Characteristic Features of Crystalline Poly(1,2-difluoroethylene): Revisiting the Old-but-Totally-Unknown Fluorine Polymer

Poly(1,2-difluoroethylene) (PVLF, −[CHFCHF]n−), a member of representative fluorine polymers, was synthesized about half a century ago by the γ-ray-irradiated radical polymerization reaction as well as by the emulsion polymerization reaction (Durrell et al.). The thus-polymerized PVLF was reported to be atactic and amorphous according to the analysis of its 19F-NMR data and X-ray diffraction data (not given). In the present report, we have successfully synthesized PVLF by a different type of radical polymerization reaction in solution. The newly measured 19F-NMR spectral profile was essentially the same as that before. The solution-cast and melt-quenched samples were almost amorphous as known from the X-ray diffraction measurements. However, once the samples were heat-treated above the glass transition temperature (about 100 °C), sharp X-ray diffraction peaks were observed to appear. The differential scanning calorimetry (DSC) data exhibited a clear endothermic peak at around 200 °C, corresponding to the melting point. In addition, the thermal, mechanical, and physicochemical properties of this newly synthesized PVLF sample have been investigated by means of wide-angle X-ray diffraction, small-angle X-ray scattering, infrared absorption spectra, DSC, and viscoelastic data. All these experimental data revealed that PVLF is a semicrystalline polymer, not totally amorphous as had been reported before, and it exhibits various characteristic behaviors. PVLF had been forgotten because of its long-time misunderstanding as an amorphous polymer species. However, the present study has allowed PVLF to become a new member of crystalline fluorine polymers.