DIELECTRIC RELAXATIONS IN COPOLYMERS OF VINYLIDENE FLUORIDE

Ferroelectric copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene have been investigated by means of broad band dielectric spectroscopy (10 mHz-1 GHz) at temperatures covering the ferroelectric, paraelectric and molten phases as well as the glassy and rubbery states. The low-temperature relaxation spectra near T(g) (approximately -40-degrees-C) consist of two processes; a WLF-type segmental mode and an Arrhenius type local mode. these processes are unified near room temperature to be transferred continuously to the high-temperature process. After showing a peak at the Curie point T(c) (70-140-degrees-C), the relaxation strength decreases gradually in the paraelectric phase until the melting point T(m) (approximately 150-degrees-C) is reached, where it decreases rapidly. It is found that the relaxation time in the molten phase lies on the Arrhenius line extended from the low-temperature local mode process. Tbis implies that related molecular motions arise from a common elementary process. Departure from this Arrhenius line in the paraelectric phase is discussed by the use of a cooperative multi-site model.