Design And Application Of Poly(Vinylidene Fluoride)-based Ferroelectric Materials

As a kind of functional material,ferroelectric materials have unique properties derived from the spontaneous polarization that can be reversed with an electric field.Ferroelectric materials are versatility.If a material proves to have ferroelectricity,it means that it must have both pyroelectricity,piezoelectricity,electrostrictivity,electrocaloric effect and electro-optic effect.Therefore,ferroelectric materials have a very wide range of applications.Ferroelectric materials can be used in the field of capacitors,non-volatile memory(RAM),piezoelectrics for ultrasound imaging,sensors,and actuators;electro-optic materials for data storage,oscillators and filters,light deflectors,modulators and displays.Polyvinylidene fluoride and its copolymers are considered to be the most valuable ferroelectric polymer materials because of their excellent dielectric and ferroelectric properties.At present,the research on the ferroelectric properties of polyvinylidene fluoride and PVDF copolymers mainly focuses on the improvement of the processing technology of polymer films and the construction of organic-inorganic composites.The research of improve the intrinsic ferroelectric properties by designing the molecular chain structure,chemical composition on molecular aspects are very few.In this thesis,a series of new polyvinylidene fluoride-based ferroelectric copolymers were prepared from the perspective of molecular design.The relationship between chemical composition,molecular chain structure and ferroelectric properties was studied,and its potential application in energy conversion was discussed.The main contents are as follows:First,we studied the ferroelectric properties of poly(vinylidene fluoride-trifluoroethylene)copolymer.We prepared a series of copolymers,and analyzed the segmentation of the copolymers through the fluoronuclear magnetic spectrum.With the introduction of trifluoroethylene monomer,a chiral center is generated in the molecular chain,and the polymer segment exhibits three kind of stereostructures,which named as isotactic,syndiotactic and atactic.We have found that a change in the composition of the copolymer causes the change in the stereoregularity of the molecular chain,which results in an ordered-disordered structural change in the crystalline region of the copolymer and a morphotropic phase boundary in a certain component region.The P(VDF-Tr FE)films in the morphotropic phase boundary region exhibits excellent piezoelectric properties,and the piezoelectric coefficient is as high as-63.5 p C/N at a vinylidene fluoride content of 50 mol%,which is maximum value in polymer material reported so far.In this part,we also found the P(VDF-Tr FE)copolymer exhibits macroscopic relaxor ferroelectricity at a vinylidene fluoride content of 50 mol% or less.However,as a relaxor ferroelectric,the above-mentioned copolymer has a dielectric constant of 20 or less,which is not advantageous in the application of a relaxor.In order to prepare high-performance relaxor ferroelectrics,we were inspired by high-entropy alloys to introduce two large-size monomers,chlorofluoroethylene and chlorotrifluoroethylene,into polyvinylidene fluoride-trifluoroethylene to prepare a series of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene-chlorotrifluoroethylene)tetrapolymer.From the infrared spectrum,the all-trans configuration in the tetrapolymer film increases with the increase of the VDF component,and the 3/1-helix configuration decreases accordingly.Observing the dielectric temperature spectrum and polarization behavior of the polymer film,it is found that the decrease of VDF content in the tetrapolymer will gradually change from the normal ferroelectric to the relaxor ferroelectric,and the dielectric constant is 46,which is much higher than the copolymer synthesized in the previous section.At the same time,the tetrapolymer exhibits excellent electrostrictive properties,which is up to 188% higher than the terpolymer in the lower electric field.After obtaining an excellent relaxed ferroelectric polymer,we applied it to an electrocaloric refrigeration device.For electrocaloric effect materials,dielectric breakdown is a key factor that must be considered,and it is strongly related to the failure probability of the electrocaloric refrigeration device.Therefore,the electrocaloric effect materials that can work efficiently under low electric field is more valuable.In order to obtain stable,reliable and high performance electrocaloric refrigeration device,the electrocaloric effect materials should not only have high electrocaloric effect,such as temperature change ?T and entropy change ?S,which should also have electrocaloric effect coefficients at low electric fields,such as ?S/?E and ?T/?E.In the tetrapolymers developed by expanding the P(VDF-Tr FE-CFE)terpolymer,the critical end point of each phase moves to the low temperature region,which makes it have a large electrocaloric coefficient under a low electric field.In addition,the low operating frequency characteristics of the cooling device provide an additional way to achieve large electrocaloric effects by exploring materials with large polarization responses at low frequencies.In summary,two novel relaxor ferroelectric polymers were synthesis by chemical design at the molecular level.In the process of constructing these two kinds of relaxor ferroelectrics,we have grasped the influence of chemical composition on the ferroelectric properties of the polymer through the regulation and analysis of the key factors such as molecular chain structure and crystal size.Among the key components in the process of polymer ferroelectricity to relaxor ferroelectricity,high performance piezoelectric,electrostrictive and electrocaloric effect materials were found.We hope that our work not only provide new materials for applications,but also lay the foundation for the design and preparation of new,high-performance ferroelectric polymers in the future.