Conformational defects and associated molecular motions in crystalline poly(vinylidene fluoride)

Electron-diffraction investigation of single crystals of γ-poly (vinylidene fluoride) grown from the melt has revealed streaking of 0k0 reflections. The three-dimensional structure of the spikes was studied by specimen tilting during electron diffraction; streaking is maximized in the b*c* view and disappears in the a*b* view, indicating that the reciprocal-lattice spikes are directed parallel to c. After exclusion of effects associated with the shape transform of our γ crystals, the streaks are attributed to stacking faults parallel to the ab plane; such faults are consistent with incorporation of α sequences as defects within γ chains during crystallization. Conformational changes between α and γ arrangements are examined using a number of models. The simplest motions (flip-flops or rigid crankshafts) require simultaneous rotation about two noncolinear axes and are, therefore, less favored than mechanisms involving flexible chain motions as described in this report.