Temperature-sensitive composite hydrogel based on sulfonated lignin and preparation method and application thereof

Abstract

The application discloses a sulfonated lignin-based temperature-sensitive composite hydrogel and a preparation method and application thereof, and belongs to the technical field of hydrogels. The preparation method of the temperature-sensitive composite hydrogel comprises the following steps: monomer acrylic acid (AA) and monomer N-isopropyl acrylamide (NIPAM) are dissolved in a solvent, then sulfonated lignin (LS) and salt, a crosslinking agent, and a thermal initiator are sequentially added, and a precursor solution is obtained by stirring, and the temperature-sensitive composite hydrogel is obtained by thermal initiation polymerization. The prepared sulfonated lignin-based temperature-sensitive composite hydrogel can adjust the LCST of the temperature-sensitive composite hydrogel through the synergistic effect of various components, so that the thermoelectric performance and the conductive performance are further improved, and the temperature-sensitive composite hydrogel can be used in the field of wearable devices.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of hydrogel, and particularly relates to a temperature-sensitive composite hydrogel based on sulfonated lignin and a preparation method and application thereof. BACKGROUND

[0002] The information disclosed in this Background section is for the purpose of increasing the understanding of the background of the application without admitting that such information constitutes prior art.

[0003] Hydrogel, as a typical soft material, is formed by the interaction of polymer network and a large number of water molecules, and can maintain a three-dimensional network structure through physical or chemical cross-linking in an aqueous environment, and exhibits excellent anti-dissolution performance and structural stability. It has become a potential matrix, carrier or soft skeleton material in the fields of biomedicine, tissue engineering, energy storage, etc. Conductive hydrogel, as an important derivative material of hydrogel, is made by adding conductive medium, and has both the characteristics of traditional hydrogel and the conductivity, and has great application potential in the fields of flexible sensing and electronic devices. The quasi-solid thermal battery based on hydrogel has become a research focus in this field due to its excellent plasticity, Seebeck coefficient, and excellent electrolyte leakage prevention performance. Under the dual challenges of global energy resources becoming increasingly scarce and the ecological environment continuing to deteriorate, exploring new environmentally friendly low-quality thermal energy conversion materials is not only a key strategy to alleviate the energy crisis, but also an important breakthrough for efficient development and utilization of low-grade heat.

[0004] Poly N-isopropyl acrylamide (PNIPAM) is a kind of temperature-sensitive functional polymer material, which has a unique molecular structure. There are both hydrophilic amide groups and hydrophobic alkyl chains, so it has temperature response behavior. During temperature change, this special molecular structure leads to dynamic changes in hydrophilic-hydrophobic interaction and intermolecular hydrogen bonds, so that PNIPAM undergoes volume phase transition near the lower critical solution temperature (LCST) of 32~33 ℃. Specifically, when the environmental temperature is lower than the LCST, the temperature-sensitive hydrogel is in a swollen state; when the temperature rises above the LCST, the temperature-sensitive hydrogel shrinks rapidly. Especially above the LCST, the intramolecular hydrogen bonds are broken, which promotes the water molecules to be expelled from the gel network. This process not only changes the ion concentration distribution in the gel, but also significantly affects the ion diffusion rate, which in turn has an important impact on the thermoelectric performance.

[0005] However, pure PNIPAM hydrogel has inherent defects such as insufficient mechanical strength and low conductivity. The composite hydrogel disclosed in the prior art has good mechanical properties, but the conductivity and Seebeck coefficient are relatively low; or the Seebeck coefficient and conductivity are high, but the mechanical properties are poor. Therefore, it is urgent to provide a composite hydrogel with high conductivity, high Seebeck coefficient and good mechanical strength. SUMMARY

[0006] In order to solve the problems of the prior art, the purpose of the present application is to provide a sulfonated lignin-based temperature-sensitive composite hydrogel and a preparation method and application thereof. The present application uses monomer acrylic acid and monomer N-isopropyl acrylamide as the gel network, sequentially adds sulfonated lignin, salt, crosslinking agent and thermal initiator in the precursor solution, and prepares the sulfonated lignin-based temperature-sensitive composite hydrogel by thermal initiation polymerization. Compared with the prior art, the present application uses the reversible oxidation-reduction characteristics of the phenolic / quinone group in sulfonated lignin to replace the traditional thermoelectric redox pair; the temperature-sensitive composite hydrogel has excellent temperature response characteristics, and the thermoelectric performance can be further improved by adjusting the phase transition temperature.

[0007] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0008] In the first aspect, the present application provides a sulfonated lignin-based temperature-sensitive composite hydrogel, which comprises monomer acrylic acid, monomer N-isopropyl acrylamide, sulfonated lignin, crosslinking agent and thermal initiator.

[0009] The preparation method of the sulfonated lignin-based temperature-sensitive composite hydrogel comprises the following steps:

[0010] The monomer acrylic acid and the monomer N-isopropyl acrylamide are dissolved in a solvent, and then the sulfonated lignin, the crosslinking agent and the thermal initiator are added and stirred to obtain a precursor solution, which is polymerized by thermal initiation to obtain the sulfonated lignin-based temperature-sensitive composite hydrogel.

[0011] The molar ratio of the monomer acrylic acid to the monomer N-isopropyl acrylamide is (3.5-4.5):(0.9-1.1).

[0012] The mass fraction of the sulfonated lignin in the solvent system composed of the monomer acrylic acid and the solvent is 1-10%.

[0013] In the second aspect, the present application provides a preparation method of the above-mentioned sulfonated lignin-based temperature-sensitive composite hydrogel, which comprises the following steps:

[0014] The monomer acrylic acid (AA) and the monomer N-isopropyl acrylamide (NIPAM) are dissolved in a solvent, and then the sulfonated lignin (LS), the crosslinking agent and the thermal initiator are added and stirred to obtain a precursor solution, which is polymerized by thermal initiation to obtain the sulfonated lignin-based temperature-sensitive composite hydrogel.

[0015] In a third aspect, the present application provides application of the above-mentioned sulfonated lignin-based temperature-sensitive composite hydrogel or the sulfonated lignin-based temperature-sensitive composite hydrogel obtained by the above-mentioned preparation method as a self-powered strain sensor in a wearable device.

[0016] In a fourth aspect, the present application provides a wearable device comprising the above-mentioned sulfonated lignin-based temperature-sensitive composite hydrogel.

[0017] One or some of the above technical solutions have the following advantages or beneficial effects:

[0018] (1) The unique molecular structure of NIPAM in the present application, the presence of hydrophilic amide groups and hydrophobic alkyl chains, makes it have temperature response behavior. During the temperature change process, this special molecular structure leads to the dynamic change of hydrophilic-hydrophobic interaction and intermolecular hydrogen bond, further improving the thermoelectric performance. Further, the present application introduces sulfonated lignin, which endows the temperature-sensitive composite hydrogel with thermoelectric performance through the reversible redox groups on the benzene ring, efficiently reuses papermaking industry waste, and relieves environmental pressure. The present application also adds sodium chloride to endow the temperature-sensitive composite hydrogel with excellent conductive performance. Therefore, the present application adjusts the LCST of the temperature-sensitive composite hydrogel through the synergistic effect of multiple components, thereby further improving the thermoelectric performance and conductive performance.

[0019] (2) The preparation method of the present application is simple, low in cost, and the obtained sulfonated lignin-based temperature-sensitive composite hydrogel has excellent tensile property and good biocompatibility.

[0020] (3) The temperature-sensitive composite hydrogel provided by the present application, when sulfonated lignin is added without salt, the elongation at break gradually increases with the increase of the amount of sulfonated lignin, and the conductivity and the Seebeck coefficient are first increased and then decreased. When the mass fraction of sulfonated lignin is 5%, good elongation at break (up to 486%) is ensured, and the conductivity (up to 0.49 S / m) and the Seebeck coefficient (up to 2.02 mV / K) are also relatively optimal.

[0021] (4) When the present application adds salt on the basis of adding sulfonated lignin, the elongation at break, the conductivity and the Seebeck coefficient are first increased and then decreased with the increase of the concentration of salt. When the concentration of salt is 1 mol / L, the hydrogen bond is weakened due to salting-out effect, the elongation at break is slightly reduced (up to 283%), the conductivity (up to 8.03 S / m) and the Seebeck coefficient (up to 3.71 mV / K) are also slightly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0022] The accompanying drawings, which form a part of this specification, are included to provide a further understanding of the application and are incorporated herein for explanation.

[0023] Figure 1 is a thermoelectric potential result diagram of embodiment 8 of the application at the phase transition temperature;

[0024] Figure 2 is a thermoelectric potential result diagram of embodiment 8 of the application at the phase transition temperature;

[0025] Figure 3 is a diagram showing the influence of different salt concentrations on the conductivity at the phase transition temperature of embodiment 8 of the application;

[0026] Figure 4 is a diagram showing the influence of different amounts of crosslinking agent on the conductivity at the phase transition temperature of embodiment 8 of the application;

[0027] Figure 5 is a diagram showing the influence of different mass fractions of sulfonated lignin on the mechanical properties of the application;

[0028] Figure 6 is a diagram showing the influence of different salt concentrations on the mechanical properties of the application. DETAILED DESCRIPTION

[0029] In the present application, unless otherwise specified, other test materials and instruments and equipment are conventional test materials in the art and can be purchased through commercial channels.

[0030] In view of the fact that existing hydrogels are difficult to have multifunctional characteristics, which limits their application range. In order to solve the above problems, the present application provides a preparation method of a composite hydrogel based on sulfonated lignin, specifically: the sulfonated lignin provides the main Seebeck coefficient of the hydrogel as a thermoelectric particle, the reversible oxidation-reduction reaction between the phenolic quinone structure in the sulfonated lignin improves the Seebeck coefficient, then salt is added to improve the conductivity and cooperatively adjust the LCST, and monomer acrylic acid (AA) and monomer N-isopropyl acrylamide (NIPAM) are polymerized as a polymer network through chemical crosslinking.

[0031] The present application adds sulfonated lignin and salt ions to the polymer network to prepare a composite hydrogel, mainly considering the following points:

[0032] (1) The surface of the sulfonated lignin has abundant catechol structures, which improve the Seebeck effect of the hydrogel and provide good thermoelectric properties for the hydrogel.

[0033] (2) Salt ions serve as conductive ions to provide good conductive properties for the thermoelectric hydrogel.

[0034] In an exemplary embodiment of the present application, a sulfonated lignin-based temperature-sensitive composite hydrogel is provided, and the raw materials thereof include monomer acrylic acid, monomer N-isopropyl acrylamide, sulfonated lignin, crosslinking agent and thermal initiator.

[0035] In another exemplary embodiment of the present application, the raw materials further include a salt. The salt includes sodium chloride (NaCl). That is, the raw materials of the sulfonated lignin-based temperature-sensitive composite hydrogel include monomer acrylic acid, monomer N-isopropyl acrylamide, sulfonated lignin, salt, crosslinking agent and thermal initiator.

[0036] The sulfonated lignin-based temperature-sensitive composite hydrogel provided by the present application is composed of a copolymerization chemical crosslinking network, sulfonated lignin, salt, crosslinking agent and thermal initiator. The copolymerization chemical crosslinking network is composed of monomer acrylic acid (AA) and monomer N-isopropyl acrylamide (NIPAM).

[0037] Lignin is a natural polymer with a three-dimensional spatial structure composed of phenylpropane units, and is a renewable polyphenol polymer compound with abundant reserves in nature. It has rich functional groups such as alcohol hydroxyl, phenolic hydroxyl and double bond. It is particularly important to note that lignin contains abundant ortho-diphenol groups. When subjected to oxidation, the phenolic structure of lignin itself will be oxidized to quinone structure, and then the phenolic / quinone groups of lignin itself can be interconverted, thereby realizing redox reaction, which makes it have the potential ability to convert low-quality thermal energy into electrical energy. Combined with the structure and performance characteristics of lignin, the addition of lignin not only realizes the efficient recycling of low-quality thermal energy, but also improves the mechanical properties of the hydrogel, and improves the water absorption, hydrophilicity, thermal stability and biocompatibility of the hydrogel.

[0038] Based on the unique temperature-sensitive response characteristics of the PNIPAM hydrogel, by grafting acrylic acid (AAc) monomer containing hydrophilic groups, the LCST of PNIPAM can be effectively regulated, and the ionic conductivity and mechanical properties of PNIPAM can be significantly improved. The sulfonated lignin (such as sodium lignosulfonate) is used as a functional additive, and the reversible oxidation-reduction characteristics of the phenolic / quinone groups are used to optimize the charge distribution of the gel system, thereby effectively enhancing the thermoelectric performance.

[0039] Therefore, based on the characteristics of the temperature-sensitive thermoelectric hydrogel and lignin, the present application develops a sulfonated lignin-based temperature-sensitive composite hydrogel, and introduces salt ions based on the Hofmeister effect, which can endow the hydrogel with high conductivity after compounding.

[0040] In the sulfonated lignin-based temperature-sensitive composite hydrogel, monomer acrylic acid (AA) and monomer N-isopropyl acrylamide (NIPAM) are used as a copolymer network, and salt ions are used as an electronic conductor. The sulfonated lignin-based temperature-sensitive composite hydrogel prepared by using sulfonated lignin and salt ions has multifunctional characteristics.

[0041] In another typical embodiment of the present application, a preparation method of the above-mentioned sulfonated lignin-based temperature-sensitive composite hydrogel is provided, which comprises the following steps:

[0042] The monomer acrylic acid (AA) and the monomer N-isopropyl acrylamide (NIPAM) are dissolved in a solvent, and then the sulfonated lignin (LS), the crosslinking agent and the thermal initiator are added and stirred to obtain a precursor solution, which is polymerized by heat initiation.

[0043] In another typical embodiment of the present application, a preparation method of the above-mentioned sulfonated lignin-based temperature-sensitive composite hydrogel is provided, which comprises the following steps: the monomer acrylic acid (AA) and the monomer N-isopropyl acrylamide (NIPAM) are dissolved in a solvent, and then the sulfonated lignin (LS) and the salt, the crosslinking agent and the thermal initiator are added and stirred to obtain a precursor solution, which is polymerized by heat initiation.

[0044] In some embodiments of the above-mentioned embodiments, the solvent is water, and the mass-volume ratio of the monomer acrylic acid and water is (1.5-2.5 g):(5-10 mL), preferably (1.8-2.0 g):(7.5-8.5 mL).

[0045] In some embodiments of the above-mentioned embodiments, the molar ratio of the monomer acrylic acid (AA) and the monomer N-isopropyl acrylamide (NIPAM) is (3.5-4.5):(0.9-1.1), preferably (3.9-4.1):(0.9-1.1), and most preferably 4:1. If only NIPAM is used, the gel is not shaped, and if only AA is used, the gel is shaped but has poor toughness and is difficult to demold for testing. In the preparation of the hydrogel, attention should be paid to the proportioning of the two. If the molar ratio of AA and NIPAM is too small, the gel is prone to be not shaped, and if the molar ratio is suitable, the gel can be shaped, but if the molar ratio is too large, the gel is shaped but has poor toughness. Therefore, in the present application, the molar ratio of AA and NIPAM is 4:1, which is the best ratio.

[0046] In some embodiments of the above-mentioned embodiments, the mass fraction of the sulfonated lignin in the solvent system composed of monomer acrylic acid and solvent is 1-10%, specifically 1%, 2%, 3%, 4%, 4.5%, 5%, 5.5%, 6%, 7%, 8%, 9%, 10%, etc., preferably 2-6%, more preferably 4-6%, and most preferably 5%. The sulfonated lignin can be sodium lignosulfonate.

[0047] In some embodiments of the above-mentioned embodiments, the crosslinking agent comprises N,N'-methylenebisacrylamide (MBA). The amount of the crosslinking agent added is 0.4-1.6% of the total mass of the monomers, specifically 0.4%, 0.5%, 0.6%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, and further preferably 0.7-0.9%. With the increase of the amount of the crosslinking agent, the conductivity shows a trend of first increasing and then decreasing, and the conductivity effect is better at the amount of 0.8%.

[0048] In some embodiments of the above-mentioned embodiments, the thermal initiator is a persulfate salt, and the persulfate salt comprises ammonium persulfate (APS). The amount of the thermal initiator added is 5-10% of the total mass of the monomers, specifically 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, etc., and preferably 7-9%.

[0049] In some embodiments of the above-mentioned embodiments, the salt is sodium chloride (NaCl). The concentration of sodium chloride in the solvent system composed of monomer acrylic acid and solvent is 0.5-1.25 mol / L, specifically 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.25 mol / L, preferably 0.8-1.2 mol / L, and most preferably 1 mol / L.

[0050] NaCl is selected as the salt ion, and Na + is a strong salting-out cation in Hofmeister effect, promotes hydrophobic interaction, reduces LCST, and also serves as a conductive ion.

[0051] In some embodiments of the above-mentioned embodiments, the temperature for thermal initiation of polymerization is 50-90°C, specifically 50°C, 60°C, 70°C, 80°C, 90°C, preferably 60-80°C, and most preferably 70°C. A temperature that is too low is not sufficient for polymerization to occur, and a temperature that is too high causes surface dehydration to form sticky substances.

[0052] In some embodiments of the above-mentioned embodiments, the time for thermal initiation of polymerization is 30-60 min, specifically 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, preferably 40-50 min, and most preferably 40 min. If the time for thermal initiation of polymerization is too long, such as 2 h, sticky substances will be formed on the surface of the hydrogel after the surface-unreacted solution is heated for a long time.

[0053] In some embodiments of the above-mentioned embodiments, the stirring speed and time are not specifically limited, as long as the components are well mixed. For example, the stirring speed is 200-1000 rpm, preferably 400-600 rpm, and more preferably 500 rpm, and the stirring time is 5-20 min, more preferably 8-12 min, and most preferably 10 min.

[0054] In some embodiments of the above-mentioned embodiments, the hydrogel raw material is monomer acrylic acid and monomer N-isopropyl acrylamide, and the cylindrical hydrogel has a bottom diameter of 2 cm and a height of 1 cm.

[0055] As a preferred embodiment, the preparation method specifically comprises: dissolving monomer acrylic acid (AA) and monomer N-isopropyl acrylamide (NIPAM) in water, then sequentially adding sulfonated lignin (LS), salt, crosslinking agent, and thermal initiator, stirring uniformly, injecting the precursor solution into a polytetrafluoroethylene mold, and reacting at a set temperature.

[0056] In another typical embodiment of the present application, the above-mentioned sulfonated lignin-based temperature-sensitive composite hydrogel or the sulfonated lignin-based temperature-sensitive composite hydrogel obtained by the above-mentioned preparation method is provided as a self-powered strain sensor for preparing a wearable device.

[0057] In another typical embodiment of the present application, a wearable device is provided, which comprises the above-mentioned sulfonated lignin-based temperature-sensitive composite hydrogel.

[0058] The temperature-sensitive composite hydrogel has excellent temperature response characteristics, and the thermoelectric performance can be further improved by adjusting the phase transition temperature, so that it is suitable for the use scenario of wearable devices.

[0059] 20~25℃ (such as 21℃, 21.5℃, 22℃, 22.5℃, 23℃, 23.5℃, 24℃, 24.5℃, 25℃) is the ambient temperature of daily life. And the best phase transition temperature is above the daily ambient temperature and below the body temperature. Therefore, the sulfonated lignin-based temperature-sensitive composite hydrogel provided by the application can regulate the phase transition temperature to 20~25℃ (preferably 22~25℃, further preferably 22~24℃, and more preferably 23~24℃) by adding lignin and salt, and is used as a wearable device when the body temperature is higher than the phase transition temperature.

[0060] Therefore, when the sulfonated lignin-based temperature-sensitive composite hydrogel provided by the application is applied to a wearable device as a self-powered strain sensor, the monomer acrylic acid (AA) carries a hydrophilic group -COOH, the hydrophilicity is enhanced, and the LCST is increased; the salt (NaCl) plays a salting-out role, precipitates water, and promotes hydrophobicity, so that the LCST is reduced; the sulfonated lignin (LS) carries a hydrophilic group -OH, so that the LCST is increased.

[0061] Regarding the thermoelectric performance, it involves ion thermal diffusion effect and redox reaction. Ion thermal diffusion effect (temperature difference affects reaction entropy difference): Na + , Cl - , AA provides mobile ions, and free protons reach the cold end faster, and accumulate positive charges at the cold end. Redox reaction (temperature difference affects reaction equilibrium): lignin phenolquinone redox, which is essentially proton-coupled electron transfer (PCET). Among them, the hot end: temperature rise, promotes the oxidation of phenolic hydroxyl group to quinone, releases electrons and protons; the cold end: temperature reduction, quinone is reduced to phenol, consumes protons and electrons.

[0062] Regarding the mechanical properties, lignin molecules are a three-dimensional network polymer. When it is combined with the matrix material, its molecular chain can form physical crosslinking points in the matrix through physical entanglement and hydrogen bonding and the like. It can effectively transmit and disperse stress, limit the slip of molecular chains, and thus improve the strength, rigidity and toughness of the material. However, high concentration of salt ions is usually harmful to mechanical properties, and salt ions will compete for water, and the presence of salt will increase the unhydrated water content, weakening the structural strength.

[0063] In order for those skilled in the art to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described in detail below with specific examples.

[0064] Example 1

[0065] Preparation of temperature-sensitive composite hydrogel based on sulfonated lignin: first, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, after the NIPAM was completely dissolved, 2 wt% of sulfonated lignin (relative to the whole solution system) was added, after the sulfonated lignin was completely dissolved, 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for 40 min of reaction, and the hydrogel was removed after preparation.

[0066] The composite hydrogel prepared in this example can achieve an elongation at break of 257%, a conductivity of 0.26 S / m, and a Seebeck coefficient of 0.72 mV·K -1 .

[0067] Example 2

[0068] Preparation of temperature-sensitive composite hydrogel based on sulfonated lignin: first, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, after the NIPAM was completely dissolved, 3 wt% of sulfonated lignin (relative to the whole solution system) was added, after the sulfonated lignin was completely dissolved, 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for 40 min of reaction, and the hydrogel was removed after preparation.

[0069] The composite hydrogel prepared in this example can achieve an elongation at break of 312%, a conductivity of 0.37 S / m, and a Seebeck coefficient of 1.57 mV·K -1 .

[0070] Example 3

[0071] Preparation of temperature-sensitive composite hydrogel based on sulfonated lignin: first, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, after the NIPAM was completely dissolved, 4 wt% of sulfonated lignin (relative to the whole solution system) was added, after the sulfonated lignin was completely dissolved, 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for 40 min of reaction, and the hydrogel was removed after preparation.

[0072] The composite hydrogel prepared in this example can achieve an elongation at break of 341%, a conductivity of 0.46 S / m, and a Seebeck coefficient of 1.85 mV·K-1 .

[0073] Example 4

[0074] Preparation of temperature-sensitive composite hydrogel based on sulfonated lignin: first, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, after the NIPAM was completely dissolved, 5 wt% of sulfonated lignin (relative to the whole solution system) was added, after the sulfonated lignin was completely dissolved, 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for reaction for 40 min, and the hydrogel was taken out after preparation.

[0075] The composite hydrogel prepared in this example can achieve an elongation at break of 486%, an electrical conductivity of 0.49 S / m, and a Seebeck coefficient of 2.02 mV·K -1 .

[0076] Example 5

[0077] Preparation of temperature-sensitive composite hydrogel based on sulfonated lignin: first, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, after the NIPAM was completely dissolved, 6 wt% of sulfonated lignin (relative to the whole solution system) was added, after the sulfonated lignin was completely dissolved, 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for reaction for 40 min, and the hydrogel was taken out after preparation.

[0078] The composite hydrogel prepared in this example can achieve an elongation at break of 524%, an electrical conductivity of 0.31 S / m, and a Seebeck coefficient of 1.72 mV·K -1 .

[0079] Example 6

[0080] Preparation of temperature-sensitive composite hydrogel based on sulfonated lignin: first, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, after the NIPAM was completely dissolved, 5 wt% of sulfonated lignin (relative to the whole solution system) was added, after the sulfonated lignin was completely dissolved, NaCl was added, after the NaCl was completely dissolved (the concentration of NaCl in the solvent system after dissolution was 0.5 mol·L -1After stirring evenly, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for reaction for 40 min, and the hydrogel was taken out after preparation.

[0081] The composite hydrogel prepared in this example can achieve an elongation at break of 154%, an electrical conductivity of 4.95 S / m, and a Seebeck coefficient of 2.38 mV·K -1 .

[0082] Example 7

[0083] Preparation of a temperature-sensitive composite hydrogel based on sulfonated lignin: First, 1.9 g of AA was added to 8.1 mL of deionized water, followed by the addition of 0.75 g of NIPAM. After the NIPAM was completely dissolved, 5 wt% sulfonated lignin (relative to the entire solution system) was added. After the sulfonated lignin was completely dissolved, NaCl was added. After the NaCl was completely dissolved (the concentration of NaCl in the solvent system after dissolution was 0.75 mol·L -1 ), 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After stirring evenly, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for reaction for 40 min, and the hydrogel was taken out after preparation.

[0084] The composite hydrogel prepared in this example can achieve an elongation at break of 295%, an electrical conductivity of 6.67 S / m, and a Seebeck coefficient of 2.93 mV·K -1 .

[0085] Example 8

[0086] Preparation of a temperature-sensitive composite hydrogel based on sulfonated lignin: First, 1.9 g of AA was added to 8.1 mL of deionized water, followed by the addition of 0.75 g of NIPAM. After the NIPAM was completely dissolved, 5 wt% sulfonated lignin (relative to the entire solution system) was added. After the sulfonated lignin was completely dissolved, NaCl was added. After the NaCl was completely dissolved (the concentration of NaCl in the solvent system after dissolution was 1 mol·L -1 ), 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After stirring evenly, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for reaction for 40 min, and the hydrogel was taken out after preparation.

[0087] The composite hydrogel prepared in this example can achieve a breaking elongation of 283%, a conductivity of 8.03 S / m, and a Seebeck coefficient of 3.71 mV·K at the phase transition temperature -1 , and a Seebeck coefficient of 2.46 mV·K at the phase transition temperature -1 .

[0088] Example 9

[0089] Preparation of a temperature-sensitive composite hydrogel based on sulfonated lignin: First, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, and after the NIPAM was completely dissolved, 5 wt% of sulfonated lignin (relative to the entire solution system) was added, and after the sulfonated lignin was completely dissolved, NaCl was added, and after the NaCl was completely dissolved (the concentration of NaCl in the solvent system after dissolution was 1.25 mol·L -1 ), 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for reaction for 40 min, and after preparation, the hydrogel was taken out.

[0090] The composite hydrogel prepared in this example can achieve a breaking elongation of 133%, a conductivity of 6.76 S / m, and a Seebeck coefficient of 2.97 mV·K at the phase transition temperature -1 .

[0091] Comparative Example 1

[0092] Preparation of a copolymer hydrogel: First, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, and after the NIPAM was completely dissolved, 0.02 g of MBA and 0.2 g of APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold and placed in a 70°C oven for reaction for 40 min, and after preparation, the hydrogel was taken out.

[0093] The hydrogel prepared in this example can achieve a breaking elongation of 184%, but the conductivity is only 0.79 S / m, and the Seebeck coefficient is only -0.71 mV·K -1 .

[0094] Comparative Example 2

[0095] Preparation of a polyacrylic acid hydrogel: First, 1.9 g of AA was added to 8.1 mL of deionized water, then 0.75 g of NIPAM was added, and after the NIPAM was completely dissolved, NaCl was added, and after the NaCl was completely dissolved (the concentration of NaCl in the solvent system after dissolution was 1.25 mol·L-1 ), then 0.02 g MBA and 0.2 g APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold, and placed in a 70°C oven for reaction for 40 min. After preparation, the hydrogel was taken out.

[0096] The hydrogel prepared in this example can achieve an elongation at break of 243%, an electrical conductivity of 6.39 S / m, and a Seebeck coefficient of 2.37 mV·K -1 .

[0097] Comparative Example 3

[0098] Unlike Example 4, the prepared hydrogel was soaked in a salt solution, specifically:

[0099] First, 1.9 g AA was added to 8.1 mL deionized water, then 0.75 g NIPAM was added, after the NIPAM was completely dissolved, 5 wt% sulfonated lignin (relative to the entire solution system) was added, after the sulfonated lignin was completely dissolved, 0.02 g MBA and 0.2 g APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold, and placed in a 70°C oven for reaction for 40 min. After preparation, the hydrogel was taken out. The hydrogel was soaked in 1 mol·L -1 NaCl for 12 h, and the final hydrogel was obtained.

[0100] Comparative Example 4

[0101] Unlike Example 8, the sulfonated lignin and salt were soaked after the preparation of the hydrogel. Specifically:

[0102] First, 1.9 g AA was added to 8.1 mL deionized water, then 0.75 g NIPAM was added, after the NIPAM was completely dissolved, 0.02 g MBA and 0.2 g APS were added, and stirred at 500 rpm for 10 min. After uniform stirring, the precursor solution was injected into a polytetrafluoroethylene mold, and placed in a 70°C oven for reaction for 40 min. After preparation, the hydrogel was taken out. The hydrogel was soaked in 5 wt% sulfonated lignin and 1 mol·L -1 NaCl for 12 h, and the final hydrogel was obtained.

[0103] Comparative Example 5

[0104] Unlike Example 1, the molar ratio of AA and NIPAM was different, such as 1:1, 2:1, 3:1, and 5:1.

[0105] The higher the molar ratio, the higher the polymer concentration (the polymer concentration refers to the total concentration of AA and NIPAM, and the increasing molar ratio means that NIPAM is unchanged and AA is increasing). When the molar ratio is small, it is not shaped, such as 1:1, 2:1, 3:1, the gel is not shaped and is in a liquid viscous state. When the molar ratio is 5:1, although the gel is shaped, the toughness is poor. Only when the molar ratio in the example is 4:1, a shaped and tough hydrogel can be obtained.

[0106] Comparative Example 6

[0107] Only monomer acrylic acid (AA) is used as the gel network, or only monomer N-isopropyl acrylamide (NIPAM) is used as the gel network.

[0108] It is found through research that: only AA is used, the gel is not shaped, and only NIPAM is used, although the gel is shaped, the toughness is very poor, and it is difficult to demold for testing.

[0109] Related test methods for elongation at break, conductivity, Seebeck coefficient and phase transition temperature:

[0110] Elongation at break: a texture analyzer is used to test the tensile and compressive properties of the gel. In the tensile test, the gel is made into a 5*1*0.1 cm long strip, cut with a mold, and the tensile strength is tested with a universal electronic tensile testing machine. After fixing the sample position, set the tensile speed to 120 mm·min -1 , until the sample breaks.

[0111] Conductivity: the gel is made into a cylinder with a radius of 1 cm and a height of 1 cm, placed between two copper sheets, and the data is recorded with an electrochemical workstation.

[0112] Seebeck coefficient: the Seebeck coefficient of the thermocell is tested by using a self-built measuring platform. A cylindrical gel thermocell with a radius of 1 cm and a height of 1 cm is prepared. One end of the thermocell is placed on a heating plate and the other end is placed on a cooling plate. The temperature change is recorded by a thermocouple and the voltage change is recorded by a multimeter.

[0113] Phase transition temperature: differential scanning calorimetry is used.

[0114] Table 1 Composition table of examples and comparative examples

[0115]

[0116] Note: “\” in Table 1 means that the component is not added.

[0117] Table 2 Elongation at break, conductivity, Seebeck coefficient and phase transition temperature of examples and comparative examples

[0118]

[0119] Note: The "\" in Table 2 indicates that the performance was not tested.

[0120] Depend on Figure 1 It can be seen that when both ends of the gel are at the phase transition temperature, the Seebeck coefficient can reach 3.71 mV·K. -1 .

[0121] Depend on Figure 2 It can be seen that when both ends of the gel are at the phase transition temperature, the Seebeck coefficient is only 2.46 mV·K. -1 The coefficient is much lower than that at the phase transition temperature, which illustrates the importance of phase transition temperature regulation for improving hydrogel thermal batteries.

[0122] Depend on Figure 3 It can be seen that the conductivity of the gel after adding 5 wt% sodium lignosulfonate and different salt concentrations was tested at the phase transition temperature and at the phase transition temperature. The results show that at the phase transition temperature, the hydrogel is in a hydrophobic state, the ion concentration increases, and the conductivity increases at the phase transition temperature.

[0123] Depend on Figure 4 It can be seen that the test was conducted with the addition of 5 wt% sodium lignosulfonate, 1 mol·L⁻¹. -1 The conductivity of the hydrogels with different crosslinking agent concentrations after NaCl was measured at the phase transition temperature and at the phase transition temperature. The results showed that the hydrogel was in a hydrophobic state at the phase transition temperature, and the conductivity increased with the increase of ion concentration at the phase transition temperature.

[0124] Depend on Figure 5 It was found that the elongation at break was tested for different concentrations of sodium lignosulfonate, and the results showed that the stress was the highest and the strength was the best when the concentration of sodium lignosulfonate was 5 wt%. Since lignin contains abundant phenolic hydroxyl groups, excessive phenolic hydroxyl groups will compete for free radical binding sites, hindering polymer chain growth and the formation of cross-linked networks, thus affecting its mechanical properties.

[0125] Depend on Figure 6 It can be seen that: the elongation at break was tested with different salt concentrations, and the results showed that the salt concentration of 1 mol·L⁻¹ was optimal. -1 The stress is greatest and the strength is best at this time. The addition of salt ions usually has a negative impact on mechanical properties. Salt ions compete for bound water, and the salt causes water to precipitate out, weakening the structural strength.

[0126] Elongation at break: Lignin molecule itself is a three-dimensional network polymer. When it is combined with polymer chain material, its molecular chain can form physical crosslinking points in the matrix through physical entanglement and hydrogen bonding and other actions. It can effectively transfer and disperse stress, limit the slip of molecular chain, thereby improving the strength, stiffness and toughness of the material. Excessive lignin will form accumulation in the gel network, thereby affecting its mechanical properties. The addition of salt ions usually has a negative impact on mechanical properties. Salt ions compete for water binding, and salt causes an increase in unbound water content, weakening the structural strength.

[0127] Thermoelectric performance: Na + is a strong salting cation in Hofmeister effect, and at the same time as the conductive ion, according to the ion thermal diffusion effect, Na + , Cl - , AA provide mobile ions, and free protons reach the cold end faster, accumulating positive charges at the cold end, while the temperature difference affects the entropy difference, and the greater the temperature difference, the greater the entropy difference, and the faster the ion movement rate, thereby improving its Seebeck coefficient and electrical conductivity. However, the addition of excessive salt ions will enhance the salting effect, and after excessive precipitation of water, the network of composite hydrogel collapses, and the ion channels become fewer, thereby affecting its electrical conductivity and Seebeck coefficient. Sodium lignosulfonate mainly affects the thermoelectric performance of hydrogel through its phenolic quinone redox reaction. The temperature difference will affect the redox reaction equilibrium, and the lignin phenolic quinone redox is a proton-coupled electron transfer (PCET) in nature. At the hot end, the temperature rises, promoting the oxidation of phenolic hydroxyl to quinone, releasing electrons and protons to the cold end. While the temperature at the cold end decreases, the quinone is reduced to phenol, consuming protons and electrons, thereby endowing the hydrogel with thermoelectric properties. The benzene ring structure of lignin forms π-π stacking, forming ion transport channels for fast ion transport, thereby improving the thermoelectric performance.

[0128] Phase transition temperature: Due to the presence of both hydrophilic amide groups and hydrophobic alkyl chains in NIPAM itself, it has temperature response behavior. During temperature changes, this special molecular structure leads to dynamic changes in hydrophilic-hydrophobic interactions and intermolecular hydrogen bonds, causing PNIPAM to undergo a phase transition near the low critical solution temperature of 32~33 ℃. With the addition of acrylic acid monomer, the side chain carries hydrophilic groups -COOH, enhancing the hydrophilicity of the polymer chain, thereby improving the LCST of the material. Lignin is an aromatic compound with a three-dimensional spatial network structure, and its benzene ring carries phenolic hydroxyl groups, which can enhance the hydrophilicity of the polymer chain, but excessive lignin concentration will affect the electrical conductivity of the composite hydrogel. After weighing the pros and cons, the final lignin concentration is determined to be 5wt%. Salt ions are initially added as conductive electrolytes, and their salting effect enhances the hydrophobicity of the material, causing the polymer chain to easily precipitate water, reducing the LCST of the material. Since the target phase transition temperature is around room temperature 24 ℃, and excessive salt will exacerbate the salting effect, affecting the mechanical properties and electrical conductivity.

[0129] The present application is to adjust the phase transition temperature to make it suitable for daily life, and the gel has good performance above the phase transition temperature, so it is necessary to determine that it is above the phase transition temperature. If the phase transition temperature is 37 DEG C, it cannot be determined that the gel can be above the phase transition temperature in the daily use environment, and the body surface temperature is usually about 33 DEG C, and 24 DEG C is exactly the environmental temperature in daily life.

[0130] The optimal phase transition temperature is above the daily environmental temperature and below the body temperature.

[0131] The present application can adjust the phase transition temperature to 24 DEG C by adding lignin and salt, and the body temperature is higher than the phase transition temperature when used as a wearable device.

[0132] The above only describes the preferred embodiments of the present application and is not used to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A sulfonated lignin-based thermosensitive composite hydrogel, characterized in that, The raw materials include monomer acrylic acid, monomer N-isopropyl acrylamide, sulfonated lignin, salt, crosslinking agent and thermal initiator; The preparation method of the sulfonated lignin-based temperature-sensitive composite hydrogel comprises the following steps: The monomer acrylic acid and the monomer N-isopropyl acrylamide are dissolved in a solvent, and then the sulfonated lignin, the salt, the crosslinking agent and the thermal initiator are added and stirred to obtain a precursor solution, and the precursor solution is subjected to thermal initiation polymerization to obtain the sulfonated lignin-based temperature-sensitive composite hydrogel; The molar ratio of the monomer acrylic acid to the monomer N-isopropyl acrylamide is (3.5-4.5):(0.9-1.1). The mass fraction of the sulfonated lignin in the solvent system composed of the monomer acrylic acid and the solvent is 1-10%. The salt comprises sodium chloride.

2. A process for the preparation of the sulfonated lignin-based thermoresponsive composite hydrogel according to claim 1, characterized by, The preparation method comprises the following steps: The monomer acrylic acid and the monomer N-isopropyl acrylamide are dissolved in a solvent, and then the sulfonated lignin, the salt, the crosslinking agent and the thermal initiator are added and stirred to obtain a precursor solution, and the precursor solution is subjected to thermal initiation polymerization to obtain the sulfonated lignin-based temperature-sensitive composite hydrogel; and the salt comprises sodium chloride.

3. The method for preparing a sulfonated lignin-based thermoresponsive composite hydrogel according to claim 2, characterized in that, The concentration of the sodium chloride in the solvent system composed of the monomer acrylic acid and the solvent is 0.5-1.25 mol / L.

4. The method for preparing a sulfonated lignin-based thermoresponsive composite hydrogel according to claim 3, characterized in that, The concentration of the sodium chloride in the solvent system composed of the monomer acrylic acid and the solvent is 0.8-1.2 mol / L.

5. The method for preparing a sulfonated lignin-based thermoresponsive composite hydrogel according to any one of claims 2 to 4, characterized in that, The solvent is water, and the mass-volume ratio of the monomer acrylic acid to water is (1.5-2.5 g):(5-10 mL). The thermal initiator comprises a persulfate salt, and the addition amount of the thermal initiator is 5-10% of the total mass of the monomers. The temperature of the thermal initiation polymerization is 50-90 DEG C, and the time of the thermal initiation polymerization is 30-60 min. The crosslinking agent comprises N,N'-methylene bisacrylamide, and the addition amount of the crosslinking agent is 0.4-1.6% of the total mass of the monomers. The mass fraction of the sulfonated lignin in the solvent system composed of the monomer acrylic acid and the solvent is 1-10%. The molar ratio of the monomer acrylic acid to the monomer N-isopropyl acrylamide is (3.5-4.5):(0.9-1.1).

6. The method for preparing a sulfonated lignin-based thermoresponsive composite hydrogel according to claim 5, characterized in that, The addition amount of the crosslinking agent is 0.7-0.9% of the total mass of the monomers. The mass fraction of the sulfonated lignin in the solvent system composed of the monomer acrylic acid and the solvent is 2-6%. The molar ratio of the monomer acrylic acid to the monomer N-isopropyl acrylamide is (3.9-4.1):(0.9-1.1).

7. The method for preparing a sulfonated lignin-based thermoresponsive composite hydrogel according to claim 6, characterized in that, The mass fraction of the sulfonated lignin in the solvent system composed of the monomer acrylic acid and the solvent is 4-6%.

8. Application of the sulfonated lignin-based temperature-sensitive composite hydrogel in a wearable device as a self-powered strain sensor, wherein the sulfonated lignin-based temperature-sensitive composite hydrogel is the sulfonated lignin-based temperature-sensitive composite hydrogel in claim 1 or obtained by the preparation method of the sulfonated lignin-based temperature-sensitive composite hydrogel in any one of claims 2-7.

9. A wearable device, comprising: The wearable device comprises the sulfonated lignin-based temperature-sensitive composite hydrogel in claim 1 or obtained by the preparation method of the sulfonated lignin-based temperature-sensitive composite hydrogel in any one of claims 2-7.

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