| Flexible force sensors are a kind of sensor that converts external mechanical stimuli such as stress or strain into electrical signals like capacitance,resistance or current,having drawn intensive attention due to their application prospects in the emerging fields of health monitoring,soft robotics,and wearable electronics.As the material basis of flexible force sensors,the development of high-performance flexible force sensing materials is the key to realize the rapid development of flexible force sensors.Flexible force sensing materials,according to conductive mechanism,are generally divided into flexible electronic conductors with electrons and holes as carriers and flexible ionic conductors with ions as carriers.Flexible ionic conductors generally possess unique features including intrinsic high stretchability,high transparency,and biocompatibility which are overwhelmingly difficult or even impossible to be achieved by electronic conductors,and thus have attracted much attention.Flexible ionic conductors,according to the type of ion carriers,can be divided into ionic conductive hydrogels/organogels,ionogels,and ionic conductive elastomers.Among them,hydrogels/organogels are a kind of gel derived from a polymer network swollen with water and/or organic solvent,having been widely used in flexible force sensors.However,the operating temperature of hydrogels/organogels is limited by the boiling point and freezing point of solvents,which faced the problems of dehydration at high temperatures and freezing at subzero temperatures,which leads to flexible force sensors with narrow operating temperature and unstable sensing signals.Therefore,the development of ionic conductors with high-temperature tolerance and anti-freezing resistance has important research and application value for the achievement of next-generation flexible force sensors.Ionogels are a kind of gel materials derived from a polymer network with confined ionic liquids(ILs),possessing high thermal/chemical stabilities,which efficiently solved the bottleneck problems of unsatisfactory extreme-environment tolerance of hydrogels/organogels.Nevertheless,conventional ionogels are sensitive to humidity because the polymer matrix and ILs are often hygroscopic,and thus are prone to swell in high-humidity environments,leading to the degradation of mechanical and electrical properties.Therefore,the development of ionogels with waterproof performance is significant for the next-generation flexible force sensing materials and devices.Liquid-free ionic conductive elastomers can realize ion migration and conductivity through the decomplexation/complexation process of alkali ions in the amorphous regions of polyether,having drawn intensive attention due to their non-leakage characteristics which efficiently overcome the leakage of ILs in ionogels.However,liquid-free ionic conductive elastomers generally exhibited unsatisfactory mechanical toughness,stretchability,moisture/heat-resistance,and self-healability,which are not able to meet the application demands of flexible force sensors under extreme environments.Therefore,the development of liquid-free ionic conductive elastomers with high stretchability,self-healability,and moisture/heat-resistant performance is very crucial research projects in the field of flexible sensors,yet still a huge challenge.This thesis mainly carries out the following research:(1)A semi-crystalline fluorinated copolymer ionogel materials(SFCI)with extremely high stretchability,under-water stability and fast self-healability was fabricated by photo-initiated copolymerization of hexafluorobutyl acrylate and oligoethylene glycol methyl ether acrylate in the ionic liquid of 1-butyl-3-methylimidazoliumbis((trifluoromethylsulfonyl)imide)([BMIM][TFSI]).Benefiting from the reversible ion-dipole interactions between the fluoroacrylate segment domains and[BMIM][TFSI]as well as the physical crosslinking effects of semi-crystalline oligoethylene glycol domains,the SFCI exhibited ultra-stretchability(>6000%),fast room-temperature self-healability(>96%healing efficiency after cutting and self-healing for 30 min),and outstanding elasticity.The SFCI also exhibited high-temperature tolerance(>300 oC),anti-freezing performance(<-35 oC)and high transparency(visible-light transmittance>93%).The as-obtained SFCI can readily demonstrate as a highly stretchable ionic conductor in wearable capacitive-type strain sensors with waterproof performance for real-time detecting physiological human activities.The resultant SFCI was assembled into a wearable capacitive-type strain sensor with waterproof performance,showing an impressive sensing performance with high sensitivity and high linearity(gauge factor up to 1.0 within a strain range of 0?200%),excellent fatigue resistance(after stretching/releasing 1000 and 5000 cycles at 50%and 5%strain respectively,retention rate of relative capacitance variation still up to 93%and 95%),fast capacitance response capability(the response time of 0.3 s under 10%strain)and high reliability in real-time monitoring of complex human motions.(2)A hydrophobicity-constrained association strategy was presented for fabricating a liquid-free ion-conducting fluorinated elastomer(ICFE)with a microphase-separated structure.ICFE is consisted of poly(hexafluorobutyl acrylate)-random-poly(oligoethylene glycol methyl ether acrylate)(PHFBA-r-OEGA)and lithium bis(trifluoromethane sulfonimide)(Li TFSI).Due to coordination and“fluorous effect”,lithium cations(Li+)and organic fluorinated anions(TFSI-)of Li TFSI were selectively distributed within the hydrophilic and hydrophobic microphases of the ICFE,respectively.The hydrophilic nanodomain among the ICFE provided a high-efficient conductive path,yielding a remarkable ionic conductivity of 3.5×10-3 S m-1 at room temperature.The hydrophobic nanodomain with abundant and reversible hydrogen bonds endowed the ICFE with superior damage-tolerant capacities including ultra-stretchability(>6000%),large toughness(17.1MJ m-3),and self-healability(?100%healing efficiency after cutting and self-healing for 12 h).In particular,a stress-induced crystallization was observed during the stretching,endowing the ICFE with high fracture strength(770 k Pa)and outstanding tearing resistance(fracture energy up to 22.3k J m-2).The liquid-free ICFE also demonstrated extremely temperature-tolerant performance within a wide temperature range of-20?300 oC and moisture resistance in a 99%relative-humidity environment.The as-obtained ICFE can readily demonstrate as a stretchable ionic conductor in wearable capacitive-type strain sensors for real-time detecting physiological human activities. |
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