Crystal structures and phase transition of tetrafluoroethylene-vinyl alcohol alternating copolymer

Abstract

Crystal structure and thermally-induced phase transition have been studied for a novel fluorine polymer or tetrafluoroethylene-vinyl alcohol (TFE-VA) alternating copolymer on the basis of the X-ray diffraction and infrared spectroscopic data analyses. The copolymer exhibits the three kinds of crystalline modification depending on the crystallization conditions. The crystal modification β is obtained by casting from the organic solution at room temperature. The molecular chains are slightly contracted from the zigzag conformation and are packed in the orthorhombic unit cell (a = 9.48 Å, b = 5.38 Å and c (chain axis) = 9.84 Å) with the space group symmetry of P21cn. The heating of the β form above 170 °C causes the irreversible transition to the high-temperature α form (αHT). The thus-created αΗΤ phase cannot change back to the β form anymore. The cooling from the melt induces the crystallization to the αHT phase at first, which transforms to the low-temperature α phase (αLT) apparently continuously and reversibly below 135 °C. The αLT form consists of the statistically disordered packing of the almost fully-extended chains in the unit cell of a = 9.58 Å, b = 5.48 Å and c (chain axis) = 5.01 Å. The αHT takes the similar crystal structure to that of the αLT, but the zigzag chains are packed in the slightly larger unit cell of a = 10.01 Å, b = 5.64 Å and c (chain axis) = 5.02 Å (at 170 °C) in more highly disordered state with thermally-activated motion of the chains. In these crystalline phases, the hydrogen bonds of F⋯H-O and O⋯H-O types are formed to stabilize the chain packing structures.