Synthesis of poly(vinylidene fluoride-co-bromotrifluoroethylene) and effects of molecular defects on microstructure and dielectric properties

A series of copolymers composed of vinylidene fluoride (VDF) and bromotrifluoroethylene (BTFE) have been synthesized via suspension polymerization up to crystallinity inhibition. P(VDF-co-BTFE) copolymers exhibit different regioregularity in comparison to previously reported PVDF based copolymers owing to differences in size and reactivity of BTFE. The polymerization of the comonomers result in molecular defects that are shown to be both included (single BTFE defects) and excluded (runs of BTFE monomers) from the crystalline phase. The effects of increasing defect concentrations determined by 19F NMR were evaluated on the resulting microstructures by using Fourier transformed infrared spectroscopy, differential scanning calorimetry, and wide-angle X-ray diffraction. Dielectric properties have been investigated in terms of complex permittivity as a function of frequency and temperature. The results indicate that the single BTFE defects are incorporated into the crystalline phase and destabilize the ferroelectric β phase, while the excluded defects reduce both lamellar and lateral crystallite sizes though also resulting in a significant drop in crystallinity. The excluded defects are found to expand the interlamellar region of the crystalline phase, which increases both temperature and frequency dependence of the dielectric β relaxation.