COCRYSTALLIZATION OF SOLVENTS WITH POLYMERS - THE X-RAY-DIFFRACTION BEHAVIOR OF SOLVENT-CONTAINING AND SOLVENT-FREE POLYACRYLONITRILE

Differences in the x-ray diffraction pattern of polyacrylonitrile (PAN ) have been observed, both here, as well as in the work reported in th e literature. Various workers have cited grossly differing unit cells for PAN and this has never been explained. In this work, it is shown t hat in dry samples, the diffraction pattern contains only two equatori al peaks, d1 almost-equal-to 5.3 angstrom and d2 almost-equal-to 3.0 a ngstrom. This is consistent with a hexagonal packing of rod-like chain s of PAN with no chain-axis order. However, in solvent-plasticized fil ms, the diffraction pattern is altered. It is proposed that certain so lvents cause a hexagonal to orthorhombic transition. However, the orth orhombic polymorph appears to be a solvated form, where the solvent ha s co-crystallized with the polymer. After removal of solvent, the stru cture reverts to hexagonal packing. Hence, it is suggested that the di screpancy in cell parameters cited in the literature, particularly whe n large cell dimensions are quoted, may be due to the inadvertent pres ence of solvent which could have been introduced during sample prepara tion. Similarities with solvated crystals in other polymeric systems a re also briefly discussed. (C) 1994 John Wiley & Sons, Inc.