Curie Transition in Poled and Unpoled Copolymer of Vinylidene Fluoride and Tetrafluoroethylene

Abstract

Copolymers of vinylidene fluoride (VDF) and tetrafluoroethylene (TeFE) with TeFE content ranging from 17.8 to 30.7 mol% were studied by thermal, dynamic mechanical, and thermally stimulated current measurements. Below the melting point, a small endotherm was observed in differential thermal analysis. Corresponding to the endotherm, a crystalline relaxation, designated as Tt, was observed by mechanical measurements. It was inferred that the endotherm and the Tt mechanical relaxation were due to the Curie transition. The Curie points of these copolymers were about 50 K lower than those of VDF-trifluoroethylene (TrFE) copolymers with the same VDF content. In X-ray measurements, it was confirmed that these copolymers took the all-trans crystal structure similar to that of PVDF form I of which the crystal lattice expanded in a- and b-axis directions and was disordered in the c-axis repeat in comparison with that for VDF-TrFE copolymers. The Curie temperature for poled samples increased by about 50 K. The poling procedure made the crystal lattice compact in a- and b-axis directions and the c-axis repeat regular; this seems to be related to the considerable increase in the Curie temperature.