Research On Phase Transition Behavior?Dielectric Property And Flexoelectric Effect Of PVDF-based Ferroelectric Copolymers

At present,the development of new energy and new materials has been a well-concerning focus.In this area,the basic research and application of functional materials are increasingly active,because of these materials show excellent optical,magnetic,electrical,force,thermal,biochemical or other properties,meanwhile,they can realize the conversion of some properties.Polymer functional materials have the advantages of light weight,easy preparation,good flexibility and stable chemical properties.The PVDF-based polymers is semi-crystalline with difeerent crystal phase structures,and these are excellent dielectric and electrical material due to its C-F bond exhibit a large dipole in some way.In 1969,the piezoelectric effect of PVDF was discovered by Kawai.Thereafter,the piezoelectric effect,pyroelectric effect,ferroelectric effect,electrostrictive effect,energy storage capacitor and electrocaloric effect have been widely research.flexoelectric effect is also a kind of electromechanical coupling effect and it can be described as the generation of an electric polarization response under a mechanical strain gradient or a mechanical response under an electric field gradient.Flexoelectric effect has been paid more and more attention by researchers.At present,Experimental results show that the flexoelectric coefficients of ferroelectric ceramics are very large.although the flexoelectric coefficients in polymers are smaller than those in inorganic materials,polymer materials have the advantages of good mechanical properties,such as flexible,tough,and resistant to catastrophic failure under external mechanical loads,desirable for study and practical applications.At the same time,the researchers found that the PVDF-based nanocomposites can effectively improve the dielectric,electrical and other properties of the matrix.In addition,further studying the crystallization behavior and phase transition law of the PVDF-based polymers is helpful to analyze the mechanism of their functional characteristics from the view of structure,and to improve the experimental controllability in the material preparation process.The first chapter mainly introduces the PVDF-based polymer and PVDF-based nanocomposites.Meanwhile,it introduces the application of these materails,such as piezoelectric effect,pyroelectric effect,ferroelectric effect,electrostrictive effect,electrocaloric effect and energy storage capacitor.In the second chapter,a series of pure film samples of P(VDF-TrFE)70/30 and P(VDF-TrFE)55/45 with different crystallization temperature and different heat treatment temperature were prepared by casting method.It is shown that heat treatment and crystallization have different influence on P(VDF-TrFE)70/30 copolymers in the term of the structure,morphology,phase transition and dielectric properties.The crystallization temperature significantly changes the morphology of the material,whereas the heat treatment temperature has a great influence on the structure of the copolymer at room temperature.Chapter 3and 4,the flexoelectric effect of PVDF-based polymers and its application were systematically researched.In Chapter 3,the effective transverse flexoelectric coefficients ?'12 of PVDF,P(VDF-CTFE),and P(VDF-HFP)polymers was measured and the p'12 of the polymers ranges from 2×10-9 to 6×10-9 C/m.The incorporation of bulky CTFE or HFP comononers significantly reduces the crystallinity level of the PVDF homopolymer.At the same time,the ?'12 of the P(VDF-CTFE)and P(VDF-HFP)copolymers is also much lower than that of the PVDF homopolymer.We propose that the difference in the flexoelectric response between these polymers may be a result of the different crystallinities of the polymers.In the fourth chapter,PVDF homopolymer is used to design the piezoelectric-flexoelectric polymer composites,including the point-ring design and the design with domed polymer.The apparent piezoelectric coefficients were measured and the corresponding flexoelectric coefficients were calculated.The temperature dependence of the apparent piezoelectric response of the composites was also measured.Comparing the piezoelectric effect of poled polymer,we found that the apparent piezoelectric coefficient is derived from the flexoelectric effect rather than the piezoelectric effect of the copolymer.However,it was found that the response decreases near the temperature where a molecular relaxation process in the crystalline phase occurs.The mechanism for the measured apparent piezoelectric response of polymer-based composites was also discussed.In the 5th chapter,the P(VDF-TrFE)copolymer/graphene nanocomposite and P(VDF-TrFE)copolymer/graphene oxide nanocomposite were prepared by casting method.It is found that the nanocomposite adding graphene oxide shows higher polarization and breakdown electric field than the pure P(VDF-TrFE)70/30 copolymer.It was systematically analyzed the different impacts of the two kinds of nano-flake particles for the crystal structure and dielectric properties to the matrix.In addition,the mechanism of interface effect was also discussed.