Molecular mechanisms for structural changes in poly(vinylidene fluoride) induced by electric field

Two structural changes in poly(vinylidene fluoride) are known to be caused by application of an electric field: a phase transition from form II to form I via polar II and a poling-induced structural change in form I. In the phase transition from form II to polar II, the molecular orientation must change. This is caused by an inversion motion near the domain boundary of the antiphase domain structure as the polar domain grows in the direction of the b-axis. In the phase transition from polar II to form I, the conformational change from TGT (G) over bar to the planar zigzag is caused by the molecular motion from GT (G) over bar to TTT via the incommensurate form. Two models have been proposed for the reorientation of the molecule in form I crystals induced by poling: the 60 degrees step rotation model and the 180 degrees reversal model. In the present study, the doubly oriented samples in which the b-axis (i.e., the direction of the dipole moment of the molecule) is parallel to the film plane and the c-axis is parallel to the machine direction were poled. No structural change could be observed due to the poling. This supports the 180 degrees reversal model.