Non-external stress in-situ real-time monitoring method for phase transition temperature of Vitrimer topology

Abstract

The invention provides an external-stress-free in-situ real-time monitoring method for the phase transition temperature of Vitrimer topology, and belongs to the technical field of sensors. The preparation method comprises the following steps: preparing a carbon nanotube dispersion liquid by taking a multi-walled carbon nanotube as a raw material; spraying the carbon nanotube dispersion liquid on a fiber web, performing heat treatment to obtain a CNT composite fiber web sensor, constructing electrodes at the two ends of the CNT composite fiber web sensor by using conductive silver paste, and connecting solid copper wires to the two ends of the electrodes to transmit electric signals responded by the sensor; a CNT composite fiber web sensor is fully soaked in Vitrimer resin to be monitored, a digital source meter synchronously collects resistance signals and temperature signals in real time, a temperature-resistance change rate curve is obtained through fitting, and the temperature corresponding to a first inflection point of the resistance change rate on the curve is the topological phase transition temperature. The invention firstly proposes that the topological phase transition temperature of the material is represented by an electric signal, and has the advantages of no invasiveness, no need of a reference substance, direct signal response, high in-situ real-time performance and the like.

Description

TECHNICAL FIELD

[0001] The application relates to a method for in-situ real-time monitoring of the topological phase transition temperature of a Vitrimer without external stress, and belongs to the technical field of sensors. BACKGROUND

[0002] Epoxy resins are widely used in various fields due to their excellent mechanical properties, but traditional thermosetting epoxy resins are difficult to reshape, process or recycle after forming a three-dimensional crosslinked network. By introducing exchangeable dynamic chemical bonds and effectively and reliably exchanging the dynamic chemical bonds at different positions of the organic polymer chain, the macroscopic flow can be realized, and the purpose of reprocessing and recycling can be achieved. The polymer network containing exchangeable chemical bonds is called covalent adaptive network (CAN), and the Vitrimer is a polymer with CAN.

[0003] Among them, the topological phase transition temperature (T T v ) of the Vitrimer resin is similar to the glass transition temperature T g and the melting point T m of ordinary polymers, T v which is both the upper limit temperature of the use of the Vitrimer resin and the lower limit temperature of recycling. The decrease of the resin viscosity follows the Arrhenius law, and the exchange reaction rate of the dynamic covalent bond is different below and above T T v . Below T T v , the exchange reaction rate of the bond is slow, and the resin behaves as a glass or a viscoelastic solid, and when the temperature is higher than T T v , the exchange reaction is accelerated, and the resin behaves as a viscoelastic liquid with the properties of reprocessing and self-repairing.

[0004] At present, T v The measurement of T T v depends on expansion experiments and rheological or stress relaxation experiments. The above-mentioned methods for monitoring the topological phase transition temperature are all carried out under the action of external force. It has been proved that the additional local force will provide additional tension on the crosslinking bond, affect the exchange rate and effective activation energy of the crosslinking bond, and cause the characterization result of the topological phase transition temperature to deviate. Experimental parameters have a great influence on the result, in other words, there is no effective method to monitor T T v of the Vitrimer under the condition of no external stress at present.

[0005] CN 110906858 A prepares carbon nanotube / non-woven fabric by impregnation method, resin curing molding by vacuum assisted resin transfer molding method, to obtain resin-based / non-woven composite sheet with carbon nanotube as functional phase, aramid / polyester non-woven fabric / resin as substrate. The obtained composite sheet has physical stimulation / piezoresistive response. Strain causes change in contact area of carbon nanotube network, and resistance responds in real time. The resistance change rate in structural member tensile strain can reach 1.9%, which can meet the structural health monitoring of large civil structures: bridge, tunnel, high-rise building, and solve the engineering problem of large-scale strain monitoring of large structures. However, its sensitivity is too low to be used for detection of Vitrimer resin topological phase transition temperature.

[0006] CN 109378110 A prepares graphene ethanol dispersion, pours solvent on the surface of cellulose gel, and hot-presses to form a film to obtain cellulose / graphene nanosheet composite sheet. The composite sheet has piezoresistive response. Pressure changes the lap joint of graphene sheet layer, and the resistance changes reversibly. The resistance change rate is 41% under 80 kgf force, which improves the cycle reliability of flexible sensing, but it cannot be applied to the detection of Vitrimer resin topological phase transition temperature. SUMMARY

[0007] The present application is aimed at the existing measurement of Vitrimer resin T v which relies on expansion experiment and rheology or stress relaxation experiment. Additional local force will cause T v measurement deviation. The present application provides a stress-free, in-situ, real-time monitoring method for Vitrimer topological phase transition temperature to solve the problem that Vitrimer resin T v cannot reflect the stress-free condition, and more accurately measure the T v of Vitrimer.

[0008] Specifically, the present application is realized by the following scheme: A stress-free in-situ real-time monitoring method for Vitrimer topological phase transition temperature, the steps are as follows: Step 1: Prepare piezoresistive sensing particles: use multi-walled carbon nanotubes (MWCNT) as raw material, and prepare carbon nanotube dispersion. Step two, preparation of composite fiber web sensor (C-GNS): carbon nanotube dispersion liquid is sprayed on the fiber web, heat treatment to obtain CNT composite fiber web sensor, and conductive silver paste is used to construct electrodes at both ends of the CNT composite fiber web sensor, solid copper wire is connected to the electrodes to transmit the electrical signal of the sensor response, and the solid copper wire and conductive silver paste are used to fix the C-GNS on the glass slide; Step three, data acquisition: the CNT composite fiber web sensor is fully immersed in the Vitrimer resin to be monitored, a heating cycle is performed, and a digital source table is used to synchronously collect the resistance signal and temperature signal of the CNT composite fiber web sensor in real time during the heating cycle; Step four, data analysis: the collected resistance signal is converted into resistance change rate, and the temperature- resistance change rate curve is fitted to obtain the topological phase transition temperature corresponding to the first inflection point of the resistance change rate on the obtained curve, The resistance change rate (RCR) satisfies: , R 0 is the initial reference resistance value, R is the instantaneous resistance at a certain time.

[0009] The above scheme prepares, sprays and connects electrodes by using carbon nanotube paste to obtain a dense and uniform carbon nanotube / glass fiber open sensing layer, which has a molecular scale piezoresistive response, and is used for monitoring the curing process and topological transition process of the Vitrimer resin. The application uses a sensing network similar in size to the polymer molecular chain to achieve monitoring of the resin curing and topological transition temperature by identifying molecular chain movement. Without external force, the resistance change rate can reach 34%, which has very high sensing sensitivity, realizes real-time monitoring of the stress-free curing of the Vitrimer resin, and determines the topological transition temperature.

[0010] Further, as preferred: The carbon nanotube dispersion liquid is obtained by dispersing raw materials, non-ionic surfactant and deionized water. More preferably, the non-ionic surfactant is Triton X-100, and the mass ratio of multi-walled carbon nanotubes to Triton is 1:3-5.

[0011] In step two, The carbon nanotube dispersion liquid is sprayed by an ultrasonic atomization spraying device. More preferably: The push injection speed of the ultrasonic atomization spraying device is 2-6 milliliters per minute, the nitrogen pressure is 0.2-0.5 megapascals, the spraying distance is 5-10 millimeters, the shaft offset is 4-6 millimeters, the ultrasonic power is 10-15 watts, and the spraying z-axis height is 50-70 millimeters.

[0012] The spraying includes three times, and the machine temperature is 60°C. After each spraying is completed, dry for 3-5 min.

[0013] The ultrasonic atomization spraying process parameters are preferably set as: spraying interval 6 mm; shaft offset 6 mm; ultrasonic power 12 W; spraying z-axis height 70 mm, spraying three times, machine temperature 60 DEG C, and drying time 5 minutes.

[0014] The heat treatment temperature is 60-75 DEG C.

[0015] In step three, The heating cycle is twice: first from room temperature to 120-130 DEG C and kept for 3 hours, then cooled to room temperature, heated to 200-220 DEG C and kept for 1 hour, and cooled to room temperature again.

[0016] In step four, The resistance change rate data and the temperature data are one-to-one corresponding through time, and the temperature-resistance change rate curve in the secondary heating process is obtained by drawing through Origin.

[0017] According to the resistance change rate curve at different temperatures, the temperature corresponding to the first inflection point of the resistance change rate is recorded as the topological phase transition temperature of the material.

[0018] In the above scheme, The obtained resistance change rate signal also includes the resistance change rate-time curve of the primary heating and curing process.

[0019] The fiber web is a glass fiber web.

[0020] The to-be-monitored Vitrimer resin is obtained by mixing bisphenol F type epoxy resin EPON 862 and 4,4'-dithiodianiline (4-DTDA) curing agent containing a disulfide bond according to a mass ratio.

[0021] Compared with the prior art, the advantages of the present application are: 1) The thermal expansion method and the stress relaxation method for measuring the topological phase transition temperature of the prior art will produce additional local force Tv offset, an improved test method is proposed to more accurately reflect the Vitrimer Tv under no external stress.

[0022] 2) The test method for the topological phase transition temperature of the glass-like resin proposed in the present application can characterize the topological phase transition temperature of any system Vitrimer, so the application range is wider.

[0023] 3) The topological phase transition temperature obtained by the test method of the present application is TvThe consistent results ensure the accuracy of the topological phase transition temperature measurement, which can effectively measure the topological phase transition temperature of Vitrimer and provide data support for the use, recycling and reprocessing of Vitrimer. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a flowchart illustrating the testing method of the present invention; Figure 2 This is a schematic diagram of the preparation of CNT composite fiber mesh sensors using ultrasonic atomization spraying. ① Ultrasonic atomization spraying site; ② Fiber mesh morphology after ultrasonic atomization spraying. Figure 3 These are optical microscope images of the CNT composite fiber web sensor morphology before and after heat treatment. ①Morphology of CNT droplets dispersed by ultrasonic atomization; ②State of CNTs dispersed by ultrasonic atomization after drying; Figure 4 This is a schematic diagram of the entire process of testing the topological phase transition temperature of Vitrimer resin. ①C-GNS structure and its layout, ② T v The C-GNS layout diagram during testing includes: ③ Electrical signal acquisition device, ④ Data processing terminal, and ⑤ Feature data output. Figure 5 The curves showing the real-time relationship between the rate of change of resistance and temperature are based on the CNT composite fiber mesh sensor (the entire process of two heating cycles of the Vitrimer sample). The solid blue line represents the curve of the rate of change of resistance over time, and the dashed red line represents the curve of temperature over time. Figure 6 It is a curve showing the direct mapping relationship between the rate of change of resistance and temperature obtained from a CNT composite fiber mesh sensor (the second heating process of the Vitrimer sample). Detailed Implementation

[0026] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the technical solutions in the embodiments of this application will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit the technical solutions of this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without creative effort are within the scope of protection of this application.

[0027] This application provides a method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress. The embodiments of this application are described below with reference to the accompanying drawings.

[0028] See Figure 1 , Figure 1 The following is a schematic diagram illustrating the implementation process of this embodiment. The specific steps are as follows: Step 1, Preparation of Piezoresistive Sensing Particles: Multi-walled carbon nanotubes, Triton X-100 and deionized water were mixed in a mass ratio of 1:3:4.3 to prepare a carbon nanotube dispersion.

[0029] Step 2, fabrication of composite fiber mesh sensor (C-GNS): combining Figure 2 Using an ultrasonic atomizing spraying device CHF-5B, the carbon nanotube dispersion obtained in step one is uniformly sprayed (nitrogen 0.2 MPa) onto a glass fiber nonwoven fabric (referred to as fiber web) via a micro-pump. After heat treatment at 60℃, a CNT composite fiber web sensor with random conductive paths is formed on a stage connected to a vacuum pump.

[0030] The morphology of the fiber web before and after the above heat treatment is shown in the attached figure. Figure 3 As shown.

[0031] Step 3: Electrodes are constructed at both ends of the 20×40 mm CNT composite fiber mesh sensor using copper electrodes and monitoring conductive silver paste, and the C-GNS is fixed on a 75×25×1 mm glass slide.

[0032] Step 4, Data Acquisition: The Vitrimer resin to be monitored is obtained by mixing bisphenol F type epoxy resin EPON 862 and a curing agent containing disulfide bonds, 4,4'-diaminodiphenyl disulfide (4-DTDA), in a certain mass ratio. The mixed liquid Vitrimer resin is dropped onto the CNT composite fiber web sensor, ensuring the sensor is fully impregnated. Two heating cycles are then initiated: first, the temperature is raised from room temperature to 130°C and cured at this temperature for 3 hours; then, after cooling to room temperature, a second heating cycle is immediately started, raising the temperature to 220°C and holding for 1 hour, followed by cooling to room temperature. A digital source meter is used to synchronously acquire the real-time temperature signal and the fiber web resistance signal during the heating process of the CNT composite fiber web sensor. A schematic diagram of the acquisition is attached. Figure 4 .

[0033] Step 5, Data Analysis: Convert the acquired resistance signal into a rate of change of resistance. Plot the rate of change of resistance and real-time temperature on the vertical axis and time on the horizontal axis, and fit the data to form a graph as shown below. Figure 5 , Figure 6 Temperature-resistivity change rate curve.

[0034] analyze Figure 5 and Figure 6 The curve shown, Figure 5 In the resistance change curve of the entire Vitrimer curing process, the resistance change rate shows an overall upward trend from 0 to 180 minutes, which is the resin curing stage. As curing progresses, the resin viscosity decreases, continuously wetting the fiber web, causing the resistance to continue to rise. Subsequently, the crosslinking density of the epoxy resin continuously increases, and the free volume continuously decreases, thereby continuously compressing the CNT piezoresistive network, generating CNT-CNT bridging conductive paths, and the resistance increase slows down until it stabilizes, at which point the crosslinking network is basically formed. In the 180-340 minute stage, the temperature drops (from 130℃ to room temperature), causing both the resin and the CNT fiber web to shrink. Due to the difference in thermal expansion coefficients, the CNT fiber web is stretched, causing the resistance to increase again. Then, in the 340-400 minute stage, the temperature rises again (from room temperature to 220℃). The relatively high stress release and strong fiber constraint in C-GNS jointly offset the increase in resistance caused by thermal expansion, resulting in a significant decrease in the overall resistance of C-GNS. When the temperature exceeds... T v At this time, the resin undergoes an accelerated exchange reaction, changing from a solid to a liquid. Its rapid volume expansion increases the electron tunneling gap between CNTs, causing a sudden change in the resistance of C-GNS. The temperature corresponding to the first inflection point of the rate of resistance change is... T v .

Claims

1. A method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress, characterized in that, The steps are as follows: Step 1: Prepare a carbon nanotube dispersion using multi-walled carbon nanotubes as raw material; Step 2: Spray carbon nanotube dispersion onto a fiber mesh, heat treat to obtain CNT composite fiber mesh sensor, and construct electrodes at both ends of the CNT composite fiber mesh sensor with conductive silver paste. Solid copper wires are connected to the two ends of the electrodes to transmit the electrical signal of the sensor response. Step 3: Fully impregnate the CNT composite fiber web sensor with the Vitrimer resin to be monitored, heat and circulate it, and use a digital source meter to synchronously acquire the resistance and temperature signals of the CNT composite fiber web sensor in real time. Step four: Convert the acquired resistance signal into a rate of change of resistance. Fit the rate of change of resistance to temperature to obtain a temperature-resistance rate of change curve. The temperature corresponding to the first inflection point of the rate of change of resistance on the curve is the topological phase transition temperature. The resistance change rate RCR satisfies: , R 0 represents the initial reference resistance value. R The instantaneous resistance at a certain moment.

2. The method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress according to claim 1, characterized in that: The carbon nanotube dispersion is obtained by dispersing the raw materials with nonionic surfactants and deionized water.

3. The method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress according to claim 2, characterized in that: The nonionic surfactant is Triton, and the mass ratio of multi-walled carbon nanotubes to Triton is 1:3~5.

4. The method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress according to claim 1, characterized in that: In step two, the carbon nanotube dispersion is sprayed using an ultrasonic atomizing spraying device.

5. The method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress according to claim 4, characterized in that: The ultrasonic atomizing spraying device has an injection speed of 2-6 ml / min, a nitrogen pressure of 0.2-0.5 MPa, a spraying spacing of 5-10 mm, an axial offset of 4-6 mm, an ultrasonic power of 10-15 watts, and a spraying z-axis height of 50-70 mm.

6. The method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress according to claim 1, characterized in that: The spraying process consists of three coats, with the machine temperature set at 60°C. Each coat is dried for 3-5 minutes after application.

7. The method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress according to claim 1, characterized in that: In step two, the heat treatment temperature is 60~75℃.

8. The method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress according to claim 1, characterized in that, In step three, the heating cycle is repeated twice: first, the temperature is raised from room temperature to 130°C and held for 3 hours, then cooled to room temperature, then raised to 200°C and held for 1 hour, and finally cooled to room temperature again.

9. A method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress, as described in any one of claims 1 to 8, characterized in that: The fiber mesh is a glass fiber mesh.

10. A method for in-situ real-time monitoring of the Vitrimer topological phase transition temperature without external stress, as described in any one of claims 1 to 8, characterized in that: The Vitrimer resin to be monitored was obtained by mixing bisphenol F epoxy resin EPON 862 and a 4,4'-diaminodiphenyl disulfide curing agent containing disulfide bonds in a certain mass ratio.

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