Understanding the Morphological and Structural Evolution of α- and γ-Poly(vinylidene fluoride) During High Temperature Uniaxial Stretching by In Situ Synchrotron X-ray Scattering

Poly(vinylidene fluoride) (PVDF) is rich in different polymorphs. Among them, β-phase PVDF is of technical importance because of its strong spontaneous polarization and desired piezoelectric properties. Phase transformation via physical deformation is demonstrated to be a facile approach to obtain a high content of electroactive β-phase from other less or nonpolar phases. Over the past few decades, deformation induced α–β transformation has been intensively researched. In contrast, there is a significant lack of study on obtaining β-phase from other important PVDF phases, i.e., γ-phase, probably due to difficulty in obtaining the pure γ-phase. In this work, the pure γ-phase was induced by an ion–dipole interaction and was subjected to uniaxial stretching along with the pure α-phase at 150 °C. By means of in situ synchrotron X-ray scattering, the evolution of the lamella morphology and crystal structure over tensile deformation was carefully investigated. Corresponding to macroscopic yielding, necking, and strain hardening shown on the stress–strain curves, the crystal superstructure underwent a microscopic transition from lamellar to fibrillar morphology, proceeding in the order of interlamellar shearing, crystallographic crystal slip, lamellae fragmentation, and block alignment. Phase transformation was observed to mainly occur when the chains in the amorphous region were extensively stretched, and crystals were sheared to rotate and slip. The γ-phase exhibited a two-step transformation with a significantly higher transformation efficiency to the β-phase at the end of the deformation. Instead, α-crystals were predominately oriented under stretching with only a small portion being converted to the β-phase. When the small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) results are combined, it is proposed that the strong yet heterogeneous stress and creation of void defects as well as the lower energy barrier for the conformational change collectively contribute to the favoring of phase transformation in the γ-phase sample. This work advances our understanding about polymorphic transformation in PVDF as well as leads the way for researching an optimal route to obtain a high content of the β-phase.