Preparation method and application of cellulose-based ion gel

Abstract

The present invention relates to a preparation method and application of a cellulose-based ion gel with high ionic conductivity, self-adhesion, self-repairing, antifreeze and excellent mechanical strength. The present invention uses imidazole-type ionic liquid as a raw material, mixes with acrylamide, and then heats and stirs to obtain a new type of low eutectic solvent. The cellulose is first dissolved in the imidazole-type ionic liquid by a step-by-step dissolution method, and then fully mixed with the prepared low eutectic solvent. The cellulose-based ion gel is prepared by photoinitiator photoreaction cross-linking and curing under ultraviolet light. The ion gel prepared by the present invention not only has good mechanical properties (tensile stress up to 1.02MPa), but also has good toughness and self-repairing properties. At the same time, the low eutectic solvent is used as a dispersion medium to make the gel have excellent electrical conductivity (conductivity can reach 0.41mS / cm at room temperature), antifreeze performance (can maintain stable performance at ‑20°C), self-adhesion and excellent strain sensitivity, and has broad application prospects in the field of flexible electronic devices.

Description

Technical Field

[0001] The invention belongs to the technical field of ion gels, and relates to a preparation method and application of a cellulose-based ion gel with high ionic conductivity, self-adhesion, self-repairing, antifreeze property and excellent mechanical strength. Background Art

[0002] Ionic liquid is a room temperature molten salt with good electrical conductivity and thermal stability. It can act as a green solvent for the synthesis of organic polymer materials. For example, in the field of gel, ionic liquid is an excellent conductive dispersion medium. Ionic gels can be synthesized by dispersing polymer monomers in ionic liquids or polymerizing polyionic liquids themselves. Compared with traditional hydrogels, ionic gels have excellent electrical conductivity, viscoelasticity, thermal stability, mechanical properties and fatigue resistance, and have received more and more attention and research in recent years. Ionic liquids also have certain defects as solvents, because most ionic liquids have high viscosity, poor fluidity, and are even solid at room temperature, which creates certain difficulties for the dissolution of polymers and their application at low temperatures after gelation. The emergence of deep eutectic solvents has solved these problems very well. As a new type of ionic liquid, deep eutectic solvent (DES) is a eutectic mixture prepared by mixing hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) at a higher temperature. It has the properties of ionic liquids such as low vapor pressure, good solubility, and wide electrochemical stability window. It also has unique properties such as simple preparation, non-toxicity, low melting point, and biodegradability, which provides a good solvent choice for synthesizing gels. In the synthesis of gels, deep eutectic solvents are dispersed in the three-dimensional network structure of polymers as a dispersion medium. They can not only build a hydrogen bond system with polymers to improve the stability and adhesion of gels, but also greatly reduce the melting point of gels, thereby improving the antifreeze property of gels and enriching their practical applicability in extreme environments. Therefore, deep eutectic solvents can be used as an excellent solvent for synthesizing gels.

[0003] At present, with the pollution of petroleum-based materials and the increasing depletion of petroleum resources, biomass resources have attracted much attention, and the combination of biomass materials and gel is a bright spot. As one of the most abundant natural resources in nature, cellulose has the characteristics of wide sources, good biocompatibility and renewable and degradable, and has a relatively broad application prospect in many fields. In the field of gel, functional gels based on cellulose are increasingly valued. Compared with gels synthesized from petroleum processed products, cellulose-based gels are more green, economical, and environmentally friendly, in line with the concept of sustainable development. However, most of the low eutectic solvents or poly-low eutectic solvents (PDES) synthesized with choline chloride as hydrogen bond acceptors reported at present have poor dissolving effects on biomass materials. The present invention adopts a two-step method to dissolve cellulose, that is, firstly using imidazole-type ionic liquids as hydrogen bond acceptors and acrylamide as hydrogen bond donors to synthesize low eutectic solvents, and using imidazole-type ionic liquids to dissolve cellulose, and then fully mixing the dissolved cellulose solution with the low eutectic solvent, thereby achieving the dissolution of cellulose in the low eutectic solvent. Since cellulose contains a large number of hydroxyl groups in its molecular structure, it can form abundant hydrogen bonds with polar groups in the ionic liquid used as a solvent, thereby building a strong gel system, improving the mechanical properties, toughness and self-healing properties of the gel, and significantly improving the performance of the prepared ion gel.

[0004] The working principle of flexible strain sensors is to convert external stimulus signals into recognizable electrical signals. As a flexible material, ion gel has excellent ionic conductivity, strain sensitivity and flexibility. It can easily adhere to the human epidermis without external force. Therefore, it can be used as a wearable strain sensor to monitor subtle changes in the wearer's body movement. It has broad application prospects in the fields of intelligent robots, electronic skin, medical health monitoring and wearable electronics. As a flexible functional polymer material, cellulose-based ion gel has good flexibility, adhesion, biocompatibility, fatigue resistance, working stability, response speed, self-healing and self-recovery properties, which make it an ideal material for flexible strain sensors. Summary of the invention

[0005] One of the purposes of the present invention is to provide a method for preparing a cellulose-based ion gel. The ion gel prepared by the invention has good electrical conductivity, self-adhesion, self-repairing, antifreeze and biocompatibility, etc., and shows good stretchability, flexibility and sensitivity when monitoring human motion. It can easily adhere to the skin at the human joints, and realize real-time monitoring of human motion with large and small strains at the human joints.

[0006] The second object of the present invention is to provide a cellulose-based ion gel prepared by the above preparation method.

[0007] The third object of the present invention is to provide an application of a cellulose-based ion gel as a strain sensor in human motion monitoring. The human motion signal monitoring includes swallowing, coughing, phonation, etc. at the throat; bending and straightening at the finger joints; bending up and down at the wrist joints; bending and straightening at the knees and elbow joints; joint movement of the neck when raising and lowering the head, etc., but is not limited to the above monitoring characteristics.

[0008] The technical solution is as follows:

[0009] Step 1: preparing a low eutectic solvent: mixing a hydrogen bond acceptor imidazole ionic liquid and a hydrogen bond donor acrylamide in a certain proportion, stirring under heating conditions until the solution becomes transparent and uniform to obtain a low eutectic solvent, and cooling to room temperature for use;

[0010] Step 2: dissolving cellulose: dissolving a certain amount of cellulose in the imidazole ionic liquid used in step 1 under heating conditions to form a transparent and uniform cellulose solution for later use;

[0011] Step 3: preparing a cellulose-based ion gel: adding the low eutectic solvent in step 1 to the cellulose solution prepared in step 2 in a certain proportion and stirring the mixture until the solution is uniform, adding a certain amount of a cross-linking agent and a photoinitiator in sequence and mixing the mixture evenly, and then pouring the obtained prepolymer solution into a sealed mold isolated from oxygen, and cross-linking and curing the solution by ultraviolet light to obtain a cellulose-based ion gel;

[0012] Step 4: Prepare the strain sensor: Cut the cellulose-based ion gel into long strips, connect the two ends of the gel by wire epitaxy, and assemble a strain sensor based on the ion gel;

[0013] Step 5: Stick the cellulose-based ion gel to the joints of the human body, connect the extended end of the wire to the Keithley 2450 digital source meter, set the parameters of the instrument, and regularly move the joints of the human body: fingers, wrists, elbows, neck, knees and throat. A curve chart of the change in relative resistance of the gel when the human joints move can be obtained from the instrument screen.

[0014] Preferably, in step 1, the hydrogen bond acceptor and hydrogen bond donor of the low eutectic solvent are 1-butyl-3-methylimidazolium chloride and acrylamide, respectively, with a molar ratio of 1:2.

[0015] Preferably, in step 1, the heating temperature for preparing the low eutectic solvent is 70°C.

[0016] Preferably, in step 2, the dissolution temperature of cellulose in the imidazolium ionic liquid is 80°C.

[0017] Preferably, in step 2, the mass fraction of cellulose dissolved in the imidazolium-type ionic liquid is 0 wt % to 4 wt %.

[0018] Preferably, in step 3, the molar ratio of the imidazole-type ionic liquid to acrylamide in the homogeneous solution obtained by mixing the low eutectic solvent with the cellulose solution and then stirring the mixture sufficiently is 1.3:1.

[0019] Preferably, in step 3, the amount of the cross-linking agent polyethylene glycol diacrylate is 0.1% of the amount of acrylamide monomer.

[0020] Preferably, in step 3, the amount of the photoinitiator 2-hydroxy-2-methyl-1-phenylpropanone is 1% of the amount of the acrylamide monomer substance.

[0021] Preferably, in step 3, the closed mold is a square mold, which is composed of two square fully transparent glass sheets with a 2 mm thick rubber sheet sandwiched in between, and the rubber sheet is hollow, and the size of the hollowed-out part is a square of 6 cm×6 cm.

[0022] Preferably, in step 3, the ultraviolet light irradiation time is 1 min to 10 min, more preferably 6 min.

[0023] Preferably, in step 4, the size of the gel strip is 4 cm×1 cm.

[0024] Beneficial effects of the present invention:

[0025] The present invention uses a low eutectic solvent as a dispersion medium and forms a three-dimensional gel network through polymerization of acrylamide monomers. Compared with traditional hydrogels, it has excellent conductivity, antifreeze, stimulus response and working stability. The addition of natural cellulose significantly improves the mechanical properties of the gel. Compared with the previously reported nanocellulose and microcrystalline cellulose as reinforcing materials for gels, natural cellulose is more environmentally friendly, more convenient to obtain, lower cost, and does not produce industrial waste, which is conducive to achieving carbon neutrality and more in line with the concept of sustainable development. In addition, the addition of cellulose further enhances the adhesion and self-healing properties of the gel. The prepared cellulose-based ion gel can be easily attached to the surface of the human body, and the sensing curve of the gel after complete self-healing is basically consistent with the original sensing curve. It can be used to construct a flexible sensor with better sensitivity, and has broad application prospects in the field of flexible electronic devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 :Photos of hydrogen bond acceptor 1-butyl-3-methylimidazole chloride and hydrogen bond donor acrylamide and the prepared deep eutectic solvent at room temperature;

[0027] Figure 2 : is a chemical reaction formula for obtaining a deep eutectic solvent by reacting a hydrogen bond acceptor 1-butyl-3-methylimidazolium chloride with a hydrogen bond donor acrylamide;

[0028] Figure 3 : Fourier transform infrared spectra of hydrogen bond acceptor 1-butyl-3-methylimidazolium chloride, hydrogen bond donor acrylamide and the prepared deep eutectic solvent;

[0029] Figure 4 : is a differential scanning calorimetry analysis diagram of the prepared deep eutectic solvent;

[0030] Figure 5 :Photos of the initial state, stretching state and knotting state of cellulose-based ion gel at room temperature;

[0031] Figure 6 :Ion gel conductivity diagram with different cellulose contents;

[0032] Figure 7 : Stress-strain curves of ion gels with different cellulose contents;

[0033] Figure 8 :Optical microscopic images of the incision of the cellulose-based ion gel after incision and at 8h, 16h, and 24h after self-healing;

[0034] Fig. 9 : The sensing curve of the regular movement of cellulose-based ion gel attached to the human wrist. DETAILED DESCRIPTION

[0035] The embodiments of the present invention will be described in detail below with reference to examples.

[0036] Example 1

[0037] This embodiment provides an ion gel, which is prepared according to the following steps:

[0038] Step 1: prepare a low eutectic solvent: mix the hydrogen bond acceptor 1-butyl-3-methylimidazolium chloride and the hydrogen bond donor acrylamide in a molar ratio of 1:2, stir at 70°C until the solution is transparent and uniform, take out and place at room temperature for use;

[0039] Step 2: dissolving cellulose: dissolving cellulose in 1-butyl-3-methylimidazolium chloride under heating conditions of 80° C. to form a transparent and clear cellulose solution, wherein the mass fraction of cellulose is 2wt%;

[0040] Step 3: Preparation of cellulose-based ion gel: 3 g of the low eutectic solvent in step 1 is added to 2.64 g of the cellulose solution obtained in step 2 and stirred thoroughly to obtain a uniform solution, wherein the molar ratio of 1-butyl-3-methylimidazolium chloride to acrylamide in the solution is 1.3:1. Subsequently, 3 μL of crosslinking agent polyethylene glycol diacrylate and 30 μL of photoinitiator 2-hydroxy-2-methyl-1-phenylacetone are added in sequence and stirred thoroughly, and the obtained prepolymer solution is poured into an oxygen-isolated sealed mold, which is a square mold, and its composition is two square fully transparent glass sheets with a 2 mm thick rubber sheet sandwiched between them, and the rubber sheet is hollow, and the hollowed-out part is a square of 6 cm × 6 cm. After irradiation with ultraviolet light for 6 minutes, the cellulose-based ion gel is obtained.

[0041] Example 2

[0042] This embodiment provides an ion gel, and the preparation method thereof is different from that of Embodiment 1 in that the mass fraction of cellulose in the cellulose solution obtained in step 2 is 0 wt %.

[0043] Example 3

[0044] This embodiment provides an ion gel, and the preparation method thereof is different from that of Embodiment 1 in that the mass fraction of cellulose in the cellulose solution obtained in step 2 is 1 wt %.

[0045] Example 4

[0046] This embodiment provides an ion gel, and the preparation method thereof is different from that of Embodiment 1 in that the mass fraction of cellulose in the cellulose solution obtained in step 2 is 3 wt %.

[0047] Example 5

[0048] This embodiment provides an ion gel, and the preparation method thereof is different from that of Embodiment 1 in that the mass fraction of cellulose in the cellulose solution obtained in step 2 is 4 wt %.

[0049] As attached Figure 7 As shown in Figure 1, the stress-strain curves of ion gels with different cellulose contents are shown. It can be seen that with the increase of cellulose content, the gel stress gradually increases, and the stress almost stops increasing after the cellulose content reaches 3wt%.

[0050] Example 6

[0051] This embodiment provides an ion gel, which is prepared according to the following steps:

[0052] Step 1: prepare a low eutectic solvent: mix the hydrogen bond acceptor 1-allyl-3-methylimidazolium chloride and the hydrogen bond donor acrylamide in a molar ratio of 1:2, stir at 70°C until the solution is transparent and uniform, take out and place at room temperature for use;

[0053] Step 2: dissolving cellulose: dissolving cellulose in 1-allyl-3-methylimidazolium chloride under heating conditions of 80° C. to form a transparent and clear cellulose solution, wherein the mass fraction of cellulose is 2wt%;

[0054] Step 3: Preparation of cellulose-based ion gel: 3 g of the low eutectic solvent in step 1 is added to 2.53 g of the cellulose solution obtained in step 2 and stirred thoroughly to obtain a uniform solution, wherein the molar ratio of 1-allyl-3-methylimidazolium chloride to acrylamide in the solution is 1.3:1. Subsequently, 2.7 μL of crosslinker polyethylene glycol diacrylate and 39 μL of photoinitiator 2-hydroxy-2-methyl-1-phenylacetone are added in sequence and stirred thoroughly, and the obtained prepolymer solution is poured into an oxygen-isolated sealed mold, which is a square mold, and is composed of two square fully transparent glass sheets with a 2 mm thick rubber sheet sandwiched between them. The rubber sheet is hollow, and the hollowed-out part is a square of 6 cm × 6 cm. After irradiation with ultraviolet light for 6 minutes, the cellulose-based ion gel is obtained.

[0055] Example 7

[0056] This embodiment provides a cellulose-based ion gel sensor, which is prepared according to the following steps:

[0057] Step 1: prepare a low eutectic solvent: mix the hydrogen bond acceptor 1-butyl-3-methylimidazolium chloride and the hydrogen bond donor acrylamide in a molar ratio of 1:2, stir at 70°C until the solution is transparent and uniform, take out and place at room temperature for use;

[0058] Step 2: dissolving cellulose: dissolving cellulose in 1-butyl-3-methylimidazolium chloride under heating conditions of 80° C. to form a transparent and clear cellulose solution, wherein the mass fraction of cellulose is 2wt%;

[0059] Step 3: Preparation of cellulose-based ion gel: Take 3g of the low eutectic solvent in step 1 and add it to 2.64g of the cellulose solution prepared in step 2 and stir thoroughly to obtain a uniform solution. The molar ratio of 1-butyl-3-methylimidazolium chloride to acrylamide in the solution is 1.3:1. Then, 3 microliters of cross-linking agent polyethylene glycol diacrylate and 30 microliters of photoinitiator 2-hydroxy-2-methyl-1-phenylacetone were added in sequence and stirred thoroughly. The obtained prepolymer solution was poured into an oxygen-isolated sealed mold. The sealed mold is a square mold, which is composed of two square fully transparent glass sheets with a 2mm thick rubber sheet sandwiched in the middle. The rubber sheet is hollow, and the hollow part is 6cm×6cm square in size. After irradiation with ultraviolet light for 6 minutes, the cellulose-based ion gel was obtained;

[0060] Step 4: Prepare strain sensor: Cut the cellulose-based ion gel into long strips, connect the two ends of the gel by wire extension, and assemble a strain sensor based on this ion gel. Fit the cellulose-based ion gel to the human joint, connect the wire extension end to the Keithley 2450 digital source meter, set the instrument parameters, bend the wrist up and down regularly, and obtain a curve of the change in relative resistance of the gel when the wrist joint moves from the instrument screen.

[0061] like Fig. 9 The figure shows the sensing curve of the regular movement of the cellulose-based ion gel attached to the human wrist. As can be seen from the figure, as the wrist bends up and down regularly, the cellulose-based ion gel can quickly monitor the movement of the human joint and convert the physical signal into an electrical signal, so that the relative resistance change curve of the gel can be clearly seen in the graph, indicating that the sensor has good sensitivity to applied strain.

Claims

1. A method for preparing a cellulose-based ion gel, characterized in that: The specific steps are as follows: Step 1: preparing a low eutectic solvent: mixing a hydrogen bond acceptor imidazole ionic liquid and a hydrogen bond donor acrylamide in a certain proportion, stirring under heating conditions until the solution becomes transparent and uniform to obtain a low eutectic solvent, and cooling to room temperature for use; Step 2: dissolving cellulose: dissolving a certain amount of cellulose in the imidazole ionic liquid used in step 1 under heating conditions to form a transparent and uniform cellulose solution for later use; Step 3: Preparation of cellulose-based ion gel: Add the low eutectic solvent in step 1 to the cellulose solution obtained in step 2 in a certain proportion and stir thoroughly until the solution is uniform. Add a certain amount of cross-linking agent and photoinitiator in sequence and mix evenly to obtain a prepolymer solution, pour it into a sealed mold isolated from oxygen, and cross-link and cure it through ultraviolet light irradiation to obtain a cellulose-based ion gel.

2. A cellulose-based ion gel is prepared according to the preparation method of claim 1.

3. The preparation method according to claim 1, characterized in that: The hydrogen bond acceptor of the low eutectic solvent in step 1 is an imidazolium-type ionic liquid, including one of 1-butyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazolium chloride, and 1-ethyl-3-methylimidazolium acetate; the hydrogen bond donor is acrylamide, and the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 2:1 to 1:

2.

4. The preparation method according to claim 1, characterized in that: The mixing temperature of the hydrogen bond acceptor and the hydrogen bond donor in the low eutectic solvent in step 1 is 65° C. to 75° C.; the mixing time of the hydrogen bond acceptor and the hydrogen bond donor is 10 min to 40 min.

5. The preparation method according to claim 1, characterized in that: The dissolution temperature of the cellulose in the imidazolium ionic liquid described in step 2 is 80°C to 90°C.

6. The preparation method according to claim 1, characterized in that: The low eutectic solvent in step 3 is mixed with the cellulose solution and then stirred to obtain a homogeneous solution in which the molar ratio of the imidazole ionic liquid to acrylamide is 1.5:1 to 1:

1.

7. The preparation method according to claim 1, characterized in that: The cross-linking agent in step 3 is polyethylene glycol diacrylate, and the amount used is 0.1% to 0.5% of the amount of acrylamide monomer substance.

8. The preparation method according to claim 1, characterized in that: The photoinitiator described in step 3 is 2-hydroxy-2-methyl-1-phenylpropanone, and the amount used is 1% to 5% of the amount of acrylamide monomer substance.

9. The preparation method according to claim 1, characterized in that: The closed mold described in step 3 is a square mold, which is composed of two square fully transparent glass sheets with a 2 mm thick rubber sheet sandwiched in the middle. The rubber sheet is hollow, and the hollowed-out part is a square of 6 cm×6 cm in size.

10. The preparation method according to claim 1, characterized in that: The ultraviolet light irradiation time described in step 3 is 1 min to 15 min.

11. A cellulose-based ion gel prepared by the preparation method according to any one of claims 3 to 10.

12. Use of the cellulose-based ion gel according to claim 11 as a strain sensor in monitoring human motion.

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