Crystallization in an electric field and piezoelectricity of poly(vinylidene fluoride)
The review indicates that piezoelectric properties of PVDF films are controlled by the crystalline structure of a polymer, i.e. the fraction of the crystalline polar phase and orientation distribution of polar crystals. Application of an electric field to the polymer induces recrystallization and/or crystallization of the oriented polar phase, whereby piezoelectric response becomes improved.Emphasis is placed on the role of the electric field in crystallization and thermodynamic stabilization of the oriented polar PVDF phase. Model computations have confirmed the empirical conclusions that the electric field induces nucleation and crystallization of the oriented polar phase. The mechanism is not that of the classical oriented crystallization from preoriented amorphous phase; it is related to the orientation-dependent electrostatic potential of polar crystals in the field. Model calculations of the orientation- dependent nucleation (homogeneous case) and of growth rates of polar phase indicate that the nucleation rate is orientation-dependent. Field effects are predicted to bevery sensitive to the degree of under cooling. For instance, to keep field effect constant, field intensity should be increased nearly proportionally with the third power of under cooling. The models predict a weak decrease of nucleation and spherulite growth for the nonpolar phase of PVDF in the electricfield, what favours formation of a structure with pronounced piezoelectric properties. Although the present kinetic models of nucleation describe homogeneous primary nucleation, they also allow qualitative conclusions to bedrawn concerning field-affected non-homogeneous primary nucleation occurring on impurities. Nevertheless, an individual model is needed to describe the effect of the electric field on non-homogeneous primary nucleation.
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