New Physical Insights into Shear History Dependent Polymorphism in Poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) is a semicrystalline polymer that exists in four crystalline phases (alpha, beta, gamma, delta). Among these, the beta-phase has received tremendous techno-commercial importance due to its higher dipole moment as compared to the other phases, and thus many strategies have been explored in the recent past to obtain the beta-polymorph of PVDF. In this study, the effect of shear history on crystalline morphology and behavior of PVDF has been investigated systematically by polarized optical microscopy coupled to a hot stage, Fourier transform infrared spectroscopy, differential thermal analysis, rheometry, and dielectric relaxation spectroscopy. Thin films of PVDF (120-150 mu m) were sheared at different temperatures ranging from 155 to 220 degrees C and were allowed to isothermally crystallize at 155 degrees C. When the samples were isothermally crystallized at 155 degrees C, a remarkable increase in beta-phase content was observed. More interestingly, this phenomenon was observed to be shear history dependent. For instance, the samples which were sheared at high temperature (220 degrees C) reflected in higher beta fraction as compared to samples which were sheared at lower temperature (155 degrees C). It is envisaged that the distance between T-shear (temperature at which the samples were sheared) and. T-cry (crystallization temperature) significantly influences the content of the beta-phase in PVDF. This study clearly demonstrates the fact that both shear history and the depth from T-c influence the conformational changes in PVDF.