Relaxation processes in the oriented polyvinylidene fluoride films with various crystalline phase composition

Oriented polyvinylidene fluoride (PVDF) films were obtained in multistage process based on melt extrusion of polymer. We investigated the results of the polymorphic α→β crystalline phase transformation in the PVDF films subjected to uniaxial stretching. During uniaxial extension both appearance of a polar piezo active crystalline structure and significant changes in the samples morphology were observed. Variations in the PVDF films morphology, polymorphic composition, and supramolecular structure upon transformation were detected with Fourier transform infrared spectroscopy, wide-angle X-ray scattering, and scanning electron microscopy techniques. Broadband dielectric spectroscopy was used to ascertain a change in molecular mobility of the polymer chains during α→β phase transformation. The relaxation processes, γ-, α_а-, α_с-, and interfacial polarization, in both α- and β-phases of PVDF were identified in the dielectric loss spectra and described with either Arrhenius or Vogel-Fulcher-Tammann equations. The analysis of the equations parameters allowed concluding that initiation of a polymorphic α→β transition through uniaxial extension results in hindering the relaxators mobility in the β-phase of PVDF samples, except γ-relaxators. This finding confirms a proposal that γ-relaxators are located in the amorphous part of PVDF. Uniaxial extension resulted in a substantial increase in the interfacial polarization, which can be attributed to the emergence of new interface boundaries.

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