| Flexible strain sensors are a kind of flexible sensor that can transform the external strain signal into electrical signals such as resistance and capacitance that are easy to monitor.Because of its light weight,portability and wearability,it is expected to be widely used in the fields of software robots,human health monitoring and wearable electronics.The flexible strains sensor based on electronic conductors due to possess high modulus and low transparency,which make the flexible sensors difficult to meet people needs in wearing comfort and visualization.In recent years,stretchable ionic conductors have become an ideal material for the preparation of flexible strain sensors because of its excellent resilience,high transparency,good biocompatibility.However,at present,ionic conductive elastomers usually show limited stretchability,low ionic conductivity,poor environmental stability and low strain sensitivity.In addition,ionic conductive elastomer composites will inevitably produce mechanical damage under the action of large external force,and produce a lot of e-waste and environmental pollution after being discarded.Therefore,in this paper,an ionic conductive elastomer composite with high stretchability,high ionic conductivity,complex environment resistance and cyclic durability was synthesized by“Salt-in-polymer”,which is expected to provide a useful reference for the development of the next generation of flexible strain sensors.This paper mainly carries out the following research:（1）A new type of ionic conductive elastomer composite（ICEC）with dense hydrogen bond network structure was synthesized by in-situ thermally initiating the polymerization of acrylic acid monomer（AA）in poly（ethylene oxide）（PEO）with lithium trifluoromethylsulfonimide（Li TFSI）salt as the conductive component.Benefiting from its covalent crosslinking and dense hydrogen bonding,ICEC-2 showed high mechanical properties（fracture strain>1100%,fracture strength1.07 MPa）,and fatigue resistance（after 500 cycles of stretching/releasing at 100%strain,the residual strain is less than 12%）.At the same time,the highly dynamic reversible hydrogen bonding endowed the composites good self-healing（after 12 hours of cutting and self-healing process,the self-healing efficiency can reach 90%）.The ICEC-2 also showed high transparency（the transmittance can reach 90%in the visible light range）and excellent high/low temperature resistance（it can be twisted and stretched in the temperature range of-20～80 oC）.In addition,due to the unique complexation decomplexation between Li+and ether oxygen bond（C-O-C）in polymer network,which ICEC-2 showed high ionic conductivity(1.05×10-4S cm-1).Based on the above excellent characteristics,the ICEC-2 can be directly assembled into a resistance strain sensor,showing an impressive sensing performance with high sensitivity in a wide strain range（the Gauge Factor value is 1.56 in the strain range of 0～300%,the Gauge Factor value is as high as 3.43 within the strain range of 300%～700%）,excellent mechanical durability（after stretching/releasing 1000 cycles at 50%strain,the retention rate of relative resistance change value still up to 90%）,and can be used as a flexible wearable strain sensor to monitor the signals generated by different moving parts of the human body in real time.（2）A hydrophobically associating network ionic conductive elastomer（ICE）was synthesized by photoinitiated copolymerization of n-butyl acrylate（BA）and acrylic acid（AA）monomer further using Li TFSI and hydrophobic ionic liquid（[BMPyrr][TFSI]）as double conductive components.Due to the ICE skeleton contains both hydrophobic n-butyl acrylate chain segment and hydrophilic acrylic acid chain segment,which exhibited significantly different optical transparency in water and polar solvent,and can realize the reversible conversion from transparent to opaque to transparent simply by soaking and drying.ICE can achieve the function of encryption-decryption-deletion of written information by soaking in deionized water and organic solvent respectively.Due to the formation of high-density hydrogen bonds and other physical crosslinking between conductive components and polymer segments,ICE-2 showed super stretchability（fracture strain>3800%）,good elasticity（recovery rate up to 95%）and mechanical strength matching with human body（～294 k Pa）.At the same time,the addition of double conductive components and the weak coordination between Li+and C=O in the conductive component Li TFSI make ICE-2 exhibited high ionic conductivity(up to 1.10×10-4S cm-1at room temperature).The ICE-2 showed excellent temperature adaptation range（-30～300 oC）and environmental stability（humidity resistant,waterproof,solvent resistant etc.）.In addition,benefiting from the dynamic reversible interaction between molecular chains,the solution reprocessing and recycling of ICE-2 can be realized by using the dissolution and regeneration process of ICE-2 in organic solvents.Based on the excellent multifunctional characteristics of ionic conductive elastomer,ICE-2 was assembled into a personalized wearable flexible strain sensor with information anti-counterfeiting and decryption functions.The sensor showed high sensitivity in a wide strain range（the Gauge Factor value is 0.91 at 0～200%,and up to 1.62 at200%～500%）,excellent durability（within 100%strain rang after 500 cycles,its relative resistance change retention rate is 90%）,and can realize real-time monitoring of human physiological signals.More importantly,the sensor can still maintain stable sensing performance after being placed in harsh environments such as high humidity,vacuum and high temperature for a period of time. |
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