Pockels effects of ferroelectric thin films of vinylidene fluoride and trifluoroethylene copolymers

摘要

A solid polymer is an extremely large molecular aggregate of chains whose individual units are covalently bonded to each other. It has a complex highly ordered structure with intermingled crystalline and amorphous regions. It was only recently when ferroelectricity was observed in polymers with such highly ordered structure, and the phrase "ferro-electric polymer" has been used since [1, 2]. This history of ferroelectric polymer research began in 1980. At that time, polyvinylidene fluoride (PVDF) attracted attention because of its high piezoelectric coefficient [1, 2]. After the vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer (P(VDF/TrFE)) was synthesized and changes in its physical constants with temperature were measured, the following findings were reported in rapid succession: peaks of heat absorption and permittivity at a certain temperature [3, 4] and rapid loss of remanent polarization related to a significant change in the crystal lattice with temperature change [5]. Based on these reports [3-5], it was recognized that the transition of P(VDF/TrFE) was a ferroelectric-to-paraelectric phase transition [2]. The structure of crystalline P(VDF/TrFE) has been studied in great detail and clarified for each composition ratio of VDF [2]. Although P(VDF/TrFE) is crystalline, film samples of 100% crystallinity cannot be obtained through conventional methods, which are combina-tions of drawing, annealing and poling, applied in sequence [2]. Amorphous components are always included to obtain complex highly ordered structures similar to those of other crystalline polymers [2]. Broussoux and MiCheron [6] and Spector and Stein [7] conducted pioneering research on the birefrin-gence and electro-optical properties of ferroelectric polymers. However, no one-to-one correspondence was found between the macro-optical propreties and crystal characteristics because the ferroelectric poly-mer film has a complex highly ordered structure with intermingled crystalline and amorphous regions [1, 2]. Recently, Ohigashi et til. [8] reported "single-crystalline" (SC) films which were obtained from a uniaxially drawn film by crystallization in the paraelectric phase with its surface free of constraint other than tensor stress along the chain axis. SC films have the following characteristics: "single-crystal" orientation, near-100% crystallinity and no lamellar structure in highly ordered structures.