Research On Preparation And Performance Of Flexible Repairable EVA Fiber-based Strain Sensors

With the popularity of smart products,wearable devices have attracted great attention from society.As an important part of wearable devices,flexible strain sensors have important application values in electronic skin,human motion monitoring,human-computer interaction,and smart textiles.Generally,the flexible strain sensors are usually composed of two units of elastic polymers and conductive fillers.However,the current polymer-based strain sensors still have the following three key problems.1)Elastic polymers have common characteristics-viscoelasticity.The existence of viscoelasticity will cause the flexible strain sensor to gradually generate residual strain during multiple stretch/relaxation processes,which will cause the sensing performance of the flexible strain sensor to attenuate or even fail.2)It is difficult for flexible strain sensors to maintain high linearity in a larger strain sensing range.3)It is difficult to create a flexible strain sensor with both high sensitivity and large strain working range.To this end,this thesis uses ethylene-vinyl acetate(EVA)as the base material to create a flexible strain sensor with repairability,large working strain and high sensitivity,and study the structure-effect relationship of the sensor.The specific work is as follows:(1)In order to give the flexible strain sensor repair function,this chapter uses the melt extrusion method to prepare EVA fibers with shape memory properties as the base material of the flexible strain sensor,and investigates the effect of VA content on the mechanical properties,elastic recovery rate and thermal properties of EVA fibers.The research results show that the EVA fiber with a VA content of 30 wt%has the largest elongation at break(188%)and the highest elastic recovery rate(retaining 92%elastic recovery rate under 80%tensile/relaxation strain),and the lowest shape recovery temperature was 61.2°C.The restults show that EVA fiber is a good base material for preparing flexible strain sensors with repair function,small size and light weight.(2)Based on the work of(1),EVA fibers with a VA content of 30 wt%were selected as flexible substrate materials,and carbon nanotube(CNT)/EVA composite fibers were prepared by the swelling-ultrasonic method.The effect of ultrasonic time on the conductivity of the CNT/EVA fiber was studied,and the fiber with the largest electrical conductivity was selected as the research focus.The micro-morphology,mechanical properties,strain sensing behavior,response speed,and sensing mechanism of the CNT/EVA composite fiber were explored.The results show that the CNTs are combined with EVA in an embedded form,and a CNT layer is formed on the surface of EVA fiber.And the CNT layer and the fiber have good bonding fastness.The CNT/EVA fiber not only has a large strain working range(?=190%)and high linear responsivity(R~2=0.995),but also has a fast response speed(312 ms).At the same time,by studying the effect of the residual strain generated by the viscoelasticity of the CNT/EVA fiber on its sensing behavior,it is proved that the excellent shape memory effect of the fiber can completely eliminate the residual strain caused by multiple cycles.The conductive network of the sensor is repaired,and the performance of the repaired strain sensor is completely consistent with the original strain sensor.This innovative point provides a new idea for the design of self-repairing strain sensor.(3)Based on the work of(2),in order to endow the EVA fiber-based strain sensor with a large strain working range and high sensitivity,the sheet structure of the conductive filler reduced graphene oxide(RGO)can be used to prepare high-sensitivity flexible strain sensor due to its easy cracking during the stretching process.Therefore,RGO is used as a conductive material and the RGO/EVA conductive fiber was prepared by the swelling-ultrasonic method.At the same time,in order to obtain a flexible strain sensor with a more stable sensing conductive network and a richer and more efficient repair method,the RGO/EVA fiber was compounded by in-situ polymerization.PDA particles with excellent adhesion properties and high light-to-heat conversion efficiency were prepared the RGO@PDA/EVA fibers.The microscopic morphology,mechanical properties,strain sensing behavior,response speed and sensing mechanism of the RGO@PDA/EVA conductive fiber were studied.The research results show that the RGO sheet is in close contact with the EVA substrate,and the PDA produced by in-situ polymerization firmly adhere the RGO sheet to the surface of EVA substrate.The structure of the RGO@PDA/EVA fiber sensor system is very stable.The fiber not only has a fast response speed(192 ms),but also has a large strain detection range(193%)and high sensitivity(GF=1801).What's more interesting is that the RGO@PDA/EVA fiber not only has excellent thermal repair effect(100%themal repair effect),but also has ultra-fast light repair speed(5 s)and super high light repair effect(100%light repair effect).The flexible strain sensor created in this subject has huge potential application value in wearable devices.For example,it has obvious advantages in human motion monitoring.By attaching the strain sensors to the joints of human motion,they can not only realize real-time and accurate monitoring of the movements of fingers,wrists and elbows with large body motion,but also It can accurately monitor minute movements such as pulse beats and mouse clicks.