Electro-mechanical Properties And Microstructure Evolution For Ferroelectric Polymers

As a representative of functional material, poly(vinylidene) fluoride (PVDF) and its copolymers have been found more and more applications in electromechanical devices to perform energy transformation between the electric and mechanical forms such as sensors, actuators, etc. Although the dielectric properties, microstructure and applications of the films have been fully studied, there is hardly any report on the mechanical behavior and its microstructure evolution under pure mechanical or combined electro-mechanical fields. The thesis carried out a systematic study on the mechanical behavior and microstructure evolution for PVDF, P(VDF-TrFE) copolymers under pure mechanical and combined electro-mechanical fields. Polarization hysteresis change during the cyclic electric field was also carried out for P(VDF-TrFE) copolymer films.At first, DSCM (digital speckle correlation method) was used to study the mechanical behavior of the polymer films. The result proved this was a suitable method. Later, studies on the stress-strain behavior under pure mechanical field were carried out for PVDF films prepared by extrusion. It was found that there existed distinct strain rate and orientation effects. With the strain rate decreased, the fracture strain increased while the maximum strength decreased. Microstructure analysis indicated that for the longitudinal films, there was distinct conformational change fromαtoβphase and its extent increased with the strain rate decreased. There was no such conformational change for the transversal films.Deformation and fracture behavior under pure mechanical tension was characterized for P(VDF-TrFE) copolymer films prepared under two different processing conditions. It was found that for the copolymer films prepared by solution casting and then annealing process, the samples undergo a typical polymeric brittle fracture feature. For the copolymer films prepared by stretching the solution-cast films and then annealing process, they exhibit typical characteristics of ductile fracture.Fracture behavior and microstructure evolution under combined electrical and mechanical loads were characterized for P(VDF-TrFE) 68/32 copolymer films prepared by stretching the solution-cast films and then annealing process. It was demonstrated that for an electric field applied parallel to the tension direction, the yield strength increased while the fracture strain and ultimate tensile strength decreased. There was no effect for the copolymer films when the electric field was applied in the film thickness direction. Structure analysis revealed that the molecular chains had the tendency to convolute, which would prevent the acquisition of a more oriented fibrillar structure than that under pure mechanical tensile stress. The electric field in the film thickness direction only caused the originally randomly oriented dipoles to rotate and to orient preferentially along the direction of external electric field. Therefore, it had no effect on the stress-strain behavior of the copolymer films.Sawyer-Tower circuits for testing the polarization loop of the ferroelectric polymer films were set up by ourselves. The polarization loops for the copolymer films prepared by the two processing methods were measured under different electric field magnitude. Changes of the polarization loop with the cycling electric number were also carried out. Preliminary experimental results revealed that there was no apparent change for the polarization loop, and it subjected to electric breakdown after certain cycling numbers for the directly annealed copolymer films. For the stretched annealed copolymer films, the remanent polarization decreased apparently with the cyclic number increased, that is, it exhibited the typical phenomenon of electric fatigue.