An explanation of switching characteristics in polymer ferroelectrics by a nucleation and growth theory

In polymer ferroelectrics, the change of polarization under an applied field is known to occur by formation of kinks or twists of polarization and by propagation of solitary waves along individual chain axes. Measurements of the switching characteristics in β-phase poly(vinylidene fluoride), β-PVDF, and in copolymers of vinylidene fluoride and trifluoroethylene have shown that the switching times of these polymers vary exponentially with the reciprocal of applied field. This exponential dependence can be explained adequately by a random domain nucleation and growth model rather than by a continuous process of twist propagation as suggested by Clark and Taylor. In addition, the nucleation and growth model is found to be consistent with the observed exponential dependence of switching time on temperature. The formation of a critical nucleus requires the realignment of polarizations of several neighboring chain segments which is accomplished by propagation of kinks along individual chain axes. The number of chain segments in the critical nucleus depends on the applied field and temperature, and. in the case of β-PVDF, it is estimated to be in the range of 6 to 23 at the applied field of 100 MV/m and at room temperature.