High-Sensitivity Flexible Piezoresistive Sensor with Multiple Conductive Contacts and Preparation Method Thereof

Abstract

The present invention provides a highly sensitive flexible piezoresistive sensing material with multiple conductive contacts and a preparation method thereof. The sensing material consists of a polymer open-cell framework and reduced graphene oxide, and has a mutually penetrating porous structure; the average pore diameter of the polymer open-cell framework does not exceed 80 μm, and the cell struts are fibrous, and some of the cell struts are broken; the reduced graphene oxide is distributed in the polymer open-cell framework. Among them, part of the reduced graphene oxide is coated on the cell struts, part of it covers the endpoints of the broken cell struts, and part of it penetrates multiple cells and simultaneously covers multiple cell struts to form cell walls, forming a pore structure with a smaller pore diameter than that of the polymer open-cell framework in the polymer open-cell framework, as well as a reduced graphene oxide conductive network interpenetrating with the polymer open-cell framework. This sensing material can form more conductive contacts and reduce the distance between conductive contacts, and can improve the sensitivity of the impregnated polymer framework conductive porous piezoelectric sensor.

Description

Technical Field

[0001] The present invention belongs to the field of flexible piezoresistive sensors, and relates to a highly sensitive flexible piezoresistive sensor with multiple conductive contacts and a preparation method thereof. Background Art

[0002] In recent years, wearable pressure sensors have shown excellent application potential in the fields of electronic bionic skin, soft robots, human health monitoring, etc. As an intelligent material that can sense, analyze external pressure signals and convert them into easily captured electrical signals, wearable pressure sensors are highly expected and are one of the key technologies for future science and technology. Although most commercially available wearable electronic products can currently monitor human activities and health, their sensitivity still needs to be improved. Therefore, developing flexible wearable pressure sensors with characteristics such as multifunctionality, high sensitivity, fast response, and low cost is still a huge challenge at present.

[0003] Based on various signal conversion mechanisms, various types of wearable flexible sensors have been widely explored, including piezoresistive, capacitive, and piezoelectric transduction types, etc. Among them, piezoresistive sensors that convert stimulus signals into resistance changes have characteristics such as easy signal acquisition, fast response, and simple circuit integration, and have become a hot material in the research and development field of wearable and highly sensitive flexible pressure sensors. Other types of sensors usually have relatively limited stretching ability and low resolution, and capacitive sensors have unstable sensor characteristics due to parasitic capacitance, so most of them cannot be used as wearable sensors.

[0004] Currently developed piezoresistive sensors include pure conductive porous material piezoresistive sensors, composite conductive porous material piezoresistive sensors, porous conductive coating piezoresistive sensors, and impregnated polymer framework conductive porous piezoresistive sensors, etc. Although pure conductive material piezoresistive sensors have ultra-low density, excellent electrical conductivity, and good chemical stability, due to their fragility, compression, bending, etc. during use will cause irreversible damage to the internal structure of the material, resulting in irreversible changes in the deformation and electrical conductivity of the material, and their sensitivity and compression strain range are limited. In the preparation process of composite conductive porous material piezoresistive sensors, conductive fillers are prone to agglomeration and uneven dispersion in the polymer matrix, affecting the construction of the conductive network and resulting in unstable sensing performance of the composite conductive porous material piezoresistive sensors. The preparation process of the coating "ink" (such as carbon nanotubes and silver, etc.) required for porous conductive coating piezoresistive sensors is complex, and there are also problems of uneven dip coating of the conductive coating and unstable combination of the conductive coating and the matrix material.

[0005] The impregnated polymer framework conductive porous piezoresistive sensor is formed by impregnating a polymer framework material in a conductive carbon material solution, and the conductive carbon material is stacked on the framework material by a reduction-self-assembly method. Such piezoresistive sensors use polymer foam materials as the framework, and the carbon materials are distributed on the polymer framework, having good mechanical resilience and being prone to forming a conductive network after the framework deforms. For example, Yao et al. prepared graphene-wrapped polyurethane (PU) foam with microstructures through dip coating and reduction, Wu et al. prepared flexible polymer-based graphene porous materials by self-assembling graphene sheets on a porous PU framework, and Guo et al. formed reduced graphene oxide (rGO) sheets (C-RGO@PU) on a PU framework sponge after in-situ reduction-self-assembly of graphene oxide (GO), and then added polyaniline nanofibers (PANIH) to PU to form a PANIH / C-RGO@PU porous sensor.

[0006] Compared with other types of piezoresistive sensors, the resilience and sensitivity of the impregnated polymer framework conductive porous piezoresistive sensor have been improved to a certain extent. However, the sensitivity of the existing impregnated polymer framework conductive porous piezoresistive sensors is still unsatisfactory. This is mainly because the existing technology mainly uses commercially available PU foams prepared by chemical foaming methods as the framework, but the pore size of this commercially available PU foam is greater than 200 μm, and the size of the edges constituting the framework is also large. The overly large pore size and overly thick edges result in graphene sheets being able to only adhere to the edges of the PU framework, rather than covering multiple edges to form thin-wall stacks, thus forming relatively limited conductive contacts, and the distance between the conductive contacts is large. This makes the graphene sheets covering the edges need to be in a greater compression state before they can come into contact with each other, resulting in the impregnated polymer framework conductive porous piezoresistive sensor requiring a greater compressive stress and a longer compression time to produce a corresponding resistance change rate, and the sensing sensitivity being relatively limited.

[0007] Therefore, if the structure of the existing impregnated polymer framework conductive porous piezoresistive sensor can be improved to make the framework material have a smaller pore size and thinner edges to form more conductive contacts and reduce the distance between the conductive contacts, it will have a positive effect on improving the sensitivity of the existing impregnated polymer framework conductive porous piezoresistive sensor. Summary of the Invention

[0008] Aiming at the problem of low sensing sensitivity caused by limited conductive contacts and too large a distance between conductive contacts in the existing impregnated polymer framework conductive porous piezoresistive sensor, the present invention provides a highly sensitive flexible piezoresistive sensor with multiple conductive contacts and a preparation method thereof to improve the sensing sensitivity of the existing impregnated polymer framework conductive porous piezoresistive sensor.

[0009] To achieve the above invention object, the technical solution adopted by the present invention is as follows:

[0010] A highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, which is composed of a polymer open-cell framework and reduced graphene oxide, and has a mutually interconnected porous structure;

[0011] The polymer open-cell framework has mutually interconnected pores, the average pore diameter of the polymer open-cell framework does not exceed 80 μm, the pore ribs are fibrous, and some of the pore ribs are broken; reduced graphene oxide is distributed in the polymer open-cell framework, and a part of the reduced graphene oxide is coated on the pore ribs, a part of the reduced graphene oxide covers the endpoints of the broken pore ribs, and a part of the reduced graphene oxide penetrates through multiple pores and simultaneously covers multiple pore ribs to form pore walls, forming a pore structure with a smaller pore diameter than that of the polymer open-cell framework in the polymer open-cell framework, and forming a reduced graphene oxide conductive network interpenetrating with the polymer open-cell framework.

[0012] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, since the reduced graphene oxide distributed in the polymer open-cell framework re-divides the pore structure of the polymer open-cell framework as pore walls, a pore structure with a smaller pore diameter than that of the polymer open-cell framework is formed in the polymer open-cell framework. Through morphological analysis and statistics of the pore structure, it is found that the average pore diameter of this flexible piezoresistive sensing material is 40% - 60% of the average pore diameter of the polymer open-cell framework.

[0013] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, the average pore diameter of the polymer open-cell framework is 50 - 80 μm.

[0014] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, in the polymer open-cell framework, the average diameter of the pore ribs is 3 - 4 μm, and some of the pore ribs are in the form of nanofibers with a diameter not exceeding 400 nm. Both the micron-scale and nano-scale pore ribs in the polymer open-cell framework are coated with reduced graphene oxide, but the finer the pore ribs, the more complete the coating of reduced graphene oxide on the ribs and the more coating sites are formed. We observed through scanning electron microscopy that for some pore ribs in the form of nanofibers, the reduced graphene oxide is coated on them in a curled manner.

[0015] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, in the flexible piezoresistive sensing material, the content of reduced graphene oxide is 9 wt.% - 11 wt.%.

[0016] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, the sheet size of the reduced graphene oxide is 0.2 to 10 μm. The sheet size here refers to the distance between the two farthest points on the sheet-shaped reduced graphene oxide.

[0017] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, within the pressure range of 1.5 to 5 kPa, the stress sensitivity of the flexible piezoresistive sensing material is 120 to 140 kPa -1 。

[0018] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, the matrix material of the polymer open-cell framework is composed of ethylene-vinyl alcohol copolymer, branched polymer, and ethylene-vinyl alcohol copolymer grafted with branched polymer; the branched polymer is a polyether polyol with a terminal group of ——NCO, and the ethylene-vinyl alcohol copolymer grafted with branched polymer is formed by the reaction of the terminal group of the branched polymer with the side hydroxyl group of the ethylene-vinyl alcohol copolymer.

[0019] Furthermore, the polymer open-cell framework is prepared by melt blending 88 to 92 parts by mass of ethylene-vinyl alcohol copolymer and 8 to 12 parts by mass of branched polymer, molding, and depressurizing and foaming with supercritical fluid; the melting point of the branched polymer is lower than the melting point of the blend formed by melt blending the branched polymer and ethylene-vinyl alcohol copolymer. Furthermore, the branched polymer is an alkylene oxide copolymer with a terminal group of ——NCO, and the number average molecular weight of the branched polymer is preferably 10 3 ~10 6 ; the molar content of ethylene segments in the ethylene-vinyl alcohol copolymer is preferably 24% to 48%.

[0020] Even further, the branched polymer is formed by reacting a polyether polyol with a substance containing a ——NCO group in a molar ratio of ——NCO group to the hydroxyl group of the polyether polyol of 1∶(0.8 to 1.1); the substance containing a ——NCO group can be a diisocyanate, and common diisocyanates can be toluene diisocyanate (TDI), 1,6-hexamethylene diisocyanate (HDI), diphenylmethane diisocyanate (MDI), isophorone diisocyanate (IPDI), etc.; the polyether polyol can be polymerized from different alkylene oxides, such as polypropylene glycol, polytetrahydrofuran glycol, and polyether polyol obtained by random copolymerization of ethylene oxide (EO) and propylene oxide (PO). The branched polymer is preferably an alkylene oxide random copolymer with a terminal group of ——NCO. For example, it can be an ethylene oxide-propylene oxide random copolymer with a terminal group of ——NCO, or other alkylene oxide random copolymers with a terminal group of ——NCO.

[0021] In the technical solution of the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, a feasible method for preparing a polymer open-cell framework is as follows:

[0022] (1) Melt-blend 88-92 parts by mass of ethylene-vinyl alcohol copolymer and 8-12 parts by mass of branched polymer to obtain a blend, and mold the obtained blend to form a green body; during the melt-blending of ethylene-vinyl alcohol copolymer and additives, the end groups of some branched polymers - NCO react with the side-chain hydroxyl groups of some ethylene-vinyl alcohol copolymers to form ethylene-vinyl alcohol copolymers grafted with additives, and the remaining additives are dispersed in the polymer matrix;

[0023] (2) Place the green body in a high-pressure chamber, introduce a gas as a blowing agent into the high-pressure chamber, control the temperature of the high-pressure chamber to be T and the pressure to be 10-25 MPa, maintain the above temperature and pressure conditions for swelling until the blowing agent reaches saturation in the green body, and then release the pressure to foam to obtain a polymer open-cell framework;

[0024] The melting point of the branched polymer < T1, where T1 is the melting point of the blend formed by melt-blending additives and ethylene-vinyl alcohol copolymer in step (1);

[0025] The temperature T of the high-pressure chamber should satisfy: T1 ≤ T < T2, where T2 is the lowest temperature at which the cell pores will completely collapse during foaming in this step, and T2 = T1 + (10-50) °C.

[0026] When preparing the polymer open-cell framework, the melt-blending time in step (1) should ensure that the material mixture is uniform and a certain degree of grafting reaction occurs between the branched polymer and the ethylene-vinyl alcohol copolymer. At the same time, the melt-blending time should be avoided being too long to cause the degradation of the side hydroxyl groups of the ethylene-vinyl alcohol copolymer. Preferably, the melt-blending time in step (1) is 1-20 min.

[0027] When preparing the polymer open-cell framework, the gas introduced into the high-pressure chamber as a blowing agent in step (2) is at least one of N2, CO2, and inert gases.

[0028] The present invention also provides a method for preparing the above-mentioned highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, including the following steps:

[0029] (1) Add a graphene oxide dispersion with a concentration of 3.5-4.5 mg / mL to a reaction vessel, immerse the polymer open-cell framework in the graphene oxide dispersion, and ultrasonicate until the graphene oxide dispersion fully infiltrates the polymer open-cell framework;

[0030] (2) Add ascorbic acid to the reaction vessel according to the mass ratio of ascorbic acid to graphene oxide of (2-3):1. After the ascorbic acid is completely dissolved, react at 90-95 °C for 1-1.5 h, then cool to room temperature, freeze until the liquid in the reaction solution is completely solidified, and then take it out and place it at room temperature until it completely melts; repeat the freezing and melting operations 1-3 times; then react at 90-95 °C for 4-6 h, take out the polymer open-cell skeleton in the reaction vessel, cool to room temperature, and then freeze-dry to obtain the flexible piezoresistive sensing material.

[0031] The main reasons for the high sensitivity of the flexible piezoresistive sensing material provided by the present invention are as follows:

[0032] The flexible piezoresistive sensing material provided by the present invention uses a polymer foam material with an average pore diameter not exceeding 80 μm and a high open-cell rate as the polymer open-cell skeleton, and reduced graphene oxide sheets are loaded in the polymer open-cell skeleton. The polymer open-cell skeleton has small pore diameters and short cell edges. After the reduced graphene oxide sheets are loaded in the polymer open-cell skeleton, it is easy to form a structure that penetrates multiple cells and simultaneously covers multiple cell edges. The reduced graphene oxide sheets that penetrate multiple cells and simultaneously cover multiple cell edges can serve as cell walls to re-divide the cell structure of the polymer open-cell skeleton; at the same time, in the polymer open-cell skeleton, the cell edges are thin, and the average diameter of the cell edges is only 3-4 μm, and there are also many cell edges in the shape of nanofibers with a diameter not exceeding 400 nm. The thin cell edges are conducive to the reduced graphene oxide sheets forming a more complete coating on the cell edges, forming more coating sites; in addition, some of the nanofiber-shaped cell edges are broken, and some of the reduced graphene oxide sheets will adhere to and cover the endpoints of the broken cell edges. The combined effects of the above aspects enable the reduced graphene oxide sheet layer to form a smaller pore structure in the polymer open-cell skeleton as the pore wall of the new pore structure, providing a large number of conductive contacts that can contact each other for the flexible piezoresistive sensing material. On this basis, due to the small pore diameter of the polymer open-cell skeleton, the distance between the conductive contacts is greatly shortened. The polymer open-cell skeleton endows the piezoresistive sensor with excellent flexibility and resilience. Through the above structural design and the mutual cooperation of various factors, the flexible piezoresistive sensing material forms a unique microstructure. This unique microstructure effectively increases the conductive contacts and effectively shortens the distance between the conductive contacts. Therefore, the flexible piezoresistive material only needs to respond to stress and undergo a small deformation to effectively increase the perfection degree of the conductive network formed by the contact between the conductive contacts, thereby significantly improving the sensitivity of the flexible piezoresistive sensing material.

[0033] From the perspective of the preparation method, to load reduced graphene oxide onto the polymer open-cell framework by means of impregnation-hydrothermal reduction, it is required that the polymer open-cell framework has a certain hydrophilicity. Otherwise, it is difficult for the aqueous graphene dispersion to be fully impregnated into the polymer framework. The polymer open-cell framework adopted in this application is formed by blending and modifying ethylene-vinyl alcohol copolymer (EVOH) as a polar matrix material containing a hydrophilic segment (vinyl alcohol segment) with a branched polymer (MDI-g-EOPO) formed by polymerizing ethylene oxide-propylene oxide random copolymer (EOPO) and 4,4'-diphenylmethane diisocyanate (MDI), and then performing supercritical fluid foaming. This polymer open-cell framework has appropriate hydrophilicity, which is conducive to uniformly loading more reduced graphene oxide in the polymer open-cell framework by means of impregnation-reduction. This is also one of the keys to improving the sensitivity of the flexible piezoresistive sensing material in this invention.

[0034] Compared with the prior art, the technical solution provided by this invention can produce the following beneficial technical effects:

[0035] 1. This invention provides a highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, which is composed of a polymer open-cell framework and reduced graphene oxide and has a mutually interconnected porous structure; the polymer open-cell framework has mutually interconnected pores, the average pore diameter of the polymer open-cell framework does not exceed 80 μm, the pore struts are fibrous, and some of the pore struts are broken; the reduced graphene oxide is distributed in the polymer open-cell framework, where part of the reduced graphene oxide coats the pore struts, part of the reduced graphene oxide covers the endpoints of the broken pore struts, and part of the reduced graphene oxide penetrates multiple pores and simultaneously covers multiple pore struts, forming a pore structure with a smaller pore diameter than that of the polymer open-cell framework in the polymer open-cell framework and forming a reduced graphene oxide conductive network interpenetrating with the polymer open-cell framework. Through the coordination of the size and distribution of the reduced graphene oxide with the pore diameter of the polymer open-cell framework and the size of the pore struts, the flexible piezoresistive sensing material is endowed with a unique microstructure. This unique microstructure effectively increases the conductive contacts and greatly shortens the distance between the conductive contacts, enabling the flexible piezoresistive material to effectively increase the perfection degree of the conductive network formed by the contact between the conductive contacts only by responding to a small stress change, thereby significantly improving the sensitivity of the flexible piezoresistive sensing material. It solves the deficiency of the low sensitivity of the existing impregnated polymer framework conductive porous piezoelectric sensing material and will play a positive role in promoting the application of the impregnated polymer framework conductive porous piezoelectric sensing material in wearable pressure sensing devices.

[0036] 2. This invention experimentally verified that the sensor assembled based on the flexible piezoresistive sensing material described in this invention has a stress sensitivity of 130.9 kPa within the stress range of 1.5 - 5 kPa-1 , the sensing sensitivity of the piezoresistive sensors reported in the existing technologies has been significantly improved. At the same time, the sensors assembled based on the flexible piezoresistive sensing material of the present invention exhibit good performance under dynamic stress within a strain range of 10%, with a high signal-to-noise ratio. After 55 loading-unloading cycles, the flexible piezoresistive sensor not only maintains a high signal-to-noise ratio, but also the current amplitude hardly changes, showing good cyclic stability. Maintaining a high signal-to-noise ratio and cyclic stability during use can increase the reliability of the flexible piezoresistive sensor in practical applications.

[0037] 3. The present invention experimentally tested the application effect of the sensors assembled based on the flexible piezoresistive sensing material provided by the present invention in human motion detection. The results confirmed that the assembled sensors can successfully capture the electrical signals generated during the bending of fingers and wrists. As the fingers and wrists bend, the generated current value rapidly increases, and when the fingers or wrists stretch, the current value returns to the initial value. At the same time, by fixing the assembled sensors on the tester's neck, the electrical signals generated when the tester speaks, drinks water, and coughs can be successfully captured. For the same action, the generated characteristic current curves are almost the same, and for different actions, there are significant differences between the generated characteristic current signals, indicating that the flexible piezoresistive sensor has good stability and excellent recognition performance. This shows that the flexible piezoresistive sensing material provided by the present invention can be applied in wearable devices and has excellent performance and great application potential in human motion detection.

[0038] 4. The present invention also provides a preparation method for a highly sensitive flexible piezoresistive sensing material with multiple conductive contacts. The preparation process of this method is simple, which is conducive to popularization and application. At the same time, since the polymer open-cell framework adopted by the present invention has appropriate hydrophilicity, more reduced graphene oxide can be uniformly loaded into the polymer open-cell framework by the impregnation-reduction method, which can solve the deficiency in the prior art that it is difficult to achieve the smooth and complex reduction of graphene oxide due to the poor hydrophilicity of the polymer open-cell framework used. Description of the Drawings

[0039] Figure 1 Figures (a)-(d) are respectively the Raman spectral curves, infrared spectra, XRD patterns, and TGA curves of GO and rGO in the rGO@EVOH sample prepared in Example 2.

[0040] Figure 2 are the XPS spectra of GO and rGO in the rGO@EVOH sample prepared in Example 2. Among them, Figure (a) is the full XPS spectrum of GO and rGO, Figure (b) is the C1s XPS spectrum of GO, and Figure (c) is the C1s XPS spectrum of rGO.

[0041] Figure 3 SEM images and pore size statistical graphs of the polymer open-cell skeletons, where the (a1)(a2), (b1)(b2), (c1)(c2), and (d1)(d2) images are SEM images and pore size statistical graphs of the polymer open-cell skeletons used to prepare rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4, respectively.

[0042] Figure 4 SEM images and pore size statistical graphs of rGO@EVOH prepared in Example 2, where the (a1)(a2), (b1)(b2), (c1)(c2), and (d1)(d2) images are SEM images and pore size statistical graphs of rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4, respectively.

[0043] Figure 5 Figures (A) - (D) of Figure 4 are partial enlarged views of figures (a1) - (d1) of , and the area enclosed by the dashed box in the figure shows the small pores formed by the stacking of rGO in the porous skeleton material.

[0044] Figure 6 SEM images of the sample obtained after thermally degrading the polymer open-cell skeleton in rGO@EVOH4 at different magnifications.

[0045] Figure 7 Figures (a) - (d) of represent the pressure response curves of the flexible piezoresistive sensors assembled based on rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4, respectively.

[0046] Figure 8 SEM images of the flexible piezoresistive sensors assembled based on rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4 before and after compression stabilization. The first row shows the SEM images before compression, and the second row shows the SEM images after compression stabilization.

[0047] Figure 9 Figures (a1)(b1) of represent the cyclic stability curves of the flexible piezoresistive sensors assembled based on rGO@EVOH2 and rGO@EVOH4, respectively. Figure 9 In figure (a2) of , the upper part of the figure is an enlarged view of the dashed box part of figure (a1), and the lower part of the figure is the stress-time change curve corresponding to the upper part. Figure 9 In figure (b2) of , the upper part of the figure is an enlarged view of the dashed box part of figure (b1), and the lower part of the figure is the stress-time change curve corresponding to the upper part.

[0048] Figure 10 Figures (a) and (b) are the electrical signals generated during the bending process of the finger and wrist. Figure 10 Figures (c), (d), and (e) are the electrical signals generated when the tester speaks, drinks water, and coughs. Figure 10 Figures (f) and (g) are photos of the flexible piezoresistive sensor fixed on the finger and wrist. Figure 10 Figure (h) is a photo of the flexible piezoresistive sensor fixed on the neck. Detailed implementation mode

[0049] The following further illustrates the high-sensitivity flexible piezoresistive sensor with multiple conductive contacts and its preparation method according to the present invention through examples. The following described examples are only a part of the examples of the present invention, rather than all examples. Based on the content and examples of the present invention, other implementation manners obtained by those of ordinary skill in the art without creative efforts all fall within the scope protected by the present invention.

[0050] In the following examples and comparative examples, reagents such as ethylene-vinyl alcohol copolymer (EVOH), polyether polyol, and diisocyanate (MDI) can be purchased from the market. The polyether polyol is an ethylene oxide-propylene oxide random copolymer (abbreviated as EOPO) formed by the random copolymerization of ethylene oxide (EO) and propylene oxide (PO), where EO accounts for 70 wt%. The diisocyanate is carbodiimide-uretonimine modified 4,4'-diphenylmethane diisocyanate, with the trade name MDI-100LL, and the content of -NCO groups is 28.0 wt.% to 30.0 wt.%; the graphene oxide (GO) has the trade name S2678, is monolayer, the sheet size is 0.2 to 10 μm, the thickness is about 1 nm, and the oxygen content > 50%.

[0051] Example 1

[0052] In this example, the preparation of the polymer open-cell framework is carried out as follows:

[0053] (1) Add polyether polyol EOPO to the reaction vessel, start stirring, heat to 120 - 125 °C under nitrogen protection and keep for 2 h for dehydration, and then cool to 40 °C. Weigh MDI according to the molar ratio of -NCO of carbodiimide-uretonimine modified 4,4'-diphenylmethane diisocyanate (MDI) to -OH of EOPO being 1:1, add it to the reaction vessel, stir at room temperature for 30 min, then slowly heat up to 80 °C to initiate the reaction and start timing, and react at 80 °C for 6 h to obtain the branched polymer MDI-g-EOPO.

[0054] (2) 90 parts by mass of EVOH with a molar content of ethylene segments of 32% and 10 parts by mass of MDI-g-EOPO were added to a mixer and melt-blended at 190 °C. The rotation speed of the mixer was controlled at 60 r / min. After melt-blending for 5 min, the mixture was taken out. The obtained blend was placed in a flat vulcanizer, preheated at 190 °C for 5 min, and then hot-pressed into a blank at 190 °C under a pressure condition of 10 MPa.

[0055] It can be seen from 1 the 1H-NMR spectrum that in step (2), the branched polymer MDI-g-EOPO and EVOH were melt-blended, and the secondary hydroxyl groups on the branched polymer and EVOH reacted, resulting in a decrease in the secondary hydroxyl groups. That is, during the melt-blending of EVOH and the branched polymer MDI-g-EOPO, the isocyanate groups of some branched polymers reacted with the side-chain hydroxyl groups of EVOH to form EVOH grafted with the branched polymer MDI-g-EOPO.

[0056] (3) The blank obtained in step (2) was placed in a high-pressure cavity, and CO2 gas as a foaming agent was introduced into the high-pressure cavity. The temperature of the high-pressure cavity was controlled at 176 °C and the pressure was 15 MPa. The blank was swollen for 2 h under the above temperature and pressure conditions until the foaming agent was saturated in the blank, and then the pressure was quickly released to foam, obtaining a polymer open-cell skeleton.

[0057] Example 2

[0058] In this example, based on the polymer open-cell skeleton, a flexible piezoresistive sensing material (rGO@EVOH) was prepared, and the steps were as follows:

[0059] (1) GO was fully dispersed in water to form GO dispersions with GO concentrations of 1 mg / mL, 2 mg / mL, 3 mg / mL, and 4 mg / mL, respectively.

[0060] Equal volumes of the above four concentrations of GO dispersions were taken and added to a reaction vessel. Then, 4 parts of the polymer open-cell skeleton prepared in Example 1 were taken and soaked in GO dispersions with GO concentrations of 1 mg / mL, 2 mg / mL, 3 mg / mL, and 4 mg / mL, respectively. The polymer open-cell skeleton was pressed below the liquid surface, and then ultrasonically oscillated until the GO dispersion completely infiltrated the polymer open-cell skeleton.

[0061] (2) According to the ratio of ascorbic acid (VC) to GO mass ratio of 2:1, add VC to each reaction vessel that has undergone the operation in step (1), and through ultrasonic and stirring until VC is fully dissolved. Then place each reaction vessel in an oil bath at 95 °C for a reduction reaction for 1 h, then take it out and cool it to room temperature, place it in a refrigerator at -18 °C for freezing for 2 h until the liquid in the reaction vessel is completely solidified, and then take it out and place it at room temperature until it completely melts; repeat the freezing and melting operations 2 times. During the freezing process, utilize the ice template formed by the freezing of the GO dispersion liquid, so that a part of the reduced graphene oxide (rGO) coats the pore edges of the polymer open-cell framework, a part of the rGO covers the endpoints of the broken pore edges, and a part of the rGO penetrates multiple pores and simultaneously covers the edges of multiple pores, forming a pore structure with a smaller pore size than the pore size of the polymer open-cell framework in the polymer open-cell framework, and obtaining a stable outer coating structure.

[0062] After that, place each reaction vessel in a water bath at 95 °C for a reaction for 5 h until GO is completely reduced, so that the formed rGO after reduction is more closely stacked and coated on the pore edges of the polymer open-cell framework. Finally, take out the polymer open-cell framework in the reaction vessel, cool it to room temperature, and freeze-dry it to obtain the flexible piezoresistive sensing material rGO@EVOH.

[0063] Immerse it in GO dispersion liquids with GO concentrations of 1 mg / mL, 2 mg / mL, 3 mg / mL, and 4 mg / mL in step (1), and the rGO@EVOH prepared through the operation in step (2) is sequentially labeled as rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4.

[0064] Example 3

[0065] In this example, the reduction situation of GO in the rGO@EVOH prepared in Example 2 is characterized.

[0066] 1. In order to monitor the structural changes of GO, such as the degree of defects, Raman spectroscopy tests are performed on GO and rGO in the rGO@EVOH sample prepared in Example 2 using a 633 nm laser, and the test wavenumber range is 1100 - 1900 cm -1 . Obtain the specific wavenumbers and peak intensities of the two characteristic peaks (D peak and G peak) of graphene.

[0067] Test GO and rGO in the rGO@EVOH sample prepared in Example 2 by total reflection Fourier transform infrared spectroscopy to analyze the functional groups of GO and rGO. The resolution of the infrared spectrum is 4 cm -1 , and the test wavenumber range is 500 - 3800 cm -1 .

[0068] 2. The GO and rGO in the rGO@EVOH sample prepared in Example 2 were tested by an X-ray diffractometer at 40 kV and 40 mA to analyze the orderliness of the microstructures of GO and rGO. Cu-Kα radiation was scanned from 2° to 90° at a step size of 1° and a step time of 1.0 s.

[0069] 3. The GO and rGO in the rGO@EVOH sample prepared in Example 2 were placed in a platinum tray and suspended in a TGA for thermogravimetric analysis testing under a nitrogen atmosphere. The heating rate during the test was 10 °C / min.

[0070] Figure 1 Figures (a) to (d) are the Raman spectrum curve, infrared spectrum, XRD pattern, and TGA curve, respectively.

[0071] As can be seen from Figure 1 Figure (a), graphene has two characteristic peaks. There is a prominent peak (D band) at 1332 cm -1 , corresponding to sp 3 carbon, structural defects, or lattice disorder. There is a prominent peak (G band) near 1597 cm -1 , corresponding to sp 2 hybridized carbon. The intensity ratio of the D band to the G band (I D / I G ) is an evaluation of the degree of disorder in the graphite structure. After calculation, the I D / I G of GO is 1.04, and the I D / I G of rGO is 1.48. The increase in I D / I G indicates the effective removal of oxygen-containing functional groups and the reduction of GO.

[0072] As can be seen from Figure 1 Figure (b), in the infrared spectrum of GO, there are characteristic peaks of -OH (3317 cm -1 ), C=O (1727 cm -1 ), C-OH (1411 cm -1 ), and C-O (1047 cm -1 ). The peak at 1632 cm -1 is a resonance peak, which can be attributed to the C-C stretching vibration and hydroxyl groups in GO. After GO is reduced, in the infrared spectrum of rGO, the -OH peak, C-OH peak, and C-O peak disappear, and a new C-O-C peak (1250 cm -1 ) appears. This indicates that the hydroxyl groups and carboxylic acids of GO are reduced, and some ether bonds are formed.

[0073] As can be seen from Figure 1As can be seen from (c) of [the relevant content], the XRD pattern of GO has a strong diffraction peak at 2θ = 10.78°, while the XRD pattern of rGO shows a broad diffraction peak at 2θ = 24.80°, and the diffraction peak at 2θ = 10.78° disappears, indicating that a large number of oxygen-containing functional groups in GO are removed during the reduction process and transformed into rGO.

[0074] As can be seen from Figure 1 (d) of [the relevant content], GO shows a gradual mass loss during the TGA test. The mass loss of about 5% below 100 °C is attributed to the removal of adsorbed water. The most significant mass loss occurs at about 200 - 400 °C, which is caused by the decomposition of the most unstable oxygen functional groups. Another mass loss can be observed at about 600 °C, which is caused by the decomposition of stable oxygen functional groups. Finally, there is a small mass loss above 900 °C, which is attributed to the decomposition of the carbon skeleton in GO. A gradual mass loss can also be observed from the TGA curve of rGO. However, since the oxygen functional groups in GO are pre-removed during the chemical reduction process, especially the less stable hydroxyl and epoxy groups, although there are still some remaining oxygen-containing functional groups, the TGA curve of rGO shows a decrease, but the degree of decrease is not as drastic as that of the TGA curve of GO.

[0075] 4. To further analyze the change of carbon-oxygen functional groups, X-ray photoelectron spectroscopy was used to perform energy spectrum analysis on the carbon and oxygen of GO and rGO in the rGO@EVOH sample prepared in Example 2. The excitation source was a monochromatic Al-Kα X-ray source. The results are as Figure 2 shown.

[0076] Figure 2 Figure (a) is the XPS full spectrum of GO and rGO, Figure (b) is the C1s XPS spectrum of GO, and Figure (c) is the C1s XPS spectrum of rGO. The oxygen content of the sample is expressed by the carbon-oxygen ratio (C / O). As can be seen from Figure 2 (a) of [the relevant content], the carbon-oxygen ratio of GO is 0.83, while that of rGO is 2.06. This is because most of the oxygen functional groups have been removed during the reduction of GO, resulting in an increase in the carbon-oxygen ratio.

[0077] To determine how the oxygen-containing functional groups change during the reduction process, the C1s core-level spectra of GO and rGO were deconvoluted. As shown in Figure 2 (b) and (c) of [the relevant content], the four main peaks centered at 284.5 eV, 286.4 eV, 287.5 eV, and 289.4 V in the figure correspond to sp 2 / sp 3 carbons (C=C and C-C), C-O bonds in epoxy / alkoxy / hydroxyl groups, carbonyl (C=O), and carboxyl (C-OO). As can be seen from Figure 2As can be seen from Figure (b), the C-O bond of GO is the single bond with the highest strength among other functional groups, accounting for 38.88%. The proportion of other oxygen-containing groups is 16.80% (C=O) and 3.38% (C-OO), indicating that GO is mainly occupied by epoxy groups and hydroxyl groups, which are the main functional groups of GO. From Figure 2 As can be seen from Figure (c), as GO is reduced, the proportion of oxygen-containing functional groups decreases as a whole, and the proportion of C=C and C-C increases to 60.07%. This shows that most of the oxygen-containing functional groups of GO are eliminated after reduction by VC, but a part of oxygen-containing functional groups still remains, which is also confirmed by the TGA curve.

[0078] Example 4

[0079] In this example, a scanning electron microscope (SEM) was used to observe the morphology of the polymer open-cell skeleton and rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4.

[0080] First, each sample was placed in liquid nitrogen for a period of time and then quenched to obtain a low-temperature fracture surface to show the porous structure. Then, gold was sputtered on the fracture surface, and finally, the porous structure of the sample and the morphology of rGO therein were observed by SEM. The ImageJ software was used to statistically analyze the cell area (S c ) and the number of cells (N c ) of the SEM images of each sample, and the average cell diameter (d) was obtained using formula (1):

[0081]

[0082] Figure 3 Figures are the SEM images and the statistical chart of cell diameters of the polymer open-cell skeleton. Among them, Figures (a1)(a2), (b1)(b2), (c1)(c2), and (d1)(d2) are the SEM images and the statistical chart of cell diameters of the polymer open-cell skeletons used to prepare rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4, respectively. It can be seen from this figure that there are many small holes with open-cell structures in the polymer open-cell skeleton, and the open-cell rate is very high. There are also many nanofibrous cell ribs, and a part of these nanofibrous cell ribs is broken. Through the statistical analysis of cell diameters, it is found that the average diameter of the polymer open-cell skeleton is between 59 and 70 μm. Further, the size of the cell ribs of the polymer open-cell skeleton was statistically analyzed, and the result shows that the average diameter of the cell ribs of the polymer open-cell skeleton is between 3 and 4 μm, and the average diameter of the cell ribs of rGO@EVOH4 is 3.71 μm.

[0083] Figure 4SEM images and cell pore size statistical charts of rGO@EVOH prepared in Example 2. Among them, the (a1)(a2), (b1)(b2), (c1)(c2), and (d1)(d2) images are SEM images and cell pore size statistical charts of rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4, respectively. Figure 5 Figures (A) to (D) of Figure 4 are partial enlarged views of the (a1) to (d1) images of Figures 4 - 5 . The area framed by the dashed box in the figure shows the small pores formed by rGO in the porous framework material. It can be seen that the graphene nanosheets enter the porous structure of the polymer open-cell framework through the aqueous medium and stack in the polymer open-cell framework to form a pore structure with smaller pore diameters.

[0084] Comparing Figure 4 the porous structures shown in the SEM images of the (a1) to (d1) images of

[0085] Example 5

[0086] In this example, rGO@EVOH4 was heated at a high temperature of 1000 °C in a nitrogen atmosphere to degrade the polymer open-cell framework in rGO@EVOH4, and a sample with only the framework composed of rGO remaining was obtained for SEM testing to understand the conductive microstructure formed by rGO in the polymer open-cell framework.

[0087] Figure 6 are SEM images of the sample obtained after the polymer open-cell framework in rGO@EVOH4 was degraded by high-temperature heating at different magnifications. The arrows in the figure indicate the morphology formed by rGO coating on the cell edges and the endpoints of the cell edges of the polymer open-cell framework. It can be seen that rGO coats on the cell edges of the polymer open-cell framework, and some rGO coats on the cell edges in a curled wrapping manner ( Figure 6 Figure 6 ​Some curly rGO can be seen, and it can also be seen that the rGO covers the endpoints of the broken cell edges of the polymer open-cell skeleton. As the new cell walls in the polymer open-cell skeleton, the rGO re-divides the pore structure of the polymer open-cell skeleton, forming smaller pore structures. The rGO distributed in the polymer open-cell skeleton and the protrusions formed by its coating on the cell edges can stack to form protrusions at the endpoints of the broken cell edges, all of which can provide abundant conductive contacts for the material, thus facilitating the formation of a more stable conductive network with a higher degree of connectivity after the material is stressed.

[0088] Example 6

[0089] In this example, the sensitivity of the flexible piezoresistive sensor assembled based on the flexible piezoresistive sensing material was tested.

[0090] Conductive double-sided tape was used to adhere copper foils to the upper and lower surfaces of the flexible piezoresistive sensing material prepared in Example 2 as positive and negative electrodes. Then, two silver wires were respectively adhered to the positive and negative electrodes through silver paste, and then encapsulated with insulating tape to obtain the flexible piezoresistive sensor.

[0091] To characterize the performance of the flexible piezoresistive sensor in stress / strain sensing applications, a high resistance meter was used in combination with a universal tensile testing machine for measurement. The change in the resistance of the flexible piezoresistive sensor during the stress / strain change process was measured using the high resistance meter, and the test voltage was 0.1V. The universal testing machine was controlled through a computer program to apply stress / strain to the flexible piezoresistive sensor, and the displacement rate was 10mm / min.

[0092] To evaluate the sensitivity of the flexible piezoresistive sensor, the flexible piezoresistive sensor was subjected to a pressure with a constant downward pressure rate of 10mm / min. Through the combination of the high resistance meter and the universal tensile testing machine, the real-time stress value and real-time resistance value experienced by the flexible piezoresistive sensor were measured. The stress sensitivity (S) was calculated through Formula (2) and Formula (3):

[0093]

[0094]

[0095] In Formulas (2) to (3), U is the measurement voltage, R is the resistance, ΔI = I - I0, I0 is the initial current value when no stress is applied, I is the real-time measured current value after stress is applied, ΔP is the applied stress, and ΔI / I0 is the current change rate.

[0096] The current change rate ΔI / I0 was calculated through Formula (2), and the stress-relative current change point diagram was plotted according to Formula (3). The points in different stress ranges were linearly fitted, and the slope of the fitted equation was the stress sensitivity (S).

[0097] To evaluate the cyclic reliability of the flexible piezoresistive sensor, a series of cyclic compression loading and unloading were performed on the flexible piezoresistive sensor at a constant strain of 10%. The mechanical testing machine was controlled to drive at a constant moving rate of 10 mm / min, and the driving device was set to generate a compressive strain of approximately 10%. The generated voltage signal of the sensor was collected by connecting the flexible piezoresistive sensor to a high-resistance meter, and the data was continuously recorded by a computer.

[0098] 1. Sensitivity test results

[0099] Through the sensitivity test, the pressure response curves of the flexible piezoresistive sensors assembled based on rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4 were obtained, as Figure 7 shown, Figure 7 Figures (a) - (d) respectively represent the pressure response curves of the flexible piezoresistive sensors assembled based on rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4, and S in the figures represents the stress sensitivity.

[0100] As Figure 7 can be seen, with the increase in the rGO content in the flexible piezoresistive sensing material, the stress sensitivity of the flexible piezoresistive sensor gradually increases. When the rGO content in the flexible piezoresistive sensor is low, there are few formed conductive contacts, and at the same time, the pore size of the flexible piezoresistive sensing material is relatively large, and a greater stress needs to be applied to make the limited conductive contacts contact, so the sensitivity is low. For the flexible piezoresistive sensor assembled based on rGO@EVOH4, as the stress increases, the pressure response curve shows three stages.

[0101] In the initial stage, the pore structure in the flexible piezoresistive sensing material is relatively large, and the contact of the conductive contacts on the polymer open-cell framework is less, and the perfection degree of the formed conductive network is relatively low, so the sensitivity is low. As the stress continues to increase and enters the second stage, the current change rate increases rapidly with the increase in stress. In this stage, the contact degree between the conductive contacts in the flexible piezoresistive sensing material increases rapidly due to the decrease in the pores of the polymer open-cell framework, and the sensitivity also increases significantly. As the stress further increases and enters the third stage, in this stage, the contact area between the conductive contacts of the flexible piezoresistive sensing material becomes the dominant factor affecting the sensitivity, so the sensitivity of ΔI / I0 to stress decreases and the sensitivity becomes low.

[0102] In the stress range of 1.5 - 5 kPa, the stress sensitivity of the flexible piezoresistive sensor assembled based on rGO@EVOH4 reached 130.9 kPa -1, This is because with the increase in the rGO content, more conductive contacts are formed in the open-pore framework of the rGO-distributed polymer, and the distribution of rGO in the open-pore framework of the polymer reduces the pore size of the flexible piezoresistive sensing material, increasing the number of contactable conductive points in the conductive network of the flexible piezoresistive sensor and reducing the distance between the conductive contacts. Only a relatively small stress is required to form a significantly more perfect conductive network, thereby rapidly increasing the current.

[0103] Figure 7 The (a) and (b) figures do not fully exhibit the pressure response in three stages because the rGO content in rGO@EVOH1 and rGO@EVOH2 is relatively low. Consequently, the number of conductive contacts in the rGO conductive framework formed is less than that in rGO@EVOH4, and the distance between the conductive contacts is relatively larger. Thus, it is necessary to apply a greater stress to make it appear. However, excessive stress will damage the fiber structure inside the flexible piezoresistive sensor. At the same time, since the current flexible piezoresistive sensors aim to have high sensitivity under low stress, in this embodiment, the sensitivity test is only carried out without damaging the flexible piezoresistive sensor.

[0104] Figure 8 The SEM images of the flexible piezoresistive sensors assembled based on rGO@EVOH1, rGO@EVOH2, rGO@EVOH3, and rGO@EVOH4 before and after stable compression are shown. The first row in the figure is the SEM image before compression, and the second row is the SEM image after stable compression. It can be seen from this figure that with the increase in the rGO content in the open-pore framework of the polymer, after stable compression, more conductive contacts in the flexible piezoresistive sensor come into contact with each other.

[0105] 2. Comparison of the sensitivity with that of the piezoresistive sensors of existing porous materials

[0106] We have statistically analyzed the sensing sensitivity data of the flexible piezoresistive sensors in this application and those reported in the prior art, as shown in Table 1.

[0107] Table 1 Statistical data of the sensing sensitivity of the flexible piezoresistive sensors in this application and those reported in the prior art

[0108]

[0109]

[0110] The literature in Table 1 is as follows:

[0111] [1] Yang, C.; Liu, W.; Liu, N.; Su, J.; Li, L.; Xiong, L.; Long, F.; Zou, Z.; Gao, Y. J. A. a. m.; Interfaces, Graphene aerogel broken to fragments for a piezoresistive pressure sensor with a higher sensitivity. 2019, 11, 33165 - 33172.

[0112] [2] Qiu, L.; Bulut Coskun, M.; Tang, Y.; Liu, J. Z.; Alan, T.; Ding, J.; Truong, V. T.; Li, D. J. A. M., Ultrafast dynamic piezoresistive response of graphene - based cellular elastomers. 2016, 28, 194 - 200.

[0113] [3] Liang, J.; Zhao, Z.; Tang, Y.; Hao, X.; Wang, X.; Qiu, J. J. C., Covalent bonds - integrated graphene foam with superb electromechanical properties as elastic conductor and compressive sensor. 2019, 147, 206 - 213.

[0114] [4] Ren, H.; Zheng, L.; Wang, G.; Gao, X.; Tan, Z.; Shan, J.; Cui, L.; Li, K.; Jian, M.; Zhu, L. J. A. N., Transfer - medium - free nanofiber - reinforced graphene film and applications in wearable transparent pressure sensors. 2019, 13, 5541 - 5548.

[0115] [5]Xiao, J.;Tan, Y.;Song, Y.;Zheng, Q. J. J. o. M. C. A., A flyweight and superelastic graphene aerogel as a high-capacity adsorbent and highly sensitive pressure sensor. 2018, 6, 9074 - 9080.

[0116] [6]Yao, H. B.;Ge, J.;Wang, C. F.;Wang, X.;Hu, W.;Zheng, Z. J.;Ni, Y.;Yu, S. H. J. A. M., A flexible and highly pressure-sensitive graphene–polyurethane sponge based on fractured microstructure design. 2013, 25, 6692 - 6698.

[0117] [7]Tewari, A.;Gandla, S.;Bohm, S.;McNeill, C. R.;Gupta, D. J. A. a. m.;interfaces, Highly exfoliated MWNT–rGO ink-wrapped polyurethane foam for piezoresistive pressure sensor applications. 2018, 10, 5185 - 5195.

[0118] [8]Ge, G.;Cai, Y.;Dong, Q.;Zhang, Y.;Shao, J.;Huang, W.;Dong, X. J. N., A flexible pressure sensor based on rGO / polyaniline wrapped sponge with tunable sensitivity for human motion detection. 2018, 10, 10033 - 10040.

[0119] [9] Wu, X.; Han, Y.; Zhang,

[0120] As can be seen from Table 1, compared with the flexible piezoresistive sensors reported in existing literature, the sensing sensitivity of the flexible piezoresistive sensor based on rGO@EVOH4 provided by the present invention is significantly improved. This is mainly because the flexible piezoresistive sensing material provided by the present invention has a unique microstructure inside, and the pore structure of the polymer open-pore skeleton matches the distribution characteristics of rGO therein well, which effectively increases the conductive contacts and reduces the distance between the conductive contacts.

[0121] 3. Reliability and dynamic responsiveness

[0122] The reliability and dynamic responsiveness of the flexible piezoresistive sensors assembled based on rGO@EVOH2 and rGO@EVOH4 were measured by multiple loading-unloading dynamic stress cycles, and the cyclic stability curves were obtained, such as Figure 9 As shown, Figure 9 Figures (a1) and (b1) represent the cyclic stability curves of flexible piezoresistive sensors assembled based on rGO@EVOH2 and rGO@EVOH4, respectively. Figure 9 The upper figure of Figure (a2) is an enlarged view of the dotted box part of Figure (a1), and the lower figure is the stress change curve corresponding to the upper figure. Figure 9 The upper figure of Figure (b2) is an enlarged view of the dotted box part of Figure (b1), and the lower figure is the stress change curve corresponding to the upper figure over time.

[0123] Flexible piezoresistive sensors assembled based on rGO@EVOH2 and rGO@EVOH4 showed good performance under dynamic stress within a strain range of 10%, indicating their reliability in practical applications. The stronger the electrical signal released by the sensor, the less likely it is to be disturbed by external conditions during the detection process, resulting in a more realistic signal. Figure 9As can be seen from Figures (a1) and (b1), within the same strain range, the current change value of the flexible piezoresistive sensor assembled based on rGO@EVOH4 is much greater than that of the flexible piezoresistive sensor assembled based on rGO@EVOH2. This is mainly because the rGO content in rGO@EVOH4 is higher, there are more conductive contacts in the conductive network, and the distance between the conductive contacts is shorter. Under stress, rGO can quickly come into contact to form a perfect conductive network, thus generating a larger current value.

[0124] At the same time, as can be seen from Figure 9 Figure (b2), when no stress is applied, the flexible piezoresistive sensor assembled based on rGO@EVOH4 has a current response of 20 - 30 μA, which indicates that when rGO@EVOH4 is not under stress, the rGO in it is interconnected to form a conductive network. After applying stress, the current response quickly increases to about 80 μA, indicating that after applying stress, a more perfect conductive network is quickly constructed in rGO@EVOH4.

[0125] Dynamic force measurement was carried out with a loading - unloading cycle of 55 times at a rate of 10 mm / min within a 10% strain range. A part of the signal was randomly intercepted from the cycle and amplified. As Figure 9 shown in Figures (a2) and (b2), the flexible piezoresistive sensor can not only maintain a high signal - to - noise ratio, but also after multiple loading - unloading cycles, the current amplitude hardly changes, showing good cycle stability.

[0126] Example 7

[0127] In this example, the application and effect of the flexible piezoresistive sensing material provided by the present invention in human motion detection are demonstrated.

[0128] Copper foil was adhered to the upper and lower surfaces of rGO@EVOH4 prepared in Example 2 with conductive double - sided tape as the positive and negative electrodes. Then, two silver wires were respectively adhered to the positive and negative electrodes through silver paste, and then encapsulated with insulating tape to obtain a flexible piezoresistive sensor.

[0129] The flexible piezoresistive sensor was fixed on a nitrile glove. After putting on the glove, five cycles of bending the index finger and wrist were performed, and the electrical signals generated during the finger and wrist bending were recorded. The flexible piezoresistive sensor was fixed on the human neck, and language and swallowing tests were carried out, and the electrical signals generated during speaking and swallowing were recorded. For all experiments, the generated voltage signals of the sensor were collected from the high - resistance meter and continuously recorded by a computer. The results are as Figure 10 shown.

[0130] Figure 10 Figures (f) and (g) are photos of the flexible piezoresistive sensor fixed on the finger and wrist.Figure 10 Figures (a) and (b) show the electrical signals generated during the bending of the fingers and wrist. As the fingers and wrist bend, the generated current value rises rapidly, and when the fingers or wrist straighten, the current value returns to the initial value.

[0131] Figure 10 Figure (h) shows a photo of the flexible piezoresistive sensor fixed on the neck. Figure 10 Figures (c), (d), and (e) show the electrical signals generated when the tester speaks, drinks water, and coughs. Figure 10 As can be seen from Figure (c), the characteristic current curves generated when repeating the same word are almost the same, indicating that the sensor has good stability. Figure 10 As can be seen from Figure (d), different changes occur in the characteristic current curve when drinking water because each action causes different vibrations of the throat muscles. The significant differences between the characteristic current signals indicate that the flexible piezoresistive sensor has excellent recognition performance. Figure 10 As can be seen from Figure (e), when coughing, the flexible piezoresistive sensor can still capture the corresponding characteristic current signal. Based on these characteristic current signals, the vibrations near the throat can be distinguished.

[0132] It can be seen from this embodiment that the flexible piezoresistive sensing material provided by the present invention can be applied in wearable devices and has excellent performance and great application potential in human motion detection.

[0133] Example 8

[0134] In this embodiment, the preparation of the flexible piezoresistive sensing material is carried out as follows:

[0135] (1) Add polyether polyol EOPO to the reaction vessel, start stirring, and heat up to 120 - 125 °C under nitrogen protection for 2 h for dehydration, then cool to 40 °C. Weigh MDI according to the molar ratio of -NCO of carbodiimide-uretonimine modified 4,4'-diphenylmethane diisocyanate (MDI) to -OH of EOPO being 1:1.1, add it to the reaction vessel, stir at room temperature for 30 min, then slowly heat up to 80 °C to initiate the reaction and start timing, and react at 80 °C for 5 h to obtain the branched polymer MDI-g-EOPO.

[0136] (2) Add 92 parts by mass of EVOH with a molar content of ethylene segments of 48% and 8 parts by mass of MDI-g-EOPO to a mixer and carry out melt blending at 190 °C, control the rotation speed of the mixer to be 60 r / min, take out after melt blending for 5 min. Place the obtained blend in a flat vulcanizer, preheat at 190 °C for 5 min, and then hot press and mold at 190 °C and a pressure of 10 MPa to obtain a blank.

[0137] (3) Place the green body obtained in step (2) into a high-pressure cavity, introduce CO2 gas as a foaming agent into the high-pressure cavity, control the temperature of the high-pressure cavity to 178 °C and the pressure to 25 MPa, keep swelling for 2 h under the above temperature and pressure conditions until the foaming agent reaches saturation in the green body, and then quickly release the pressure for foaming to obtain a polymer open-cell framework.

[0138] (4) Fully disperse GO in water to form a GO dispersion with a GO concentration of 3.5 mg / mL. Take the GO dispersion and add it to a reaction vessel. Then take the polymer open-cell framework prepared in step (3) and soak it in the GO dispersion. Press the polymer open-cell framework below the liquid level, and then ultrasonically oscillate until the GO dispersion completely wets the polymer open-cell framework.

[0139] (5) According to the mass ratio of VC to GO of 3:1, add VC to the reaction vessel that has undergone the operation in step (1). Dissolve VC completely by ultrasonic and stirring, then place the reaction vessel in an oil bath at 90 °C for a reduction reaction for 1.5 h. Then take it out and cool it to room temperature, place it in a refrigerator at -18 °C and freeze for 2 h until the liquid in the reaction vessel is completely solidified, and then take it out and place it at room temperature until it completely melts; repeat the freezing and melting operations once. Then place the reaction vessel in a water bath at 90 °C and react for 4 h until GO is completely reduced. Then take out the polymer open-cell framework in the reaction vessel, cool it to room temperature, and freeze-dry it to obtain a flexible piezoresistive sensing material.

[0140] Example 9

[0141] In this example, the preparation of the flexible piezoresistive sensing material is as follows:

[0142] (1) Add polyether polyol EOPO to a reaction vessel, start stirring, heat up to 120 - 125 °C under nitrogen protection and keep for 2 h for dehydration, and then cool to 40 °C. According to the molar ratio of -NCO of carbodiimide-uretonimine modified 4,4'-diphenylmethane diisocyanate (MDI) to -OH of EOPO of 1:0.8, weigh MDI and add it to the reaction vessel, stir at room temperature for 30 min, then slowly heat up to 80 °C to initiate the reaction and start timing, and react at 80 °C for 4 h to obtain a branched polymer MDI-g-EOPO.

[0143] (2) Add 88 parts by mass of EVOH with a molar content of ethylene segments of 24% and 12 parts by mass of MDI-g-EOPO to a mixer and carry out melt blending at 190 °C, control the rotation speed of the mixer to 60 r / min, take it out after melt blending for 5 min. Place the obtained blend in a flat vulcanizer, preheat it at 190 °C for 5 min, and then hot press it at 190 °C under a pressure condition of 10 MPa to obtain a green body.

[0144] (3) Place the green body obtained in step (2) into a high-pressure cavity, introduce CO2 gas as the blowing agent into the high-pressure cavity, control the temperature of the high-pressure cavity at 180 °C and the pressure at 10 MPa, keep swelling for 2 h under the aforementioned temperature and pressure conditions until the blowing agent reaches saturation in the green body, and then rapidly release the pressure for foaming to obtain a polymer open-cell skeleton.

[0145] (4) Disperse GO fully in water to form a GO dispersion with a GO concentration of 4.5 mg / mL. Take the GO dispersion and add it to a reaction vessel. Then take the polymer open-cell skeleton prepared in step (3) and soak it in the GO dispersion. Press the polymer open-cell skeleton below the liquid level, and then ultrasonically oscillate until the GO dispersion completely wets the polymer open-cell skeleton.

[0146] (5) According to the mass ratio of VC to GO of 2:1, add VC to the reaction vessel that has undergone the operation in step (1). Ultrasonicate and stir until VC is fully dissolved. Then place the reaction vessel in an oil bath at 95 °C for a reduction reaction for 1 h. Then take it out and cool it to room temperature. Place it in a refrigerator at -18 °C and freeze for 2 h until the liquid in the reaction vessel is completely solidified. Then take it out and place it at room temperature until it completely melts; repeat the freezing and melting operations 3 times. After that, place the reaction vessel in a water bath at 95 °C and react for 6 h until GO is completely reduced. Then take out the polymer open-cell skeleton in the reaction vessel, cool it to room temperature, and freeze-dry it to obtain a flexible piezoresistive sensing material.

Claims

1. A highly sensitive flexible piezoresistive sensing material with multiple conductive contacts, characterized in that: The flexible piezoresistive sensing material is composed of a polymer open-cell framework and reduced graphene oxide, and has a mutually interconnected porous structure; The polymer open-cell framework has mutually interconnected cells. The average pore diameter of the polymer open-cell framework does not exceed 80 μm. The cell edges are fibrous, and some of the cell edges are broken; Reduced graphene oxide is distributed in the polymer open-cell framework. Part of the reduced graphene oxide coats the cell edges, part of the reduced graphene oxide covers the endpoints of the broken cell edges, and part of the reduced graphene oxide penetrates through multiple cells and simultaneously covers multiple cell edges to form cell walls, forming a pore structure with a smaller pore diameter than that of the polymer open-cell framework in the polymer open-cell framework, and forming a reduced graphene oxide conductive network interpenetrating with the polymer open-cell framework.

2. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to claim 1, characterized in that: The average pore diameter of the flexible piezoresistive sensing material is 40% - 60% of the average pore diameter of the polymer open-cell framework.

3. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to claim 1, characterized in that: The average pore diameter of the polymer open-cell framework is 50 - 80 μm.

4. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to claim 1, characterized in that, In the polymer open-cell framework, the average diameter of the cell edges is 3 - 4 μm, and some of the cell edges are nanofibrous with a diameter not exceeding 400 nm.

5. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to claim 1, characterized in that, In the flexible piezoresistive sensing material, the content of reduced graphene oxide is 9 wt.% - 11 wt.%.

6. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to any one of claims 1 to 5, characterized in that The sheet size of the reduced graphene oxide is 0.2 - 10 μm.

7. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to any one of claims 1 to 5, characterized in that, In the pressure range of 1.5 to 5 kPa, the stress sensitivity of this flexible piezoresistive sensing material is 120 to 140 kPa -1 .

8. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to any one of claims 1 to 5, characterized in that: The matrix material of the polymer open-cell framework is composed of ethylene-vinyl alcohol copolymer, branched polymer, and ethylene-vinyl alcohol copolymer grafted with a branched polymer; The branched polymer is a polyether polyol with a terminal group of -NCO, and the ethylene-vinyl alcohol copolymer grafted with a branched polymer is formed by the reaction of the terminal group of the branched polymer with the side hydroxyl group of the ethylene-vinyl alcohol copolymer.

9. The highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to claim 8, characterized in that: The polymer open-cell framework is prepared by melt blending 88 - 92 parts by mass of ethylene-vinyl alcohol copolymer and 8 - 12 parts by mass of branched polymer, molding, and supercritical fluid depressurization foaming; The melting point of the branched polymer is lower than the melting point of the blend formed by melt blending the branched polymer and the ethylene-vinyl alcohol copolymer.

10. The method for preparing a highly sensitive flexible piezoresistive sensing material with multiple conductive contacts according to any one of claims 1 to 9, characterized in that: It includes the following steps: (1) Add a graphene oxide dispersion with a concentration of 3.5 - 4.5 mg / mL to a reaction vessel, immerse the polymer open-cell framework in the graphene oxide dispersion, and ultrasonically treat until the graphene oxide dispersion fully wets the polymer open-cell framework; (2) Add ascorbic acid to the reaction vessel according to the mass ratio of ascorbic acid to graphene oxide of (2 - 3):

1. After the ascorbic acid is completely dissolved, react at 90 - 95 °C for 1 - 1.5 h, then cool to room temperature, freeze until the liquid in the reaction solution is completely solidified, and then take it out and place it at room temperature until it completely melts; Repeat the freezing and melting operations 1 - 3 times; Then react at 90 - 95 °C for 4 - 6 h, take out the polymer open-cell framework in the reaction vessel, cool to room temperature, and then freeze-dry to obtain the flexible piezoresistive sensing material.

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