. Pure hydrogel (no EG and LiCl added, the following pure hydrogels are the same as the addition here) showed a relatively weak state, with a maximum stress of only 8kPa, while the stress of the hydrogel after adding EG A clear enhancement was shown, with a nearly three-fold increase in the stress of the hydrogel at a concentration of 30% compared to pure hydrogel. Due to the presence of a large number of -N ⁺ (CH ₃ ) ₂ and -SO ₃ ^- groups on the zwitterions, the HEMA structure also contains a large number of -OH groups. A large number of hydrogen bonds will be formed between these polar groups and ethylene glycol, and these hydrogen bonds will increase the rigidity of the hydrogel to a certain extent, so that the stress of the hydrogel will be enhanced. In general, a large increase in stress leads to a decrease in strain. However, after the introduction of organic molecules into the hydrogel, it was found that the stress of the hydrogel increased while the strain did not decrease sharply, and still maintained good mechanical properties. It is worth noting that when the EG concentration exceeds 40%, the EG continues to increase but weakens the mechanical properties of the hydrogel. It may be that the high EG content makes the hydrogel show brittleness, which makes the hydrogel fragile and easy to break. Therefore, after comparison, it was found that the optimal EG content was 30%-40%, and the hydrogel with 40% EG content showed excellent mechanical properties of 20kPa stress and 520% strain, so in the next experiment, 40% EG was selected. % concentration as EG addition.

As shown in Figure 1(b), the addition of EG will indeed enhance the mechanical properties of the hydrogel, but it will also have a greater impact on its electrical conductivity. It was found that the hydrogel had a high conductivity of 42 mS cm ^-1 without adding any EG, but the conductivity of the hydrogel decreased rapidly after adding EG. When the content is 20%, the conductivity is reduced to 60% of the pure hydrogel, and when the content is 60%, the conductivity is 7.9mS·cm ^-1 , which is only 19% of the pure hydrogel. This shows that the addition of EG will indeed reduce the conductivity of the hydrogel, and the higher the EG content, the more obvious the reduction effect. This may be due to the increase of EG content and the corresponding decrease of water content, and the transmission rate of conductive ions in ethylene glycol is much lower than that in water. In addition, the enhancement of the interaction in the system also hinders the movement rate of ions. All of these lead to a rapid decrease in conductivity.

The addition of ethylene glycol not only affects the mechanical properties and electrical conductivity of the hydrogel, but more intuitively is the tolerance of the hydrogel to extreme environments. As shown in Figure 2(a), at -20 °C, the pure hydrogel had completely frozen, while the EG-added hydrogel remained transparent and did not freeze. Hydrogels with different EG contents have different tolerances to low temperatures. 20% of the hydrogels freeze at -30°C, and 0%, 20%, and 40% when the temperature continues to decrease to -60°C , 60% of the hydrogel has been completely frozen. As a strongly hydrophilic molecule, EG produces hydrogen bond interactions after contact with water molecules, and these interactions will destroy the three-dimensional hydrogen bond structure between water molecules, thereby inhibiting the generation of ice lattices. Furthermore, the binding of these organic molecules to water reduces the amount of free water in solution. Therefore, the organic hydrogel exhibits excellent antifreeze performance, and can even endure the low temperature of -50°C when only EG is added (60% EG addition). High concentration of salt is also an effective strategy in the preparation of antifreeze hydrogels, and LiCl is also widely used in antifreeze hydrogels due to its good solubility. As a strong hydration ion, lithium salt can form ion clusters with water in water. These clusters also destroy the hydrogen bond between water molecules, reduce the content of free water, and achieve the effect of freezing point depression. Here we introduced 4M LiCl into the organic hydrogel, and found that its antifreeze performance was better than that of the anhydrous gel with the same EG content. Even better than the hydrogel with 60% EG content, it remained transparent when the organic hydrogel was completely frozen at -60 °C, which indicated that under the joint action of lithium salt and EG, the polySH-EG hydrogel exhibited more low freezing point.

Due to the formation of stable molecular clusters between ethylene glycol and water molecules, which compete with the hydrogen bonds in water, the saturation vapor pressure of the solvent in the entire system decreases. This enables the hydrogel to also exhibit excellent water retention. Place the hydrogel at room temperature (25°C, 30% humidity) to volatilize naturally, and then use W/W ₀ (W ₀ is the initial weight of the hydrogel, W is the weight at different times) to evaluate the hydrogel Water retention. As shown in Figure 2(b), for pure hydrogels, the water in the hydrogels has basically evaporated within 1 day after being placed, and after stabilization, the mass is only 42% of the initial value. However, this value is still higher than the theoretical weight retention after complete evaporation of water (33.3%). This is because the amphoteric groups in the polymer structure also have a certain hydration ability, so that part of the water becomes "structural water" and prevents volatilization. The addition of EG enhanced the water retention capacity of the hydrogel, and with the increase of EG content, the water retention capacity gradually increased. The hydrogel with 60% content still maintains more than 80% weight ratio. More importantly, the addition of LiCl also enhanced the water retention capacity of the hydrogel. The hydrogel with 40% EG content has only 64% weight retention rate, while the hydrogel with 40%-4M has a weight retention rate of 80%. LiCl, as a strong water and salt, prevents the interaction with water volatilization of water molecules. Therefore, under the influence of polymer structure, EG, LiCl, etc., the polySH-EG hydrogel exhibited good resistance to moisture volatilization.

In general, the conductivity of a solution has a certain relationship with the concentration of conductive ions. In order to obtain organic hydrogels with higher conductivity, the content of EG was fixed at 40%, and different concentrations of LiCl were introduced into the system to observe the effect of LiCl concentration on the conductivity of hydrogels. Due to the high solubility of LiCl, it can still be dissolved at 6M, but the hydrogel appears liquid at this time, so the LiCl concentration of the samples prepared in the experiment is up to 5M. As shown in Fig. 3(a), at 25 °C, the conductivity of the hydrogel increases with the increase of LiCl content. When the LiCl concentration is 5M, it can reach 25.5mS·cm ^-1 . When the LiCl concentration exceeds 3M, continuing to increase the salt content does not cause a sharp increase in conductivity, indicating that the conductivity is slowly approaching saturation at this time. After cooling down, the conductivity of the hydrogels decreased sharply, and at 0 °C, the conductivity of all hydrogels had been reduced by half. However, compared with low-salt-concentration hydrogels, the high-salt-concentration hydrogel still maintained a higher conductivity. At -30°C, since the hydrogel is not frozen at this time, even the hydrogel with a LiCl concentration of 1M still has a high conductivity of 0.83mS·cm ^-1 . At this time, the conductivity of the hydrogels with a LiCl concentration of 3-5M is not much different, being 1.71mS·cm ^-1 , 1.84mS·cm ^-1 , and 1.85mS·cm ^-1 . Low temperature reduces the transport rate of lithium ions, which leads to a decrease in electrical conductivity. The addition of high-concentration LiCl not only improves the antifreeze performance of the hydrogel, but also improves the conductivity of the hydrogel. However, the addition of LiCl also brought some negative effects to the hydrogel system. Due to the existence of amphoteric groups, in order to maintain the electrical neutrality in the whole system, the amphoteric groups attract each other electrostatically and become -N ⁺ (CH ₃ ) ₂ SO ₃ ^- . The mechanical properties of glue. After adding LiCl, the original electrostatic balance is broken, and the interaction between -N ⁺ (CH ₃ ) ₂ SO ₃ ^- becomes -N ⁺ (CH ₃ ) ₂ Cl ^- , -SO ₃ ^- Li ⁺ respectively, affecting mechanical properties of the hydrogel. As shown in Fig. 3(b), polySH-EG ₄₀ -0M exhibited the highest stress, and with the increase of salt concentration, the hydrogel tended to be soft, the stress decreased and the strain increased. The stress of polySH-EG ₄₀ -5M is only 6.5kPa, while the strain reaches 840%. Since LiCl breaks the original electrostatic interaction of the group, the toughness of the hydrogel is weakened. Due to the hydrogen bond formed between ethylene glycol and the hydrogel structure, the sacrificial bond can effectively dissipate the deformation energy during the strain process, so although the stress of the hydrogel is reduced, the strain has been improved to a certain extent.

Table 1 Comparison of experimental data of hydrogels with different contents of EG

the	PolySH- EG0	PolySH-EG 20	PolySH-EG 30	PolySH-EG 40	PolySH-EG 60
Tensile Stress(kPa)	8.0	18.0	23.0	20.0	16.0
Stretch Strain(%)	500	450	450	520	440
Conductivity at room temperature (mS cm -1 )	42.0	26.0	20.0	15.0	7.9
Antifreeze (℃)	0	-20	-40	-50	-60

Table 2 Comparison of experimental data of LiCl hydrogels with different concentrations under 40% EG content

the	EG 40-1M	EG 40 -2M	EG 40-3M	EG 40-4M	EG 40-5M
Tensile Stress(kPa)	15.0	12.5	11.7	8.5	6.5
Stretch Strain(%)	480	590	520	680	840
Conductivity at room temperature (mS cm -1 )	10.0	15.0	22.5	23.5	25.5

Based on the data in Table 1 and Table 2, after comparing the mechanical properties, electrical conductivity, antifreeze performance, etc., we choose polySH-EG ₄₀ -4M hydrogel (from then on, the following hydrogels are polySH-EG ₄₀ -4M Hydrogel) as a further experimental object. Stress relaxation rate is an important parameter for the application of hydrogel materials, and here we test the recovery of hydrogels in response to deformation by cyclic tensile tests. As shown in Figure 4, the polySH-EG ₄₀ -4M hydrogel was subjected to 30 stretching cycles, and the hydrogel was subjected to recovery treatment at room temperature for 5 minutes between each cycle. After 15 cycles, the hysteretic curve of the hydrogel decreased slightly, and the area of the hysteretic curve represented the dissipated energy of the hydrogel in the face of strain. After completing 30 cycles, it was found that the dissipated energy of the hydrogel can still be maintained at a high level, which indicates that the polySH-EG ₄₀ -4M hydrogel exhibits excellent strain recovery performance. The relationship between stretching rate and resistance of the organic hydrogel electrolyte of the present invention, as well as its strain recovery ability, make it applicable to various test devices, such as stress and strain recording devices. Moreover, we found that the organic hydrogel electrolyte of the present invention also has excellent adhesion, which allows the organic hydrogel electrolyte to be directly adhered to certain parts of the human body or specific parts of an object for detection stress strain.

In addition to good fatigue resistance, the polySH-EG ₄₀ -4M hydrogel also exhibited excellent adhesion. As shown in Figure 5(a), the hydrogel exhibited a high adhesion of 500N·m ^-1 when using cotton cloth as the substrate, and 200N·m ^- 1 when using an aluminum sheet. ¹ high adhesion. Even for PTFE with low surface energy, the hydrogel still exhibits an adhesion of 20N·m ^-1 . Figure 5(b) shows the adhesion of polySH-EG ₄₀ -4M hydrogel to different solid objects, no matter it is facing glass, stainless steel, PTFE or stone surface (the sequence is from upper left to lower right in the figure), the hydraulic coagulation All glues showed excellent adhesion. This excellent adhesion comes from the dipole-dipole interaction between groups such as -OH, -COOH, -NH ₂ on the surface of the substrate and the anion and cation groups on the one hand, and on the other hand from the interaction between these groups and Hydrogen bonding between ethylene glycol.

As shown in Fig. 6, due to the excellent electrical conductivity and good stretchability of polySH-EG ₄₀ -4M hydrogels, the hydrogels can be used as strain-responsive elements after combining these properties. Stretch the hydrogel at different ratios, and use a multimeter to record its resistance change. The hydrogel is stretched to 100%, 200%, 300%, and 400%, respectively. When the hydrogel is stretched, the transmission path of lithium ions becomes longer, which leads to an increase in resistance. Different stretching distances have different resistance changes, and when stretched to 400%, the resistance of the hydrogel increases by nearly 20 times. And when the stretching length is kept constant, the electrical resistance of the hydrogel also remains stable. This shows that as the polySH-EG ₄₀ -4M hydrogel is strained, its electrical resistance changes accordingly, a property that allows the hydrogel to be used to record stress-strain behavior.

Since polySH-EG ₄₀ -4M has good adhesion, it can be adhered to different parts of the human body to record the strain of human body movements. As shown in Figure 7, the hydrogel is made into a strip film and tightly attached to the elbow joint of the arm, and the resistance of the hydrogel is monitored in real time. With the flexion and extension of the arm, the hydrogel also deformation, resulting in a change in resistance. When flexing and stretching, the resistance change rate of the hydrogel changes accordingly. After repeated cycles, the electrical resistance of the hydrogel also returned to its initial value. As shown in Figure 8, the hydrogel is made into a strip film and applied closely to the knuckles. Unlike the flexion and extension of the elbow joint, the range of flexion and extension of the knuckle joint is small, so the resistance change of each flexion and extension is small. However, the rate of change of resistance can always remain stable during the flexion-extension cycle. More importantly, when the hydrogel is applied to the throat of the human body, the swallowing action of the person can be recorded. The same as the previous steps, the hydrogel was made into a strip film and then applied tightly to the throat, as shown in Figure 9, due to the slightness of the swallowing action, the resistance of the hydrogel changed slightly, but during the whole process The resistance change remains basically stable. This shows that polySH-EG ₄₀ -4M hydrogel can record various human actions and is suitable for human body action detection.

Since polySH-EG ₄₀ -4M hydrogel has good conductivity and antifreeze, it can be used as an ion conductor when connecting it to a circuit. As shown in Figure 10(a), connect the hydrogel to the circuit and use it to light a small light bulb, and the small light bulb emits dazzling light at room temperature. Since its resistance is greatly affected by temperature, the brightness of the small bulb begins to gradually decrease at low temperatures. It is worth noting that at -30°C, although the luminescence of the small bulb is weak, it can still light up normally, indicating that the hydrogel still has good conductivity at low temperatures. Taking advantage of the different conductivity of the hydrogel at different temperatures, it can be used as a temperature sensor. As shown in Figure 10(b), when the temperature is changed in the range of -20-25 °C, the hydrogel shows different resistance changes, see Table 3 (calculate the rate of change based on the resistance at 25 °C, that is, R ₀ , the calculation formula is: resistance change rate = (RR ₀ )/R ₀ ; R ₀ is the resistance at 25°C, and R is the resistance at the measured temperature). And when the temperature is kept constant, its resistance also remains stable.

Table 3 Resistance change rate of polySH-EG40-4M hydrogel at different temperatures

temperature(°C)	25°C	0°C	-10°C	-20°C
Resistance change rate	0	1.8	4.0	8.8

The polySH-EG ₄₀ -4M hydrogel also exhibited good stability when subjected to 100% stretching cycles. As shown in Figure 11, stretching cycles were carried out on the hydrogel at -20°C, -10°C and 25°C respectively, and 16 stretching cycles were carried out at each temperature, and the calculation formula was: resistance change rate = (RR ₀ )/R ₀ ; where R ₀ is the resistance when it is not stretched, and R is the resistance when it is stretched by 100%. It is found that the resistance of the hydrogel changes significantly at room temperature, but at low temperatures, the change rate of the hydrogel resistance decreases, and the lower the temperature, the smaller the resistance change rate during stretching, which is only 0.065 at -20°C. This suggests that as the temperature decreases, the sensitivity of the hydrogel to stretching at low temperatures also decreases. In conclusion, the hydrogel exhibits excellent performance both as an ion conductor and as a strain sensor, and more importantly, its good antifreeze property extends this application to sub-zero temperatures as well.

Table 4 Resistance changes of polySH-EG40-4M hydrogels subjected to 100% stretching cycles at different temperatures

temperature(°C)	25°C	-10°C	-20°C
Resistance change rate	0.254	0.100	0.065

Due to the good electrical conductivity of polySH-EG ₄₀ -4M hydrogel, it also has a promising application in supercapacitors (SC). As in Example 5, the hydrogel film was sandwiched between two AC electrodes to form a sandwich structure, and an electric double layer capacitor was prepared. Electrochemical tests were carried out at high and low temperatures to test its workability.

CV cycling was first performed on the supercapacitors to test the successful composition of the capacitors. As shown in Fig. 12-Fig. 14, regardless of the low scan rate of 50mV s ^-1 or the high scan rate of 1000mV s ^-1 , the shapes of the CV curves at the three temperatures all exhibit a rectangular shape. Indicates the ideal capacitive behavior of the fabricated supercapacitors. At 25°C and 60°C, the deformation of the CV curve is relatively slight, while at -20°C, the deformation is more obvious, indicating that low temperature has a greater impact on the performance of supercapacitors. At the same time, the CV cycle shows that the supercapacitor can work normally at different temperatures in the range of -20°C to 60°C.

As shown in Figure 15, due to the antifreeze property and certain water retention capacity of the hydrogel, the supercapacitor also has good performance at high temperature, so the CV curve maintains a good rectangular shape no matter at low temperature or high temperature. As the temperature decreases, the area of the CV curve decreases, indicating that the capacity of the capacitor is decreasing. As shown in Figure 16, the GCD curve of the capacitor also maintains a triangular shape. As the temperature decreases, the discharge time of the supercapacitor decreases and the voltage drop gradually increases. The deterioration of the electrochemical performance of the capacitor is inseparable from the decrease in the conductivity of the hydrogel electrolyte at low temperature. As shown in Fig. 17, the impedance curve in the low frequency region is approximately parallel to the vertical axis, indicating good ion diffusion ability in the capacitor system. At room temperature, the internal resistance of the capacitor is only 7.7Ω, and as the temperature decreases, the internal resistance gradually increases, and increases to 30Ω at -20°C. The increase in temperature makes the ion diffusion rate faster, so there is only an internal resistance of 6.6Ω at 60°C.

Table 5 Impedance of polySH-EG40-4M hydrogel-based supercapacitor at different temperatures

temperature(°C)	60	25	0	-10	-20
Internal resistance (Ω)	6.6	8.2	13.6	19.0	31.1

Thanks to the good adhesion of the polySH-EG ₄₀ -4M hydrogel, which enables close contact between the electrolyte and the electrodes, the supercapacitor exhibits a small charge transfer resistance. It is only 1.0Ω at 25°C and increases to 8.0Ω at -20°C. This may be due to the shrinkage of the hydrogel electrolyte at low temperatures, resulting in less close contact with the electrodes, which is also one of the reasons for the decline in the electrochemical performance of supercapacitors at low temperatures. According to the GCD discharge time, the mass specific capacity of the supercapacitor at different temperatures was calculated. As shown in Figure 18, at a current density of 0.2A·g ^-1 at 25°C, the specific capacity is 49.6F·g ^-1 , maintaining the same current density at 60°C as 53.6F·g ^-1 , at It was 24.2 F·g ^-1 at -20°C, 108% and 48.8% of that at 25°C, respectively. This shows that the effect of low temperature on the electrochemical performance of supercapacitors is more obvious, which is mainly due to the decrease in the conductivity of the hydrogel electrolyte at low temperature. Overall, the supercapacitors exhibited excellent electrochemical performance in the range of -20-60°C, indicating that the supercapacitors are suitable for working applications in a wide temperature range.

Claims (14)

1. A preparation method of an organic hydrogel electrolyte, the steps are as follows:

   1) Dissolve SBMA and HEMA monomers in aqueous ethylene glycol solution, place the solution in an ice bath and stir, then add LiCl and mix well;

   2) adding an initiator to the solution prepared in step 1), stirring evenly in an ice bath, and ultrasonically removing air bubbles to obtain a precursor solution;

   3) inject the precursor solution prepared in step 2) into a mold, and polymerize in a sealed environment to obtain an organic hydrogel electrolyte.

   Preferably, the temperature of the ice bath in step 1) is 0°C to 5°C.
2. The preparation method according to claim 1, wherein the concentration of the aqueous ethylene glycol solution in step 1) is 20%-60% by volume; more preferably, the aqueous ethylene glycol concentration is 30%-40% ,Volume ratio.
3. The preparation method according to claim 1, characterized in that, in step 1), the mol ratio of SBMA and HEMA is SBMA:HEMA=1:0.5～1:4; in step 1), the solution is stirred in an ice bath The time is 0.2h-0.8h; the concentration of the added LiCl after dissolution is 1M-5M.
4. The preparation method according to claim 1, characterized in that, in step 2), the amount of initiator added is equivalent to 0.5wt%-2wt% of the total mass of the monomer; the initiator is persulfide or oxide; said In step 2), the ultrasonic time for ultrasonically removing air bubbles is 5min-15min.
5. The preparation method according to claim 1, characterized in that, the polymerization condition in the sealed environment in the step 3) is to polymerize in a sealed environment at 35°C-40°C for 10h-15h.
6. The organic hydrogel electrolyte prepared by the method according to any one of claims 1-5.
7. The organic hydrogel electrolyte according to claim 6, characterized in that its tensile stress ranges from 6.5kPa to 23.0kPa at 25°C and 30%RH. The tensile strain range of the organic hydrogel electrolyte is 400%-840% at 25° C. and 30% RH.
8. The organic hydrogel electrolyte according to claim 6 or 7, which has an electrical conductivity of 7.9 mS·cm ^-1 to 42.0 mS·cm ^-1 at 25°C and 30% RH.
9. The organic hydrogel electrolyte as claimed in claim 6 or 7, when the hydrogel electrolyte is stretched to 400% at normal temperature, the resistance of the organic hydrogel electrolyte is increased by 20 times compared with the unstretched state; When stretching 100%, the resistance change rate at 25°C is 0.254, the resistance change rate at -10°C is 0.100, and the resistance change rate at -25°C is 0.065.
10. The application of the hydrogel electrolyte according to any one of claims 6-9, for human body motion detection, strain response element or in supercapacitor.
11. A supercapacitor is characterized in that the supercapacitor is an electric double layer capacitor, and an organic hydrogel electrolyte is sandwiched between two AC electrodes to form a sandwich structure.
12. The supercapacitor according to claim 11, wherein the steps of assembling the supercapacitor are as follows:

    1) Preparation of activated carbon (AC) electrode:

    Disperse activated carbon, conductive carbon black and PVDF in NMP at a mass ratio of 8:1:1 to prepare a uniform dispersion slurry, coat the slurry on carbon cloth and dry it in a vacuum oven at 180°C for 24 hours. After drying, an AC electrode is obtained;

    2) Assemble the supercapacitor:

    Take two pieces of AC electrodes with the same loading area, cover them with the organic hydrogel electrolyte on both sides to form a sandwich structure to prepare a supercapacitor; then add 1 drop to 4 drops of the hydrogel electrolyte to the electrodes on both sides of the supercapacitor before The bulk solution was used to wet the electrodes; the total thickness of the prepared supercapacitor was 1 mm–1.5 mm, and the thickness of the organic hydrogel electrolyte was half of the thickness of the supercapacitor.
13. The supercapacitor according to claim 11 or 12, wherein the internal resistance of the supercapacitor is 8.2Ω at 25°C, 31.1Ω at -20°C, and 6.6Ω at 60°C ; At a current density of 0.2A·g ^-1 , the mass specific capacity is 49.6F·g ^-1 at 25°C, 53.6F·g ^-1 at 60°C, and 24.2 at -20°C F·g ^-1 .
14. A stress-strain recording device comprising an organic hydrogel electrolyte.

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