Study On The Preparation And Flexible Strain Sensing Performance Of Fluorinated Polymer-based Ionogels

Flexible strain sensors are a class of electronic sensor devices that can convert external strains into easily processed electrical signals.Due to their flexibility,deformation resistance and lightweight features,flexible strain sensors are expected to be used as wearable devices for monitoring physiological signals and complex movements of human,and have broad application prospects in the fields of health monitoring,human-machiene interfaces,soft robotics and flexible electronics.The key to the future development of flexible strain sensors is how to prepare materials with high stretchability and excellent conductive properties.Compared with the electronconducting flexible materials,ion-conducting flexible materials with anions and cations as carriers usually exhibit high stretchability,high transparency and high biocompatibility.Ion-conductive flexible materials can be classified into conductive hydrogels,conductive organogels,ionogels and ion-conductive elastomers depending on the differences of ion transport medium.Among them,conductive hydrogels and organogels are limited by the large amount of solvents they contain and have bottlenecks such as a narrow temperature tolerance range.Therefore,it is important to develop new-type ion-conductive flexible materials that can withstand high and low temperature environments.Ionogels are solid mixtures with ionic conductivity formed by mixing polymer chains and ionic liquids that can be electrolyzed as ions,which usually have the advantages of high thermal stability,non-volatile solvent composition,fast ion migration,and wide electrochemical window.However,in practical applications,ionogels,whose polymer networks and ionic liquids usually contain large amounts of polar groups,would absorb water and swell in humidity environments,inevitably leading to problems such as the vollume increase and significant degradation of mechanical and conductive properties of the resultant ionogels.Therefore,the development of new-type ionogels with unique hydrophobic properties is of significance for the development of flexible strain-sensing materials and devices with high environmental stability.The following research advances were made in this thesis:Hydrophobic fluoropolymer ionogels(SFIG)with high stretchability,self-healing and self-adhesive performances were obtained by one-step photoinitiated polymerization using the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate([BMIM][BF4])as the conductive medium,acrylic acid and hexafluorobutyl acrylate as the copolymer monomers,and polyethylene glycol(PEG)as the compatibilizer for the chain segments of polyacrylic acid and poly(hexafluorobutyl acrylate).The ionogels exhibited high stretchability(～1587% strain),self-adhesive and self-healing properties due to the high density of hydrogen bonding interaction between the polyacrylic acid and PEG chain segments in the SFIG,and the presence of microcrystalline regions due to the crystallization of both PEG molecular chains and poly(hexafluorobutyl acrylate)chain segments gave the ionogels high mechanical strength(up to 8.4 MPa).The ion-dipole interaction formed between the fluorine-containing copolymer and [BMIM][BF4]allowed the ionogel to exhibit excellent environmental stability and high resistance to ambient water.Due to the presence of PEG molecules as compatibilizers,the fluorinated copolymer chain segments had good compatibility with [BMIM][BF4],and the SFIG exhibited high transparency(average transmittance of > 97% in the visible wavelength range).A flexible capacitive strain sensor was constructed using SFIG as a flexible ion-conductive material,which exhibited high sensitivity and good fatigue resistance(Gauge factor maintained at 2 after 500 of 10% strain stretch-recovery cycles)in the moderate strain range(5%-70% strain).The prepared capacitive strain sensor could be used to monitor the motions of different joints of the human body,and due to the good hydrophobic properties of SFIG,the sensor could be also used for underwater strain sensing.(2)Hexafluorobutyl acrylate,acrylic acid and polyethylene glycol methyl ether methacrylate(PEGMA)were further utilized as copolymer monomers for one-step photoinitiated polymerization in hydrophobic ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide([BMIM][TFSI])to obtain fluoropolymer ionogels(TFIG)with microphase-separated structures.In the TFIG,the terpolymer fluorinated copolymer molecular chain had good compatibility with[BMIM][TFSI],and the polyacrylic acid molecular chain could be used as a unique compatibilizer to achieve molecular-level compatibility between the soft segments of hydrophilic poly(ethylene glycol)methyl ether methacrylate(PEGMA)and the hard segments of hydrophobic poly(hexafluorobutyl acrylate).The hydrophobic ionic liquid[BMIM][TFSI] was freely dissociated and acted as a carrier to achieve ion migration and conductive function in the TFIG polymer network.The prepared TFIG had high stretchability(～1086%),high mechanical strength(～1.76 MPa)and excellent water resistance,and could produce highly sensitive conductivity changes in response to external stimuli such as different temperatures and humidity.The flexible resistive strain sensor exhibited three stepwise increasing Gauge factors(1.43,2.97 and 5.60,respectively)in the range of 0 ～ 200% strain when subjected to large strain deformation,as well as excellent fatigue resistance(relative resistance changes maintained at 30%after 1000 of 10% strain stretch-recovery cycles)and fast response to small strain(120ms).The sensor had a fast response time(120 ms)to small strains,and could monitor in real time the large strain sensing signals generated at human joints and small strain sensing signals generated by human breathing and pulse.