Self-initiated polymerization lignin-based temperature-sensitive gel and preparation method and application thereof

Abstract

The present application relates to a kind of self-initiated polymerization lignin-based temperature-sensitive gel and its preparation method and application, belong to polymer functional material technical field.To solve the problem of polymerization obstruction caused by lignin radical scavenging effect under the condition of high lignin content, the present application provides a kind of preparation method of self-initiated polymerization lignin-based temperature-sensitive gel, steps include preparation lignin sulfonate ester and preparation lignin-based temperature-sensitive gel.The present application realizes 20~70 seconds ultrafast gelation under room temperature condition by constructing lignin sulfonate ester (LSME) -Fe 3+ -APS redox synergistic autocatalytic system, constructs the double-network structure with chemical covalent bond and physical dynamic bond, makes the obtained hydrogel have excellent mechanical strength, electric conductivity and temperature response performance, can be used as flexible strain sensor to detect the multiscale movement signal of human body, also can be used for real-time temperature detection and physiological signal acquisition, show good reliability and application potential.

Description

Technical Field

[0001] This invention belongs to the field of polymer functional materials technology, and particularly relates to a self-initiated polymerized lignin-based thermosensitive gel, its preparation method and application. Background Technology

[0002] Hydrogels are three-dimensional network materials formed by cross-linking of polymer chains. Due to their structural similarity to soft tissue, they are widely used in biomedicine and wearable devices. However, traditional single-network hydrogels generally suffer from poor mechanical properties and insufficient fatigue resistance, mainly due to the inhomogeneity of the network structure and limited energy dissipation mechanisms. In recent years, the performance of hydrogels has been improved by introducing dynamic interactions such as hydrogen bonding and ion coordination, but most preparation processes rely on non-renewable resources or high-energy-consuming methods, which do not meet the needs of sustainable development.

[0003] Lignin, the second most abundant natural polymer, possesses a wealth of active functional groups that can enhance the mechanical and electrical properties of hydrogels through multiple interactions. For example, the introduction of lignin sulfonate (LS) can promote hydrogen bond formation and crystallization induction, significantly improving the toughness of the material. However, existing research often relies on energy-intensive processes such as freeze-thaw cycles or high-temperature polymerization, which can easily lead to limited reaction rates and structural inhomogeneities.

[0004] Rapid gelation technology based on polyphenol-metal coordination offers a new approach to solving the above problems. For example, LS and Fe 3+ The redox coordination of lignin can form multifunctional hydrogels under mild conditions, exhibiting both electrical conductivity and UV resistance. However, it is important to note that when the lignin content is high (≥1.5 wt%), its inherent free radical scavenging properties significantly inhibit free radical accumulation, hindering the polymerization reaction. This drawback limits the doping ratio of lignin in high-performance hydrogels, thus affecting the improvement of the overall material performance. Summary of the Invention

[0005] To address the problems of high energy consumption, long processing time, limited mechanical properties, and polymerization inhibition caused by lignin free radical scavenging under high lignin content conditions in existing gel preparation processes, this invention provides a self-initiated polymerized lignin-based thermosensitive gel, its preparation method, and its application.

[0006] The technical solution of the present invention: A self-initiated polymerized lignin-based thermosensitive gel comprises the following components in parts by weight: 0.2-0.4 parts lignin sulfonate, 2.4 parts acrylic acid, and 1.6 parts N-isopropylacrylamide.

[0007] A method for preparing a self-initiated polymerized lignin-based thermosensitive gel, comprising the following steps: Step 1: Preparation of lignin sulfonate esters: Lignosulfonate was dissolved in dimethyl sulfoxide to obtain a ligninsulfonate solution. Maleic anhydride was added to the ligninsulfonate solution and stirred until completely dissolved. Then, 1-methylimidazole catalyst was added in batches. The esterification modification reaction was completed under heating and stirring conditions. The resulting reaction solution was added dropwise to anhydrous ethanol to precipitate. After centrifugation, washing and vacuum drying, ligninsulfonate ester was obtained. Step 2: Preparation of lignin-based thermosensitive gel: The lignin sulfonate obtained in step one was placed in deionized water and stirred in an ice bath until completely dissolved. Acrylic acid and N-isopropylacrylamide were added to the lignin sulfonate solution and stirred in an ice bath until evenly dispersed. Crosslinking agent and initiator were added in sequence and stirred evenly in an ice bath. A salt solution containing polyvalent metal ions was added and stirred for 12-15 seconds before being poured into a mold. Self-initiated polymerization was carried out at room temperature to obtain lignin-based thermosensitive gel.

[0008] Furthermore, the concentration of the lignin sulfonate solution in step one is 10 wt%, and the amount of maleic anhydride added is calculated based on the molar ratio of aliphatic hydroxyl groups in the lignin sulfonate to maleic anhydride being 1:3.33.

[0009] Furthermore, in step one, the amount of 1-methylimidazole catalyst added is calculated based on a mass-to-volume ratio of lignin sulfonate to 1-methylimidazole of 5 g: 1 mL, and 1-methylimidazole is added in 10 equal portions; the temperature of the esterification modification reaction is 60 °C, and the reaction time is 3 h.

[0010] Furthermore, the volume of anhydrous ethanol in step one is 10 times the volume of the reaction liquid, the washing is performed with anhydrous ethanol for a total of 3 times, and the vacuum drying is performed at 50°C for 24 hours.

[0011] Furthermore, in step two, the mass-to-volume ratio of lignin sulfonate ester, deionized water, acrylic acid, N-isopropylacrylamide, crosslinking agent, initiator, and salt solution containing polyvalent metal ions is 0.2~0.4g:8mL:2.4g:1.6g:0.01g:0.1g:2mL.

[0012] Furthermore, the salt solution containing polyvalent metal ions mentioned in step two is FeCl3 solution, ZnCl2 solution, or AlCl3 solution, and the molar concentration of the metal ions in the salt solution containing polyvalent metal ions is 0.074 mmol / mL.

[0013] Furthermore, the crosslinking agent in step two is one of N,N'-methylenebisacrylamide, polyethylene glycol diacrylate, or polyethylene glycol dimethacrylate, and the initiator is one of ammonium persulfate, potassium persulfate, or sodium persulfate.

[0014] Furthermore, the self-initiated polymerization time described in step two is 20~70s.

[0015] An application of a self-initiated polymerized lignin-based thermosensitive gel in the field of sensors, including smart wearable medical diagnostic monitoring sensors, ambient temperature detection sensors, and human-computer interaction interface temperature monitoring sensors.

[0016] The beneficial effects of this invention are: 1. Highly efficient and energy-saving room temperature rapid polymerization technology This invention achieves ultra-fast gelation at room temperature (20-70 seconds) by constructing a synergistic redox autocatalytic system of lignin sulfonate ester (LSME), multivalent metal ions, and ammonium persulfate (APS). This system utilizes the quinone structure formed by the complexation of LSME with multivalent metal ions. During this process, the multivalent metal ions undergo a redox reaction with the initiator ammonium persulfate (APS), promoting the decomposition of APS to generate sulfate radicals (SO42-). - ·), and further react with water to generate hydroxyl radicals (·OH) and singlet oxygen (·). 1 Reactive oxygen species such as O2. These free radicals can participate in chain initiation and chain growth reactions in the system, thereby promoting the rapid polymerization of LSME, acrylic acid (AA), and N-isopropylacrylamide (NIPAM) monomers. The polymerization rate can be precisely controlled by adjusting the LSME content (5-10 wt%).

[0017] The synergistic catalytic system provided by this invention enables efficient gelation under low energy consumption conditions, significantly improving polymerization efficiency and the uniformity of network structure. The method of this invention has advantages such as simple and controllable process, inexpensive and readily available raw materials, environmental friendliness, and good biocompatibility. It not only provides a new pathway for the high-value utilization of lignin but also opens up a new technical route for the rapid preparation of multifunctional hydrogels, which can be further used to prepare flexible functional materials with self-healing properties, excellent conductivity, and temperature response performance.

[0018] 2. Composite structure with multiple dynamic network collaborative enhancements LSME exhibits a dual function in the system: on one hand, it participates in free radical generation as a redox co-initiator; on the other hand, the unsaturated double bonds in its molecules can copolymerize with AA and NIPAM to form a three-dimensional cross-linked network. Simultaneously, the polyphenolic groups in the LSME molecule can react with polyvalent metal ions (Fe2+, Fe ... 3+ Zn 2+ Or Al 3+ A dynamic coordination bond network is formed, thereby constructing a dual network structure that combines chemical covalent bonds and physical dynamic bonds, giving the resulting hydrogel excellent mechanical strength, electrical conductivity and temperature response.

[0019] 3. High-precision flexible sensing and multi-scale motion monitoring The lignin-based thermosensitive gel prepared by this invention can be used as a flexible strain sensor to detect multi-scale motion signals of the human body (such as finger joint bending, limb stretching, etc.). It can also produce a stable and repeatable electrical response to minute deformations, realizing highly sensitive human motion monitoring and providing a highly reliable sensing material for wearable electronic devices.

[0020] 4. Wide-temperature-range intelligent response and stable thermal signal output The lignin-based thermosensitive gel prepared in this invention can rapidly respond to changes in human body surface temperature over a wide temperature range, with a stable and reversible resistance signal, and can be used for real-time temperature detection and physiological signal acquisition. The sensor constructed from this lignin-based thermosensitive gel can detect temperature changes in the human body over a wide temperature range and stably output a resistance signal. It can be applied in fields such as wound fever early warning and intelligent temperature-controlled dressings, achieving long-term stable monitoring of physiological thermal signals, demonstrating good reliability and application potential. Attached Figure Description

[0021] Figure 1 The images shown are physical and microscopic images of the LS and LSME samples in Example 1. (a) is a physical image of the LS sample, (b) is a physical image of the LSME sample, (c) is a physical image of the water-soluble LS sample, (d) is a physical image of the water-soluble LSME sample, (e) is a SEM image of the LS sample, and (f) is a SEM image of the LSME sample. Figure 2 The following are the 1H NMR characterization diagrams of LS and LSME in Example 1: (a) from left to right are schematic diagrams of the chemical structures of p-nitrobenzaldehyde, LS and LSME, the internal standard for NMR testing; (b) is the 1H NMR spectrum of LS; and (c) is the 1H NMR spectrum of LSME. Figure 3 The following are the carbon NMR characterization diagrams of LS and LSME in Example 1: (a) from left to right are schematic diagrams of the basic structural units of the NMR test internal standard p-nitrobenzaldehyde, LS and LSME; (b) is the 13C NMR spectrum of LS; and (c) is the 13C NMR spectrum of LSME. Figure 4 The infrared spectra of LS and LSME in Example 1 are shown in (a) and (b) are shown in (4000-400 cm⁻¹) infrared spectra of LS and LSME. -1 (b) shows the infrared spectra of LS and LSME (2000-400 cm⁻¹). -1 ); Figure 5 The XPS spectra of LS and LSME in Example 1 are shown in (a) and (b) respectively. The XPS spectra of LS and LSME are shown in (c) respectively. Figure 6 Infrared spectra (4000-400 cm⁻¹) of AA, NIPAM, LSME, the gel prepared in Comparative Example 1, and the thermosensitive gel prepared in Example 2. -1 ); Figure 7 The XPS spectra of the thermosensitive gel prepared in Example 2 are shown in (a) as the full XPS spectrum, (b) as the C1s XPS fine spectrum, (c) as the O1s XPS fine spectrum, (d) as the N1s XPS fine spectrum, and (e) as the S2p XPS fine spectrum. Figure 8 The following are comparison charts showing the tensile properties of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 1-3: (a) is the tensile stress-strain curve, (b) is the tensile stress bar chart, (c) is the tensile strain bar chart, (d) is the stress-strain curve of the gel compressed to 80%, (e) is the stress bar chart of the gel compressed to 80%, and (f) is the stress-strain curve of the gel compressed to 60% after 10 cycles. Figure 9 The following are comparative graphs showing the thermal response performance of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 1-3 before and after heating on a hot stage at 37°C for 5 min: (a) is a comparison graph of gel weight loss rate; (b) is a comparison graph of gel size change rate; (c) from left to right are the actual dimensions of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 2-3 before heating; (d) from left to right are the actual dimensions of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 2-3 after heating. Figure 10 Comparison of the performance of freeze-dried samples of gel prepared in Comparative Example 1 before and after heating at 37℃ for 5 min: (a) SEM image of sample before heating, (b) SEM image of sample after heating, (c) pore size distribution of sample before heating, (d) pore size distribution of sample after heating. Figure 11 The following are comparison images of the freeze-dried samples of the thermosensitive gel prepared in Example 2 before and after heating at 37°C for 5 min: (a) is the SEM image of the sample before heating, (b) is the SEM image of the sample after heating, (c) is the pore size distribution of the sample before heating, and (d) is the pore size distribution of the sample after heating. Figure 12 The figures show a comparison of the swelling properties of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 1-3; (a) is the swelling curve of the gel sample in deionized water, (b) is the dimensional change rate of the gel sample after swelling, and (c) is the swelling kinetic curve of the gel sample. Figure 13The images show a comparison of the actual dimensions of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 2-3 before and after swelling; (a) from left to right: actual dimensions of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 2-3 before swelling; (b) from left to right: actual dimensions of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 2-3 after swelling. Figure 14 A comparison of the conductivity of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 1-3 is shown in the figure. Figure 15 The ΔR / R0 curves of the thermosensitive gel prepared in Example 2 under different strains are shown in (a) for 50%, 75%, and 100% tensile strain, and (b) for 10%, 40%, and 60% compressive strain. Figure 16 The following are the ΔR / R0 curves of the thermosensitive gel prepared in Example 2 under human motion strain at different scales: (a) knee flexion, (b) upper arm flexion, (c) neck flexion, (d) wrist flexion, (e) finger flexion, and (f) Adam's apple vibration. Figure 17 A comparison of the conductivity of the thermosensitive gel prepared in Example 2 at different temperatures; Figure 18 The images show the actual temperature response of the thermosensitive gel prepared in Example 2, where (a) is for water bath temperature ≤ 36℃ and (b) is for water bath temperature > 37℃. Figure 19 This is a photograph of the thermal response of the thermosensitive gel prepared in Example 2 to human body temperature. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0023] Example 1 This invention provides a method for preparing lignin sulfonate esters, the preparation steps of which are as follows: Weigh 5g of LS and dissolve it in 50mL of dimethyl sulfoxide to prepare a 10wt% LS solution; analyze the solution by nuclear magnetic resonance spectroscopy (1H NMR spectroscopy). 1The aliphatic hydroxyl content in LS was determined by ¹H NMR. Based on the molar ratio of aliphatic hydroxyl to maleic anhydride in LS of 1:3.33, 0.066 mol of maleic anhydride was added to the LS solution and stirred until completely dissolved. 1 mL of 1-methylimidazole was added to the LS solution as a catalyst, calculated at a mass-to-volume ratio of lignin sulfonate to catalyst of 5 g:1 mL. The catalyst was added in 10 portions of 0.1 mL each, calculated at a mass-to-volume ratio of lignin sulfonate to 1-methylimidazole of 5 g:1 mL. The esterification modification reaction was then completed by heating and stirring at 60 °C for 3 h. After the reaction was completed, the resulting reaction solution was slowly added dropwise to 10 times its volume of anhydrous ethanol for precipitation. The precipitate was then collected by centrifugation and washed with anhydrous ethanol, 100 mL each time, for a total of 3 washes. The precipitate was then vacuum dried at 50 °C for 24 h to obtain LSME, which was collected in a sample vial for later use.

[0024] In this embodiment, LS quantification 1 The H NMR testing conditions are as follows: 60 ± 1 mg of LS or LSME and 6 ± 1 mg of internal standard p-nitrobenzaldehyde (NBA) were quantitatively dissolved in 0.6 mL of deuterated dimethyl sulfoxide (DMSO-d6), and spectral data were acquired on a Bruker Avance III HD 500 MHz NMR spectrometer. The LS hydroxyl content was calculated using formula (1), and the peak area was normalized before calculation (using the proton hydrogen peak of the benzene ring of NBA as a reference): (1) In formula (1) F The content is hydroxyl content (mmol / g). I The peak area corresponding to the hydroxyl group. W NBA For the quality of the NBA, 4 represents the number of proton hydrogen atoms on the benzene ring of the NBA. I NBA The area of ​​the NBA score (after normalization, the area is 1). W LS 151 is the relative molecular mass of NBA, and 1000 is the conversion factor from mmol / mg to mmol / g.

[0025] Figure 1 The images shown are physical and microscopic images of the LS and LSME samples in Example 1. Figure 1 As shown, the microstructure demonstrates the success of the esterification modification.

[0026] Figure 2 and Figure 3 The images shown are, in order, the 1H NMR spectrum and the 1C NMR spectrum of the LS and LSME samples from Example 1. Figure 2and Figure 3 The results indicate that the aliphatic hydroxyl signal in LS was significantly weakened or disappeared after esterification, while new C=C, ester carbonyl and acryloyl-CH3 peaks appeared, while the aromatic hydroxyl groups remained unchanged, which fully proves that the unsaturated ester has been successfully grafted onto the lignin skeleton.

[0027] Figure 4 and Figure 5 The images shown are, in order, the infrared and XPS spectra of LS and LSME from Example 1. Figure 4 and Figure 5 Together, they demonstrate that ATR-IR shows reduced hydroxyl absorption and the appearance / enhancement of characteristic peaks for ester C=O, C–O, and C=C, while XPS shows a positive shift of the O–C=O peak and a weakening of the C–O signal, which together prove that maleic anhydride / methacrylic anhydride was successfully esterified and grafted onto LS.

[0028] Figure 1-5 This demonstrates the successful esterification modification of lignin sulfonate. Based on macroscopic and microscopic morphology, combined with characterization methods such as 1H NMR, 1C NMR, IR, and XPS, the successful preparation of lignin sulfonate is confirmed.

[0029] Example 2 This embodiment provides a method for preparing a lignin-based thermosensitive gel with an LSME content of 5 wt%. The preparation steps are as follows: Weigh 0.2g of LSME prepared in Example 1 and place it in 8mL of deionized water. Stir in an ice-water bath until completely dissolved to obtain a homogeneous LSME aqueous solution. Weigh 2.4g of AA and 1.6g of NIPAM and add them to the LSME aqueous solution and dissolve in an ice-water bath. Then, add 0.01g of crosslinking agent N,N'-methylenebisacrylamide MBA and 0.1g of initiator APS in sequence and stir until a homogeneous solution is formed. Add 2mL of FeCl3 solution with a molar concentration of 0.074 mmol / mL to the obtained homogeneous mixed solution. After stirring for 15s, quickly inject the mixture into a mold and allow it to undergo a redox autocatalytic polymerization reaction at room temperature for 50s to obtain a lignin-based thermosensitive gel with an LSME content of 5wt%.

[0030] Example 3 This embodiment provides a method for preparing a lignin-based thermosensitive gel with an LSME content of 10 wt%. The preparation steps are as follows: Weigh 0.4g of LSME prepared in Example 1 and place it in 8mL of deionized water. Stir in an ice-water bath until completely dissolved to obtain a homogeneous LSME aqueous solution. Weigh 2.4g of AA and 1.6g of NIPAM and add them to the LSME aqueous solution and dissolve in an ice-water bath. Then, add 0.01g of crosslinking agent N,N'-methylenebisacrylamide MBA and 0.1g of initiator APS in sequence and stir until a homogeneous solution is formed. Add 2mL of FeCl3 solution with a molar concentration of 0.074 mmol / mL to the obtained homogeneous mixed solution. After stirring for 15s, quickly inject the mixture into a mold and allow it to undergo a redox autocatalytic polymerization reaction at room temperature for 42s to obtain a lignin-based thermosensitive gel with an LSME content of 10wt%.

[0031] Example 4 This embodiment provides a method for preparing a lignin-based thermosensitive gel with an LSME content of 5 wt%. The preparation steps are as follows: Weigh 0.2g of LSME prepared in Example 1 and place it in 8mL of deionized water. Stir in an ice-water bath until completely dissolved to obtain a homogeneous LSME aqueous solution. Weigh 2.4g of AA and 1.6g of NIPAM and add them to the LSME aqueous solution and dissolve in an ice-water bath. Then, add 0.01g of crosslinking agent N,N'-methylenebisacrylamide MBA and 0.1g of initiator APS in sequence and stir until a homogeneous solution is formed. Add 2mL of ZnCl2 solution with a molar concentration of 0.074 mmol / mL to the obtained homogeneous mixed solution. After stirring for 15s, quickly inject the mixture into a mold and allow it to undergo a redox autocatalytic polymerization reaction at room temperature for 55s to obtain a lignin-based thermosensitive gel with an LSME content of 5wt%.

[0032] Example 5 This embodiment provides a method for preparing a lignin-based thermosensitive gel with an LSME content of 5 wt%. The preparation steps are as follows: 0.2 g of LSME prepared in Example 1 was weighed and placed in 8 mL of deionized water. The mixture was stirred in an ice-water bath until completely dissolved to obtain a homogeneous LSME aqueous solution. 2.4 g of AA and 1.6 g of NIPAM were weighed and added to the LSME aqueous solution and dissolved in an ice-water bath. Then, 0.01 g of crosslinking agent N,N'-methylenebisacrylamide MBA and 0.1 g of initiator APS were added sequentially and stirred until a homogeneous solution was formed. 2 mL of AlCl3 solution with a molar concentration of 0.074 mmol / mL was added to the resulting homogeneous mixed solution. After stirring for 15 s, the mixture was quickly injected into a mold. A redox autocatalytic polymerization reaction occurred at room temperature for 63 s, resulting in a lignin-based thermosensitive gel with an LSME content of 5 wt%.

[0033] Comparative Example 1 This comparative example provides a method for preparing a lignin-based gel with an LS content of 5 wt%, the preparation steps of which are as follows: 0.2 g of LS was weighed and placed in 8 mL of deionized water. The mixture was stirred in an ice-water bath until completely dissolved to obtain a homogeneous LS aqueous solution. 2.4 g of AA and 1.6 g of NIPAM were weighed and added to the LS aqueous solution and dissolved in an ice-water bath. Then, 0.01 g of crosslinking agent N,N'-methylenebisacrylamide MBA and 0.1 g of initiator APS were added sequentially and stirred until a homogeneous solution was formed. 2 mL of FeCl3 solution with a molar concentration of 0.074 mmol / mL was added to the resulting homogeneous solution. After stirring for 15 s, the mixture was quickly injected into a mold. A redox autocatalytic polymerization reaction was carried out at room temperature for 120 s to obtain a lignin-based thermosensitive gel with an LS content of 5 wt%.

[0034] Comparative Example 2 This comparative example provides a method for preparing a lignin-based thermosensitive gel with an LSME content of 15 wt%. The preparation steps are as follows: 0.6 g of LSME prepared in Example 1 was weighed and placed in 8 mL of deionized water. The mixture was stirred in an ice-water bath until completely dissolved to obtain a homogeneous LSME aqueous solution. 2.4 g of AA and 1.6 g of NIPAM were weighed and added to the LSME aqueous solution and dissolved in an ice-water bath. Then, 0.01 g of crosslinking agent N,N'-methylenebisacrylamide MBA and 0.1 g of initiator APS were added sequentially and stirred until a homogeneous solution was formed. 2 mL of FeCl3 solution with a molar concentration of 0.074 mmol / mL was added to the resulting homogeneous mixed solution. After stirring for 15 s, the mixture was quickly injected into a mold. A redox autocatalytic polymerization reaction occurred at room temperature for 25 s, resulting in a lignin-based thermosensitive gel with an LSME content of 15 wt%.

[0035] Comparative Example 3 This comparative example provides a method for preparing a lignin-based thermosensitive gel with an LSME content of 20 wt%. The preparation steps are as follows: 0.8 g of LSME prepared in Example 1 was weighed and placed in 8 mL of deionized water. The mixture was stirred in an ice-water bath until completely dissolved to obtain a homogeneous LSME aqueous solution. 2.4 g of AA and 1.6 g of NIPAM were weighed and added to the LSME aqueous solution and dissolved in an ice-water bath. Then, 0.01 g of crosslinking agent N,N'-methylenebisacrylamide MBA and 0.1 g of initiator APS were added sequentially and stirred until a homogeneous solution was formed. 2 mL of FeCl3 solution with a molar concentration of 0.074 mmol / mL was added to the resulting homogeneous mixed solution. After stirring for 15 s, the mixture was quickly injected into a mold. A redox autocatalytic polymerization reaction occurred at room temperature for 20 s, resulting in a lignin-based thermosensitive gel with an LSME content of 20 wt%.

[0036] The mechanical properties and conductivity of the gel materials prepared in Examples 2-5 and Comparative Examples 1-3 were tested, and the results are shown in Table 1.

[0037] Table 1

[0038] A comparison of the data from Examples 2-3 and Comparative Examples 2-3 in Table 1 shows that as the LSME content increases, the polymerization time of the gel decreases from 50 s to 20 s. This indicates that increasing the LSME content effectively improves the polymerization rate and shortens the polymerization time. The tensile strength and tensile strain rate of Examples 2-3 are significantly higher than those of Comparative Examples 2-3, indicating that appropriately increasing the LSME content can significantly improve the mechanical properties of the gel material. However, when the LSME content is too high, the tensile strength and tensile strain rate of the gel decrease significantly. This is because excessive LSME disrupts the cross-linking network of the gel, thus affecting its mechanical properties. The electrical conductivity data shows that the electrical conductivity of Example 2 is significantly higher than that of Comparative Examples 1-3, indicating that the introduction of LSME significantly improves the electrical conductivity of the gel material. In summary, lignin-based thermosensitive gels with an LSME content of 5-10 wt% exhibit excellent mechanical and electrical properties and have high application value.

[0039] Figure 6 Infrared spectra of the gels prepared for AA, NIPAM, LSME, Comparative Example 1, and the thermosensitive gel prepared for Example 2 are shown below. Figure 6 As shown, FT-IR reveals a significant decrease in the C=C stretching peak at 1712 cm⁻¹. -1 The broadened carbonyl peak was enhanced, the O–H / N–H absorption was weakened, and the S–O characteristic peak appeared, indicating that the hydrogel polymerization was completed and LSME was successfully introduced into the network.

[0040] Figure 7 The XPS spectrum of the thermosensitive gel prepared in Example 2 is shown below. Figure 7As shown, C1s, O1s, and N1s in XPS exhibit typical C–N, C=O, and amide bonding morphologies, confirming the successful copolymerization of AA and NIPAM; the presence of the S=O characteristic peak in S2p further indicates that LSME has been effectively introduced into the hydrogel network.

[0041] Figure 8 The graph shows a comparison of the tensile properties of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 1-3. Figure 8 As shown, the appropriate introduction of LSME significantly improves the tensile and compressive strength of the hydrogel and maintains good recovery under cyclic loading, exhibiting excellent toughness and energy dissipation capacity. However, when the LSME content is too high, the network continuity is disrupted, stress concentration is aggravated, and the mechanical properties deteriorate. Mechanistically, the unsaturated double bonds in LSME participate in copolymerization to form additional covalent cross-linking points. Its hydrophilic groups such as –SO3H and –OH, as well as aromatic structures, construct multiple reversible physical cross-links through hydrogen bonding, electrostatic interactions, and π–π stacking, achieving a synergistic effect of strengthening and energy dissipation. Overall, this system outperforms most existing PNIPAM-based hydrogels in terms of both high strength and large elongation, providing a solid material foundation for wearable sensing and human motion monitoring.

[0042] Figure 9 The graph shows a comparison of the thermal response properties of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 1-3 before and after heating on a hot stage at 37°C for 5 minutes. Figure 9 As shown, at 37°C, an appropriate amount (approximately 5%) of LSME significantly improved the weight loss and dimensional change rate of the hydrogel, resulting in a more sensitive thermal response. The mechanism is that the hydrophilic groups of LSME facilitate hydration at low temperatures, while the aromatic rings and unsaturated structures enhance hydrophobic association and π-π stacking above the LCST, thereby promoting dehydration and network reconstruction. However, as the LSME content continues to increase, excessive crosslinking and dense aggregation restrict chain segment movement and water migration, leading to a decrease in thermal response indicators. This indicates that only at a moderate dosage can the optimal balance between hydrophilic-hydrophobic interactions and dynamic physical crosslinking be achieved.

[0043] Figure 10 and Figure 11 The figures show a comparison of the freeze-dried sample properties of the gel prepared in Comparative Example 1 and the thermosensitive gel prepared in Example 2 before and after heating on a hot stage at 37°C for 5 minutes. Figure 10 and Figure 11As shown, SEM results indicate that both hydrogels underwent significant pore structure shrinkage before and after heating: the gel in Comparative Document 1 shrank from a loose porous network at room temperature to a denser structure, while the thermosensitive gel in Example 2 exhibited a denser microporous network in its initial state, with the pore size further decreasing and the cross-linking becoming more compact after heating. Combining pore size distribution statistics and schematic diagrams, the reversible transition from a hydration-expanded state to a dehydration-shrinked state during heating is clearly visible, and the introduction of LSME makes this thermoinduced shrinkage behavior more significant and controllable.

[0044] Figure 12 and Figure 13 This is a comparison chart showing the swelling properties of the thermosensitive gels prepared in Examples 2-3 and the gels prepared in Comparative Examples 1-3; as shown. Figure 12 and Figure 13 The equilibrium swelling results show that the hydrogel swells rapidly in deionized water in the initial stage after the addition of LSME, and the overall swelling capacity is significantly improved. 5 wt% LSME not only enhances the hydration capacity, but also maintains good network flexibility and swelling kinetics. As the LSME content further increases, although the equilibrium swelling ratio continues to rise and the volume expands significantly, the formation of dense microregions induced by hydrophobicity and π–π interactions slows down the chain segment relaxation. The swelling process tends to be "larger but slower", reflecting the transformation of the network structure from "moderately flexible" to "high expansion and low kinetics".

[0045] Figure 14 This is a comparison chart of the conductivity of the thermosensitive gels prepared in Examples 2-5; as shown. Figure 14 As shown, conductivity tests revealed that the hydrogel conductivity initially increased significantly with the introduction of LSME, reaching a maximum at 5 wt%, and then gradually decreased with further increases in content. This indicates that an appropriate amount of lignin-derived hydrophilic / ionic groups in LSME is beneficial for constructing continuous hydrated ion channels and enhancing ion migration capacity, while excessive amounts result in an overly dense network or localized aggregation, thereby weakening conductivity.

[0046] Figure 15 The ΔR / R0 curves of the thermosensitive gel prepared in Example 2 under different strains are shown below. Figure 15 The strain test results show that the relative resistance of the L5%AN hydrogel increases monotonically with strain during both tension and compression, indicating that it is extremely sensitive to deformation and exhibits a stable signal response, capable of simultaneously identifying tensile and compressive loads. Combined with its high conductivity and good cyclic stability, this hydrogel holds promise as a wearable strain sensor for real-time monitoring of both large-amplitude and subtle human movements.

[0047] Figure 16 The ΔR / R0 curves of the thermosensitive gel prepared in Example 2 under human motion strain at different scales are shown below. Figure 16The human wearing test shown demonstrates that the hydrogel sensor can stably recognize various large-amplitude movements, including knee flexion, arm, neck, wrist, and fingers. It also responds well to subtle vibrations of the larynx during vocalization, distinguishing the vocal cord vibration characteristics corresponding to different sentences. Overall, the sensor is highly sensitive to changes in amplitude and velocity, and its signal output is clear and stable, laying a solid foundation for its application in wearable real-time monitoring of human movement and physiological signals.

[0048] Figure 17 This is a comparison graph showing the conductivity of the thermosensitive gel prepared in Example 2 at different temperatures. Figure 17 As shown, the electrical conductivity of the hydrogel continuously increases with increasing temperature, exhibiting a significant and predictable temperature dependence. Heating accelerates the migration of free ions and lowers the ion transport energy barrier, giving the system excellent thermoelectric response characteristics, making it very suitable as a temperature sensing element.

[0049] Figure 18 The image shows the actual temperature response of the thermosensitive gel prepared in Example 2. Figure 18 The device demonstration results show that the hydrogel can achieve a clear "off-flicker-on" LED switching behavior through resistance changes when approaching the response temperature range: the resistance is high at lower temperatures and the circuit is open; the signal begins to transition when approaching LCST; and the resistance drops significantly at higher temperatures and the circuit is open. This clear temperature-controlled switching characteristic intuitively verifies the application potential of hydrogel as a thermal triggering element and temperature threshold sensor.

[0050] Figure 19 The image shows the thermal response of the thermosensitive gel prepared in Example 2 to human body temperature. Figure 19 The body temperature monitoring demonstration shown indicates that the circuit remains closed when the hydrogel is applied to the skin at room temperature, but the LED lights up immediately and the response is reversible after contact with a preheated glove. This demonstrates that it can provide rapid and clear electrical signal feedback for minute temperature differences close to human body temperature, and is feasible as a wearable real-time body temperature sensing and early warning element.

Claims

1. A self-initiated polymerized lignin-based thermosensitive gel, characterized in that, It includes the following components in parts by weight: 0.2-0.4 parts lignin sulfonate, 2.4 parts acrylic acid and 1.6 parts N-isopropylacrylamide; The preparation method of the self-initiated polymerized lignin-based thermosensitive gel includes the following steps: Step 1: Preparation of lignin sulfonate esters: Lignosulfonate was dissolved in dimethyl sulfoxide to obtain a ligninsulfonate solution. Maleic anhydride was added to the ligninsulfonate solution, and the amount of maleic anhydride added was calculated according to the molar ratio of aliphatic hydroxyl groups in ligninsulfonate to maleic anhydride of 1:3.

33. The mixture was stirred until completely dissolved, and then 1-methylimidazole catalyst was added in batches. The esterification modification reaction was completed under heating and stirring conditions. The resulting reaction solution was added dropwise to anhydrous ethanol to precipitate the lignosulfonate ester after centrifugation, washing and vacuum drying. Step 2: Preparation of lignin-based thermosensitive gel: The lignin sulfonate obtained in step one was placed in deionized water and stirred in an ice bath until completely dissolved. Acrylic acid and N-isopropylacrylamide were added to the lignin sulfonate solution and stirred in an ice bath until evenly dispersed. Crosslinking agent and initiator were added in sequence and stirred evenly in an ice bath. A salt solution containing polyvalent metal ions was added and stirred for 12-15 seconds before being poured into a mold. Self-initiated polymerization was carried out at room temperature to obtain lignin-based thermosensitive gel.

2. The self-initiated polymerized lignin-based thermosensitive gel according to claim 1, characterized in that, The concentration of the lignin sulfonate solution in step one is 10 wt%.

3. The method for preparing a self-initiated polymerized lignin-based thermosensitive gel according to claim 2, characterized in that, The amount of 1-methylimidazole catalyst added in step one is calculated according to the mass-volume ratio of lignin sulfonate to 1-methylimidazole of 5g:1mL, and 1-methylimidazole is added in 10 equal portions; the temperature of the esterification modification reaction is 60℃ and the reaction time is 3h.

4. The self-initiated polymerized lignin-based thermosensitive gel according to claim 3, characterized in that, The volume of anhydrous ethanol in step one is 10 times the volume of the reaction liquid. The washing is performed with anhydrous ethanol for a total of 3 times. The vacuum drying is performed at 50°C for 24 hours.

5. The self-initiated polymerized lignin-based thermosensitive gel according to claim 4, characterized in that, In step two, the mass-to-volume ratio of lignin sulfonate ester, deionized water, acrylic acid, N-isopropylacrylamide, crosslinking agent, initiator, and salt solution containing polyvalent metal ions is 0.2~0.4g:8mL:2.4g:1.6g:0.01g:0.1g:2mL.

6. The self-initiated polymerized lignin-based thermosensitive gel according to claim 5, characterized in that, The salt solution containing polyvalent metal ions mentioned in step two is FeCl3 solution, ZnCl2 solution, or AlCl3 solution, and the molar concentration of the metal ions in the salt solution containing polyvalent metal ions is 0.074 mmol / mL.

7. The self-initiated polymerized lignin-based thermosensitive gel according to claim 6, characterized in that, The crosslinking agent in step two is one of N,N'-methylenebisacrylamide, polyethylene glycol diacrylate, or polyethylene glycol dimethacrylate, and the initiator is one of ammonium persulfate, potassium persulfate, or sodium persulfate.

8. The self-initiated polymerized lignin-based thermosensitive gel according to claim 7, characterized in that, The self-initiated polymerization time described in step two is 20~70s.

9. The application of the self-initiated polymerized lignin-based thermosensitive gel as described in claim 1 in the field of sensors, characterized in that, The sensors include intelligent wearable medical diagnostic monitoring sensors, ambient temperature detection sensors, and human-computer interaction interface temperature monitoring sensors.

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