Influence of crystallization conditions on the microstructure and electromechanical properties of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymers

The crystallization process and its influence on the microstructures and in turn the ferroelectric and electromechanical properties of poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer were investigated. It was found that, in addition to the crystallites formed at the isothermal crystallization temperature T_x, further crystallites form during the rapid cooling from T_x to room temperature. The proportion of lower melting crystallites increases with T_x, and for T_x = 142°C, this proportion is more than 90%. The experimental results show that the lower melting crystallites have a higher fraction of the all-trans conformation (and thus more polar-phase component) than the crystallites formed at T_x. As a result, the higher T_x samples show an increased polarization hysteresis and reduced electric field induced strain response. For example, the strain is reduced from -5.9 to -4.2% as T_x is increased from 112 to 142°C. In addition, the experimental results in combination with other reported results indicate that CFE units are included in the crystalline lattice. Consequently, the influence of CFE on the ferroelectric behavior of the polymer is through the defects induced in the crystal lattice.