Crystal Structure\nand Phase Transition Behavior of\nPoly(1,2-difluoroethylene)

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