Effects of annealing on the structure and switching characteristics of VDF/TrFE copolymers

Structure-property relationship has been investigated in concern with the ferroelectric switching characteristics of a VDF(78)/TrFE(22) copolymer. Unoriented and oriented film samples prepared by melt-quenching and melt-extrusion, respectively, were subjected to annealing and resulting structural changes were characterized by SEM, X-ray, density and DSC measurements. It was found that annealing just above the Curie point (120-130 degrees C) caused a marked increase in crystallinity, whereas annealing in the paraelectric state (130-145 degrees C) induced a crystal growth into thick lamellae. Such structural changes resulted in the ferroelectric switching characteristics with larger remanent polarization and shorter switching time.:The unoriented sample yielded a smaller remanent polarization than the oriented sample by a factor of pi/4, which substantiated that polarization reversal occurred as a result of rotation of molecules rotation about their chain axis. The melt-extruded and well-annealed sample consisted of regularly stacked large lamellae, thus providing information on the switching process occurring in constituent crystallites. Analysis of the observed switching transient has shown that polarization reversal progresses through a conventional nucleation-growth mechanism with a large waiting time.