Crack type flexible strain sensitive composite material and preparation method thereof

Abstract

The invention belongs to the technical field of flexible conductive composite materials, and particularly relates to a crack type flexible strain sensitive composite material and a preparation method thereof.The preparation method comprises the following steps that multi-walled carbon nanotubes (MWCNTs) serve as a conductive layer, maleic anhydride grafted styrene-ethylene-butylene-styrene copolymer SEBS-g-MAH serves as a matrix layer, and the matrix layer is prepared through a sol-gel method; and the composite material mCSM with high stretchability is prepared. On the basis of an mCSM, the mCSM is pre-stretched to a certain pre-strain, a controllable digital crack array is etched on a conducting layer of the mCSM through a femtosecond laser process, and the strain is released, so that composite materials PC-L and PC-S with different crack parameters can be obtained. Wherein the composite material PC-S3 with the crack width of 30 [mu] m, the crack length of 1.4 mm and the crack gap of 300 [mu] m when the pre-strain is 100% has high sensitivity (GF = 938.62), a wide sensing range (661.72%), rapid response under 0.3% strain, small hysteresis effect and capability of maintaining long-term stability under large strain (stable circulation for 500 times under 100% strain).

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible conductive composite materials, and in particular relates to a crack-type flexible strain-sensitive composite material and a preparation method thereof. Background Art

[0002] With the rapid development of science and technology, flexible force-sensitive materials hold great promise for development in fields such as smart healthcare, wearable devices, human-machine interfaces, soft robotics, and artificial intelligence. However, maintaining stable signals over long periods of time in complex environments is currently a challenge. Therefore, achieving stable and sensitive strain / stress signal detection under large strains is a significant challenge.

[0003] In order to prepare highly stretchable conductive composite materials, people have used composite materials of different forms, such as percolation networks formed by mixing conductive fillers with polymer matrices, isolated structures, and layered structures. Among them, layered composite materials can take into account both high conductivity and flexibility, with flexible and controllable processes, and can maintain structural stability and consistency, thus achieving multifunctional sensing. Although the composite materials prepared by the above methods have improved conductivity and stretchability, the interface between the conductive layer and the matrix is not good, so the filler will slide off the matrix during stretching, resulting in unstable resistance response.

[0004] To achieve strain response, the key to strain / stress sensing lies in the response mechanism, which includes piezoresistive effects, tunneling effects, and crack propagation. Crack propagation mechanisms provide high sensitivity. This ultra-high mechanical sensitivity is due to the gradual expansion of cracks under strain or vibration, which leads to a break-and-reconnect process at nanoscale crack junctions, resulting in a destruction-and-recovery process of the internal conductive network. These microscopic changes ultimately give the material ultra-high sensitivity at the macroscopic level.

[0005] Crack formation is crucial in the crack propagation mechanism. In recent years, crack preparation methods have included template transfer, mechanical cutting, and pre-stretching (ACS Appl. Mater. Interfaces, 2022, 14, 34, 39230–39239; Nat Commun, 2024, 15, 3752; Materials Today Physics 48, 2024, 101562; Small 19, 50, 2023, 2304033). Pre-stretching to create microcracks and adjusting crack density can significantly improve sensor sensitivity. Current research has enhanced the sensitivity of flexible sensing materials by manipulating crack structures. However, to further optimize their sensitivity, the controllable preparation of cracks and their stable response remain key challenges.

[0006] However, the crack structures fabricated in previous work often have difficulties such as randomness and uncontrollability, which easily lead to instability in the preparation process and sensing performance, thus limiting the widespread application of sensing materials based on crack structures. Summary of the Invention

[0007] In order to solve the above-mentioned deficiencies in the prior art, the present invention aims to provide a crack-type flexible strain-sensitive composite material and a preparation method thereof.

[0008] The crack structure of the flexible strain-sensitive composite material is controllable, and it exhibits a wide sensing range, high sensitivity, fast response at 0.3% strain, low hysteresis effect, and cyclic stability under large strain.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A crack-type flexible strain-sensitive composite material is provided. The crack-type flexible strain-sensitive composite material consists of a conductive layer and a base layer. The conductive layer is multi-walled carbon nanotubes (MWCNTs), and the base layer is maleic anhydride grafted styrene-ethylene-butylene-styrene copolymer (SEBS-g-MAH). A crack array is etched on the conductive layer of the crack-type flexible strain-sensitive composite material.

[0011] The present invention also provides a method for preparing the above-mentioned crack-type flexible strain-sensitive composite material, the preparation method comprising the following steps:

[0012] (1) SEBS-g-MAH is subjected to internal mixing and first hot pressing to obtain a SEBS-g-MAH membrane; MWCNTs are mixed with DMF and ultrasonically treated to obtain a mixed solution; the mixed solution is coated on the SEBS-g-MAH membrane and dried to obtain a CSM composite material;

[0013] (2) hot pressing the CSM composite material obtained in step (1) for a second time to obtain an mCSM composite material;

[0014] (3) After the mCSM composite material obtained in step (2) is stretched to a certain pre-strain, a crack array is etched on the conductive layer using a femtosecond laser process, and the pre-strain is released to obtain a crack-type flexible strain-sensitive composite material.

[0015] Furthermore, in the step (1), the banburying conditions are banburying at 150-170°C for 5-10 min (preferably banburying at 160°C for 5 min); the first hot pressing conditions are hot pressing at 150-170°C and 5-10 MPa for 5-10 min (preferably hot pressing at 160°C and 5 MPa for 10 min).

[0016] Furthermore, in the step (1), the mass ratio of MWCNTs to DMF is 3:200.

[0017] Furthermore, in step (1), the ultrasonic treatment condition is 650W ultrasonic treatment for 20-40 minutes (preferably 30 minutes); the drying condition is drying at 20-40°C for 2-6 hours (preferably drying at 30°C in a forced air drying oven for 4 hours).

[0018] Furthermore, the second hot pressing is performed at 150-170° C. and 2-4 MPa for 10-12 min (preferably at 160° C. and 2 MPa for 10 min).

[0019] Furthermore, the pre-strain is 100%; the crack array is a mutually staggered comb-shaped crack array, the crack length is 1.4-2.8 mm, the crack width is 30 μm, and the crack gap is 300-700 μm.

[0020] Furthermore, the specific operation of etching the crack array on the conductive layer is: using the femtosecond laser damage threshold difference window to etch the MWCNTs layer on the surface of the mCSM composite material, so that the etching gap is controlled at 30μm and the incision ablation is reduced.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0022] 1. The preparation method of the present invention first prepares a layered mCSM composite material by coating and hot pressing. The MWCNTs in the conductive layer of the mCSM composite material are deeply embedded in the matrix, which improves the stretchability of the mCSM and increases its effective sensing range to 1545% (see Figure 2 f) in the figure can also respond effectively under 500% strain.

[0023] 2. PC-L (GF = 802.07) has higher sensitivity than UC-L (GF = 5.38), but the sensing range (113.1%) is lower than that of UC-L (945%) (see Figure 4 In Figure c), after releasing the tensile strain from the cracked structure fabricated under 100% prestrain, the MWCNTs on either side of the crack can reclose, restoring the conductive path. This crack closure reduces the initial resistance (R0) of the composite and accelerates the resistance response, laying the foundation for high sensitivity.

[0024] 3. By shortening the crack length to build a PC-S structure, the working range is increased to 1001.9% (see Figure 5 f), and after further optimizing the crack gap, the material PC-S3 achieves a balance between sensitivity (GF = 938.62) and working range (661.72%) (see Figure 6 h in the figure). PC-S3 exhibits excellent resilience and rapid self-recovery at 50% and 100% strains; it exhibits rapid response (response time <700ms) at a small strain of 0.3%, and can achieve effective and stable sensing in the strain gradient range of 0.3%-10%; it can stably cycle 500 times at 100% strain (see Figure 7 ). BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the customized inter-digital pattern composed of interlaced comb-shaped crack arrays in Examples 1-4;

[0026] Figure 2 The cross-sectional SEM morphology, tensile properties, and electrical properties of the CSM in Comparative Example 1 and the mCSM in Comparative Example 2 are shown;

[0027] Figure 3 The SEM morphology images of mCSM in Comparative Example 2, UC-L in Comparative Example 3, and PC-L in Example 1 are shown;

[0028] Figure 4 The tensile properties and electrical properties of mCSM in Comparative Example 2, UC-L in Comparative Example 3, and PC-L in Example 1;

[0029] Figure 5 The SEM morphology, tensile properties, and electrical properties of PC-S1 in Example 2 and PC-L in Example 1 are shown;

[0030] Figure 6 The SEM morphology, tensile properties, and electrical properties of PC-S1 in Example 2, PC-S2 in Example 3, and PC-S3 in Example 4 are shown;

[0031] Figure 7 These are the tensile and electrical properties of PC-S3 in Example 4.

[0032] Figure 8 The resistance response of PC-S3 in Example 4 within the strain range of 0.3%-10%. DETAILED DESCRIPTION

[0033] The technical solutions of the present invention are described in detail below through specific embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0034] MWCNTs (50 μm in length, 8–15 nm in diameter, Xianfeng Nanotechnology Co., Ltd.); DMF (99.5%, Sinopharm Chemical Reagent Co., Ltd.); SEBS-g-MAH particles (brand 3356, with a weight-average molecular weight (Mw) of 100,000–200,000 g / mol; MAH content approximately 2 wt%).

[0035] Example 1

[0036] A method for preparing a crack-type flexible strain-sensitive composite material comprises the following steps:

[0037] (1) SEBS-g-MAH particles were kneaded in a torque rheometer at 160°C and 50 rpm for 5 min, cut into blocks with scissors, and then hot-pressed in a hot press at 160°C and 5 MPa for 10 min to obtain a 10 mm × 10 mm × 1 mm SEBS-g-MAH membrane; MWCNTs were mixed with DMF and ultrasonically treated at 650 W for 30 min to obtain a mixed solution with a mass ratio of MWCNTs to DMF of 3:200; the mixed solution was coated on the SEBS-g-MAH membrane and placed in a forced air drying oven at 30°C for 4 h to evaporate the solvent DMF to obtain a CSM composite material; the CSM composite material was placed in a hot press mold and hot-pressed at 160°C and 2 MPa for 10 min to obtain an mCSM composite material.

[0038] (2) The mCSM composite material was cut into a rectangular film with a size of 16 mm × 4 mm × 1 mm, fixed on a glass plate, stretched to 100% pre-strain, and placed in the space of the laser machine. After setting the laser beam power (i.e., 0.08 mW), the surface MWCNTs layer (i.e., the conductive layer) was etched using the femtosecond laser damage threshold difference window to control the etching gap to 30 μm and the etching depth to about 67 μm, and to reduce the incision ablation. A customized interdigital pattern (e.g., a comb-like crack array interlaced with each other) was etched on the conductive layer using a laser. Figure 1 As shown in the figure), while the matrix layer remains unchanged, the pre-strain is released to promote the local contact recovery of the cut crack, and finally a flexible composite material PC-L with a crack width of 30μm, a crack length of 2.8mm and a crack gap of 700μm is obtained when the strain is 100%.

[0039] Example 2

[0040] A method for preparing a crack-type flexible strain-sensitive composite material comprises the following steps:

[0041] (1) SEBS-g-MAH particles were kneaded in a torque rheometer at 160°C and 50 rpm for 5 min, cut into blocks with scissors, and then hot-pressed in a hot press at 160°C and 5 MPa for 10 min to obtain a 10 mm × 10 mm × 1 mm SEBS-g-MAH membrane; MWCNTs were mixed with DMF and ultrasonically treated at 650 W for 30 min to obtain a mixed solution with a mass ratio of MWCNTs to DMF of 3:200; the mixed solution was coated on the SEBS-g-MAH membrane and placed in a forced air drying oven at 30°C for 4 h to evaporate the solvent DMF to obtain a CSM composite material; the CSM composite material was placed in a hot press mold and hot-pressed at 160°C and 2 MPa for 10 min to obtain an mCSM composite material.

[0042] (2) The mCSM composite material was cut into a rectangular film with a size of 16 mm × 4 mm × 1 mm, fixed on a glass plate, stretched to 100% pre-strain, and placed in the space of the laser machine. After setting the laser beam power (i.e., 0.08 mW), the surface MWCNTs layer was etched using the femtosecond laser damage threshold difference window to control the etching gap to 30 μm and the etching depth to about 67 μm, while reducing the incision ablation. A customized interdigital pattern (e.g., a comb-like crack array interlaced with each other) was etched on the conductive layer using a laser. Figure 1 As shown in the figure), while the matrix layer remains unchanged, the pre-strain is released to promote the local contact recovery of the cut crack, and finally a flexible composite material PC-S (PC-S1) with a crack width of 30 μm, a crack length of 1.4 mm, and a crack gap of 700 μm is obtained when the strain is 100%.

[0043] Example 3

[0044] A method for preparing a crack-type flexible strain-sensitive composite material comprises the following steps:

[0045] (1) SEBS-g-MAH particles were kneaded in a torque rheometer at 160°C and 50 rpm for 5 min, cut into blocks with scissors, and then hot-pressed in a hot press at 160°C and 5 MPa for 10 min to obtain a 10 mm × 10 mm × 1 mm SEBS-g-MAH membrane; MWCNTs were mixed with DMF and ultrasonically treated at 650 W for 30 min to obtain a mixed solution with a mass ratio of MWCNTs to DMF of 3:200; the mixed solution was coated on the SEBS-g-MAH membrane and placed in a forced air drying oven at 30°C for 4 h to evaporate the solvent DMF to obtain a CSM composite material; the CSM composite material was placed in a hot press mold and hot-pressed at 160°C and 2 MPa for 10 min to obtain an mCSM composite material.

[0046] (2) The mCSM composite material was cut into a rectangular film with a size of 16 mm × 4 mm × 1 mm, fixed on a glass plate, stretched to 100% pre-strain, and placed in the space of the laser machine. After setting the laser beam power (i.e., 0.08 mW), the surface MWCNTs layer was etched using the femtosecond laser damage threshold difference window to control the etching gap to 30 μm and the etching depth to about 67 μm, while reducing the incision ablation. A customized interdigital pattern (e.g., a comb-like crack array interlaced with each other) was etched on the conductive layer using a laser. Figure 1 As shown in the figure), while the matrix layer remains unchanged, the pre-strain is released to promote the local contact recovery of the cut crack, and finally a flexible composite material PC-S (PC-S2) with a crack width of 30 μm, a crack length of 1.4 mm, and a crack gap of 400 μm is obtained when the strain is 100%.

[0047] Example 4

[0048] A method for preparing a crack-type flexible strain-sensitive composite material comprises the following steps:

[0049] (1) SEBS-g-MAH particles were kneaded in a torque rheometer at 160°C and 50 rpm for 5 min, cut into blocks with scissors, and then hot-pressed in a hot press at 160°C and 5 MPa for 10 min to obtain a 10 mm × 10 mm × 1 mm SEBS-g-MAH membrane; MWCNTs were mixed with DMF and ultrasonically treated at 650 W for 30 min to obtain a mixed solution with a mass ratio of MWCNTs to DMF of 3:200; the mixed solution was coated on the SEBS-g-MAH membrane and placed in a forced air drying oven at 30°C for 4 h to evaporate the solvent DMF to obtain a CSM composite material; the CSM composite material was placed in a hot press mold and hot-pressed at 160°C and 2 MPa for 10 min to obtain an mCSM composite material.

[0050] (2) The mCSM composite material was cut into a rectangular film with a size of 16 mm × 4 mm × 1 mm, fixed on a glass plate, stretched to 100% pre-strain, and placed in the space of the laser machine. After setting the laser beam power (i.e., 0.08 mW), the surface MWCNTs layer was etched using the femtosecond laser damage threshold difference window to control the etching gap to 30 μm and the etching depth to about 67 μm, while reducing the incision ablation. A customized interdigital pattern (e.g., a comb-like crack array interlaced with each other) was etched on the conductive layer using a laser. Figure 1As shown in the figure), while the matrix layer remains unchanged, the pre-strain is released to promote the local contact recovery of the cut crack, and finally a flexible composite material PC-S (PC-S3) with a crack width of 30 μm, a crack length of 1.4 mm, and a crack gap of 300 μm is obtained when the strain is 100%.

[0051] Comparative Example 1

[0052] A method for preparing a flexible strain-sensitive composite material comprises the following steps:

[0053] (1) SEBS-g-MAH particles were kneaded in a torque rheometer at 160°C and 50 rpm for 5 min, cut into blocks with scissors, and then hot-pressed in a hot press at 160°C and 5 MPa for 10 min to obtain a 10 mm × 10 mm × 1 mm SEBS-g-MAH membrane; MWCNTs and DMF were mixed and ultrasonically treated at 650 W for 30 min to obtain a mixed solution with a mass ratio of MWCNTs to DMF of 3:200; the mixed solution was coated on the SEBS-g-MAH membrane and placed in a forced air drying oven at 30°C for 4 h to evaporate the solvent DMF to obtain the CSM composite material.

[0054] Comparative Example 2

[0055] A method for preparing a flexible strain-sensitive composite material comprises the following steps:

[0056] (1) SEBS-g-MAH particles were kneaded in a torque rheometer at 160°C and 50 rpm for 5 min, cut into blocks with scissors, and then hot-pressed in a hot press at 160°C and 5 MPa for 10 min to obtain a 10 mm × 10 mm × 1 mm SEBS-g-MAH membrane; MWCNTs were mixed with DMF and ultrasonically treated at 650 W for 30 min to obtain a mixed solution, with a mass ratio of MWCNTs to DMF of 3:200; the mixed solution was coated on the SEBS-g-MAH membrane and placed in a forced air drying oven at 30°C for 4 h to completely evaporate the solvent DMF to obtain a CSM composite material, which was then placed in a hot press mold and hot-pressed at 160°C and 2 MPa for 10 min to obtain an mCSM composite material.

[0057] Comparative Example 3

[0058] A method for preparing a crack-type flexible strain-sensitive composite material comprises the following steps:

[0059] (1) SEBS-g-MAH particles were kneaded in a torque rheometer at 160°C and 50 rpm for 5 min, cut into blocks with scissors, and then hot-pressed in a hot press at 160°C and 5 MPa for 10 min to obtain a 10 mm × 10 mm × 1 mm SEBS-g-MAH membrane; MWCNTs were mixed with DMF and ultrasonically treated at 650 W for 30 min to obtain a mixed solution with a mass ratio of MWCNTs to DMF of 3:200; the mixed solution was coated on the SEBS-g-MAH membrane and placed in a forced air drying oven at 30°C for 4 h to evaporate the solvent DMF to obtain a CSM composite material; the CSM composite material was placed in a hot press mold and hot-pressed at 160°C and 2 MPa for 10 min to obtain an mCSM composite material.

[0060] (2) The mCSM composite material was cut into a rectangular film with a size of 16 mm × 4 mm × 1 mm, fixed on a glass plate, and placed in the space of the laser machine. After setting the laser beam power (i.e., 0.08 mW), the surface MWCNTs layer was etched using the femtosecond laser damage threshold difference window to control the etching gap to 30 μm and the etching depth to about 67 μm, while reducing the incision ablation. A customized interdigital pattern consisting of an array of interlaced comb-like cracks (e.g., Figure 1 As shown), a flexible composite material UC-L with a crack width of 30 μm, a crack length of 2.8 mm, and a crack spacing of 700 μm was obtained.

[0061] Sensing performance test

[0062] The tensile properties and electrical properties of the composite materials prepared in Examples 1-4 and Comparative Examples 1-3 were tested respectively. The specific test conditions are as follows: the cross-section and cross-sectional morphology of the composite material after brittle fracture in liquid nitrogen and gold spraying treatment were characterized by scanning electron microscopy; the composite material was prepared into a dumbbell-shaped specimen with a spline size of Type I, the sample size was 12mm×4mm×1mm, the tensile speed was 20mm / min, multiple tests were performed and the intermediate value was taken; a desktop digital multimeter UT804 was used in combination with a universal tensile testing machine and connected to the dumbbell-shaped specimen, and the electrical changes and mechanical properties of the sample were measured at a tensile speed of 20mm / min; the .... The UT804 digital multimeter was used in combination with a universal tensile testing machine to measure the cyclic electromechanical properties of the sample at tensile rates of 5 mm / min, 10 mm / min, 20 mm / min, 50 mm / min, and 100 mm / min and strains of 300% and 500%. The PC-S3 composite material prepared in Example 4 was connected to the throat, fingers, elbows, wrists, and knees using copper wire and tape, and the resistance of repeated motion at a tensile rate of 20 mm / min was recorded using a desktop digital multimeter UT804 to conduct a test for human motion monitoring.

[0063] Figure 2 Figures a and b are the cross-sectional morphologies of the CSM prepared in comparative example 1 and the mCSM prepared in comparative example 2, respectively. It can be seen that the MWCNTs in the CSM are only attached to its surface, while the MWCNTs in the mCSM composite material are embedded in the SEBS-g-MAH matrix. Figure 2 Figure c shows that the elongation at break of the mCSM after hot pressing (1589.08%) is significantly higher than that of the original CSM (1251.94%), and the tensile strength of both exceeds 7.5 MPa, indicating that hot pressing enhances the ductility of the material while maintaining high mechanical strength. Figure 2 Figures d and e show that the resistance of the composite material increases with increasing strain, but the working range of mCSM (1545%) far exceeds that of CSM (536.25%), which is attributed to the fact that MWCNTs are semi-embedded in the matrix after hot pressing, and the conductive path remains connected under large strain. Figure 2 The f and g in Figure 3 show that the resistance of mCSM is stable at 300% and even 500% strain, while the resistance of CSM fluctuates significantly after 100% strain. The data of mCSM composite materials and other materials reported in the literature are processed by formula (1) and compared, as shown in Figure 3. Figure 2 As shown in h, it can be found that the mCSM composite material has a higher y value among similar materials, and has a wider working range and a stable working range.

[0064]

[0065] Where W sIt represents the working range, W0 is defined as the stable working range, and the ratio of the two is defined as y. It can be seen that the larger the y value is, the better the stability of the material.

[0066] Figure 2 The original data sources of the materials in h are as follows:

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[0080] This work: mCSM composite material in Comparative Example 2.

[0081] like Figure 3 As shown, Figure 3 a in the table is the mCSM composite material in Comparative Example 2, Figure 3 b is the flexible composite material UC-L in Comparative Example 3, Figure 3 Figure c is an SEM image of the flexible composite material PC-L from Example 1. It can be seen that the MWCNTs on the composite's surface are evenly dispersed across the substrate surface. Cracks approximately 30 μm wide appear on the surface of the UC-L. When the strain is 100%, a crack approximately 30 μm wide also appears on the surface of the PC-L. Upon strain release, the crack closes, and the MWCNTs on either side of the crack gradually converge until they overlap, demonstrating the successful fabrication of controlled cracking.

[0082] like Figure 4 As shown in Figure 1, the cracks in PC-L in Example 1 partially closed after the pre-strain was released, and the edges of the MWCNTs overlapped to reconstruct the conductive network, making its resistance (941.37Ω) lower than that of UC-L (3161.56Ω). The tensile test showed that PC-L had the highest sensitivity (802.07, Figure 4 c), but the crack is too long, making its conductive network easily destroyed under large strain, and the working range is reduced to 113.1%; while UC-L relies on the conductive path in the uncracked area to maintain a high working range (945%).

[0083] like Figure 5 As shown in a, when the strain is 100%, the crack length of PC-S1 in Example 2 is only half of that of PC-L in Example 1, and both cracks are partially closed after the strain is released ( Figure 5 b1-b2 in the figure), which is due to the shrinkage of the matrix causing the MWCNTs on both sides of the crack to approach each other; at the same time, the high elongation at break and tensile strength ( Figure 5 c) in the figure lays the foundation for its multifunctional sensing application. Although the initial resistance of the two is similar ( Figure 5 d), but during the stretching process, the resistance of PC-L increases faster due to longer cracks and more MWCNTs overlapping the crack edges ( Figure 5 e in). However, the working range of PC-S1 (1001.9%) far exceeds that of PC-L (113.1%) ( Figure 5 f), which is attributed to the fact that its short crack structure can still maintain the conductive network through the conductive paths in the uncracked areas after the MWCNTs are completely separated, thereby significantly improving the working range.

[0084] Figure 6 The results show the effect of the crack gaps (S1 = 700 μm, S2 = 400 μm, S3 = 300 μm) of the PC-S1 composite material in Example 2, PC-S2 in Example 3, and PC-S3 in Example 4 on the sensing performance of the prepared composite materials when the strain is 100%. The actual crack gaps measured in the experiment are 722 μm, 401.54 μm, and 306.99 μm, respectively. Figure 6 bd in the figure is basically consistent with the design parameters, and the cracks all overlap partially. After strain release, the crack closure rate increases significantly with the decrease of the gap (S1 = 45.24%, S2 = 55.32%, S3 = 70.31%), indicating that the smaller the gap, the higher the degree of closure ( Figure 6 e), among which PC-S3 has the lowest initial resistance due to the largest number of MWCNTs overlapping the crack edge ( Figure 6 f in the tensile test. The tensile test shows that the resistance of the three materials increases with the increase of strain ( Figure 6g), but PC-S3 has the fastest resistance growth rate in the 0-100% strain range because the number of MWCNTs overlapping at the crack is the largest. When stretched, the MWCNTs separate along the two sides of the crack, resulting in a significant reduction in the conductive path and the most significant resistance change. Among them, PC-S3 has high sensitivity (GF=938.62), wide working range (661.72) ( Figure 6 h) in the text.

[0085] Figure 7 It is shown that Example 4 (composite material PC-S3 with a crack width of 30 μm, a crack length of 1.4 mm, and a crack gap of 300 μm when the pre-strain is 100%) has a stable resistance response at different tensile rates and different strain rates, a fast response at 1% strain, a small hysteresis effect, and cyclic stability under strain.

[0086] By testing PC-S3 composite materials at different tensile rates (5-200mm / min) ( Figure 7 a) and different strain amplitudes ( Figure 7 The cyclic sensing behavior of b) in Figure 1 verifies its excellent sensing performance. The results show that the sensor can output stable and repeatable resistance responses at different rates ( Figure 7 a and b in ), and the maximum ΔR / R0 changes periodically with the increase of strain amplitude ( Figure 7 b), it maintains good stability under strain up to 300%. The small strain (1%) test shows that its response time and recovery time are 1400ms and 700ms respectively. Figure 7 c) in the above example. Figure 7 The d in the figure shows the hysteresis of PC-S3 at 50% strain and 100% strain. According to formula (2), the hysteresis of the resistance response of PC-S3 under 50% and 100% strain in one cycle of stretching / releasing is about 4% and 6.7%, respectively, indicating its rapid resilience under large strain. After 500 cycles of 100% strain test ( Figure 7 In figure e), the sensor shows no obvious drift and the response curves are highly consistent, which confirms its long-term dynamic stability and reliability, making it suitable for irregular human motion monitoring.

[0087]

[0088] Among them, A L and A U are the areas under the curves of the resistance response of stretching / releasing under one cycle of strain in the loading path and unloading path, respectively, and H is the hysteresis.

[0089] Figure 8ai and lm are the resistance responses of the PC-S3 composite material of Example 4 in the strain range of 0.3%-10%, respectively, demonstrating that the PC-S3 composite material can recognize ultra-low strain below 0.3% and achieve effective sensing in the strain gradient range of 0.3%-10%.

[0090] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A crack-type flexible strain-sensitive composite material, comprising a conductive layer and a matrix layer, wherein the conductive layer is multi-walled carbon nanotubes (MWCNTs) and the matrix layer is maleic anhydride-grafted styrene-ethylene-butylene-styrene copolymer (SEBS-g-MAH), characterized in that: A crack array is etched on the conductive layer of the crack-type flexible strain-sensitive composite material.

2. A method for preparing the crack-type flexible strain-sensitive composite material according to claim 1, characterized in that: The preparation method comprises the following steps: (1) SEBS-g-MAH is subjected to internal mixing and first hot pressing to obtain a SEBS-g-MAH membrane; MWCNTs are mixed with DMF and ultrasonically treated to obtain a mixed solution; the mixed solution is coated on the SEBS-g-MAH membrane and dried to obtain a CSM composite material; (2) hot pressing the CSM composite material obtained in step (1) for a second time to obtain an mCSM composite material; (3) After the mCSM composite material obtained in step (2) is stretched to a certain pre-strain, a crack array is etched on the conductive layer using a femtosecond laser process, and the pre-strain is released to obtain a crack-type flexible strain-sensitive composite material.

3. The preparation method according to claim 2, characterized in that In the step (1), the banburying condition is 150-170° C. for 5-10 min; the first hot pressing condition is 150-170° C., 5-10 MPa for 5-10 min.

4. The preparation method according to claim 2, characterized in that In the step (1), the mass ratio of MWCNTs to DMF is 3:

200.

5. The preparation method according to claim 2, characterized in that In the step (1), the ultrasonic treatment condition is 650W ultrasonic treatment for 20-40 minutes; the drying condition is drying at 20-40°C for 2-6 hours.

6. The preparation method according to claim 2, characterized in that In the step (2), the second hot pressing is performed at 150-170°C and 2-4 MPa for 10-12 minutes.

7. The preparation method according to claim 2, characterized in that In the step (3), the prestrain is 100%; the crack array is a mutually staggered comb-shaped crack array, the crack length is 1.4-2.8 mm, the crack width is 30 μm, and the crack gap is 300-700 μm.

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