Crystal Structure and Phase Transition Behavior of Poly(1,2-difluoroethylene)

For several decades, poly(1,2-difluoroethylene) [PVLF, −(CHFCHF)n−] had been believed to be atactic and amorphous. However, our recent study has revealed that PVLF is an atactic but partially crystalline polymer. In the present report, the crystal structure of PVLF, as the most important basic structural information, was determined by the quantitative analysis of 2D wide-angle X-ray diffraction (WAXD) data measured at room temperature. The two chains pass through the orthorhombic unit cell of a = 9.20 Å, b = 5.32 Å, c (chain axis) = 4.97 Å with the space group Pcam. The polymer chains take the deflected zigzag conformation of TS′TS̅′ (torsional angles, T: 180° and S′: 142°), which is contracted slightly from the fully extended all-trans form. The temperature-dependent measurement of the WAXD patterns revealed that the unit cell size increases almost linearly with temperature but shows a small deflection point at around 100 °C. This deflection corresponds to the reversible second-order phase transition between the low-temperature (LT) phase and the high-temperature (HT) phase. The molecular dynamics calculation revealed an occurrence of the thermally activated translational and torsional motion of molecular chains in the HT phase. In parallel, temperature-dependent small-angle X-ray scattering data analysis revealed that the lamellar stacking structure changes irreversibly at the transition point as a result of such thermally enhanced motions of molecular chains.