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

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 beta 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 angstrom, b = 5.38 angstrom and c (chain axis) = 9.84 angstrom) with the space group symmetry of P2(1)cn. The heating of the beta form above 170 degrees C causes the irreversible transition to the high temperature alpha form (alpha(HT)). The thus-created alpha(HT) phase cannot change back to the beta form anymore. The cooling from the melt induces the crystallization to the alpha(HT) phase at first, which transforms to the low-temperature alpha phase (alpha(LT)) apparently continuously and reversibly below 135 degrees C. The alpha(LT) form consists of the statistically disordered packing of the almost fully-extended chains in the unit cell of a = 9.58 angstrom, b = 5.48 angstrom and c (chain axis) = 5.01 angstrom. The alpha(HT) takes the similar crystal structure to that of the alpha(LT), but the zigzag chains are packed in the slightly larger unit cell of a = 10.01 angstrom, b = 5.64 angstrom and c (chain axis) = 5.02 angstrom (at 170 degrees C) in more highly disordered state with thermally-activated motion of the chains. In these crystalline phases, the hydrogen bonds of F center dot center dot center dot H-O and O center dot center dot center dot H-O types are formed to stabilize the chain packing structures.

Automated summary

A novel fluorine polymer, tetrafluoroethylene-vinyl alcohol (TFE-VA) copolymer, has been studied for its crystal structure and thermally-induced phase transition using X-ray diffraction and infrared spectroscopy. The copolymer exhibits three types of crystalline modification under different crystallization conditions, with transitions between beta, high-temperature alpha(HT), and low-temperature alpha(LT) forms observed. The chain packing structures are stabilized by hydrogen bonds of F···H-O and O···H-O types in these crystalline phases.