Electric-Field-Induced Phase Transition and Crystal Structural Change of the Oriented Poly(vinylidene Fluoride) β Form as Clarified by the In Situ Synchrotron Wide-Angle X-ray Diffraction Measurement

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In situ synchrotron wide-angle X-ray diffraction measurements have been performed successfully for the oriented poly(vinylidene fluoride) β form under the periodically changed high electric field. (i) The β form existing under the low electric field (β low- E ) was found to transform to the β high- E phase when subjected to an electric field higher than ca. ± 150 MV/m, as known from the remarkable X-ray diffraction profile change between two phases. In addition, (ii) the inversion of the polar b -axis (or the CF 2 CH 2 dipoles) and (iii) the titling of the zigzag-chain stems were found to occur simultaneously, as already reported in our previous paper ( Macromolecules 2021, 54, 2334). These three kinds of phenomena (i–iii) have been found to occur systematically in a cooperative manner. Once the β high- E phase was created in the highest electric field strength, it was reserved in the process of decreasing the electric field strength to 0 MV/m, during which, however, the stepwise rotation of the polar b -axis and the chain tilting from the oriented direction occurred again. Once when the electric field strength reached ca. −100 MV/m after passing through 0 MV/m, in addition to the cooperative phenomena of the stepwise b -axial rotation and c -axial tilting, the partial transition from the β high- E to β low- E phases occurred, and then, the β high- E phase was stabilized again at −300 MV/m. In the recovery process from −300 to 0 MV/m, the β high- E phase was reserved but the b -axial rotation and c -axial tilting still occurred as before. The stepwise b -axial rotation and the cooperative c -axial tilting are speculated to occur through the generation and propagation of TG-kink defects along the chain axis (T: trans and G: gauche C–C torsions), as reported in the previous paper. The quantitative analysis of the observed X-ray diffraction data, in combination with the lattice energy optimization calculations, has allowed us to derive the crystal structure of the β high- E phase in addition to the structure of the well-known β low- E phase. The zigzag chains are packed in parallel along the b -axis similar to the β low- E phase. However, the unit cell is deformed slightly from the original rectangular shape of the β low- E phase to the monoclinic type. On the basis of the thus-collected experimental information, the images of the structural changes occurring under the electric field have been derived concretely.