Thermal-responsive Dielectric Property In Polyvinylidene Fluoride Fiber Composites Induced By Ionic Conductivity

In recent years,with the rapid development of the intelligent electronic equipment industry,the State Council has made new sensors and actuators as key development areas in the "14th Five-Year Plan" development plan.Most of the thermally responsive dielectric materials are ferroelectric crystals and complexes,however,the dielectric response temperature of these materials is generally not in the room temperature range,and their mechanical properties cannot meet the needs of some practical applications,and there are also problems of complex synthesis methods and high costs.Although some nanocomposites with temperature-dependent phase change characteristics also exhibit thermally responsive dielectric properties,the lack of shape stability and self-supporting properties limits their application.In this thesis,polyvinylidene fluoride/1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide(PVDF/MMITFSI)nanofibers were firstly prepared by solution blow spinning technique,and the fiber structure and electrical properties were characterized.The results show that the introduction of MMITFSI and solution injection drafting can effectively promote the production of polar ?-crystals in PVDF,with the percentage of ?-crystals reaching ?85%.Compared with pure PVDF,the low frequency dielectric constant of the composite fibers was significantly higher and showed a stable temperature-dielectric constant linear response in the tested temperature range.In order to optimize the comprehensive performance of the composite fiber membranes,dense structured nanofiber membranes were prepared by a laminated cold pressing process.The results show that the dense structure of the fibrous membrane was obtained at 10 MPa pressure,the pores in the fibrous membrane were basically removed and the mechanical properties of the PVDF fibrous membrane were effectively improved.After cold pressing the PVDF/MMITFSI composite fibers,it was found that the percentage of ?-crystals of PVDF in the composite fiber membranes increased significantly,up to ?93%.In addition the percolation threshold of the composite membranes was reduced and the dielectric constant was increased by a factor of ?4.Finally,to further explore the mechanism of the dielectric thermal response based on ionic conductivity,PVDF/MMITFSI hot pressed membranes,polyvinylidene fluoride/ polyacrylonitrile(PAN)/MMITFSI composite fibrous membranes were prepared by combining 1-butyl-3-methylimidazolium tetrafluoroborate(BMIMBF4),and 1-butyl-3-methylimidazolium hexafluorophosphate(BMIMPF6)and 1-hexadecyl alkyl-3-methylimidazolium hexafluorophosphate(C16MIMPF6)were compounded with PVDF to prepare composite fibrous membranes and to investigate the effects of fiber structure,matrix polarity,ionic liquid structure and fiber matrix/ionic liquid interactions on the thermal response dielectric.The results show that the highly polar polymeric fiber matrix contributes to the dielectric response performance,and the influence of the ionic liquid on the dielectric response behavior is mainly dependent on the size of the anion/cation pair and its polarity.All the above systems exhibit stable and reversible temperature-dielectric constant linear response behavior,which can provide a reference for the design of linear thermos-responsive dielectric sensing materials.