Ferroelectric polarization in polymers

Previous work has shown that a model of oriented dipoles accounts quite well for observed piezoelectric and pyroelectric behavior of amorphous and semicrystalline polymers. The process whereby the dipoles become oriented is the subject of this paper. Here we summarize a model and calculated results for ferroelectric reorientation of molecular dipoles in the all-trans polar crystal phase (β phase or Form I) of polyvinylidene fluoride. By ferroelectric we mean a material with polar crystals where the polarization direction can be reversed with an applied electric field. In the model, the orientations of molecular dipoles depend on the electric field and the history of the field application. The quantities <cosΘ>, <cos2Θ>, and f(Θ) can be calculated where Θ is the angle between a dipole and the applied field, f(Θ) is the fraction of dipoles at angle Θ and the brackets <> indicate a spacial average. The experimental physical quantities of interest for comparison with the model are polarization (a measure of <cosΘ>), infra red transmission intensity (a measure of <cos2Θ>) and x-ray pole figure data (a measure of f(Θ)). Since only one adjustable parameter is used in the model, these various comparisons are especially meaningful.