ABSTRACT
PVOF has a very simple chemical formula, -CH:z-CF:z-, intermediate between polyethylene (PE) -CH:z-CH:z-and polytetrafiuoroethylene (PI'FE) -CF:z-CF:z-. This simplicity of chemical structure gives both high flexibility (as much as PE) and some sterochemical constraint (as seen in PTFE) to the main-chain structure of PVOF. Because of these structural characteristics, therefore, PVOF takes many types of molecular and crystal structures, which change depending on the preparation conditions of the samples. The crystal form which exhibits ferroelectricity, form I or ~, is only of these crystalline modifications. Depending on the molecular conformation and the chain packing in the unit cell, the mechanical and electric properties are affected sensitively. These crystalline modifications can be transformed to each other reversibly or irreversibly under suitable external conditions. The transition mechanism is complicated but can be understood qualitatively on the basis of trans-gauche conformational exchange.
