Crystal Structure of the Trigonal Modification (α Form) of Syndiotactic Polystyrene

Abstract

A model of packing for the ordered modification of the α form of syndiotactic polystyrene is suggested, through comparisons between calculated structure factors and experimental intensities observed in the X-ray fiber and electron diffraction patterns. The present analysis improves the preliminary model proposed in previous papers. Trans planar chains are arranged in triplets and packed in the unit cell with axes a = b = 26.26 Å, c = 5.04 Å, according to a statistical rhombohedral symmetry for the disordered α‘ modification and a trigonal, quasi-rhombohedral, symmetry for the ordered α‘‘ modification. The glide plane symmetry of the isolated chain is not maintained in the lattice; we suggest that the space group is R3̄ for the statistically disordered α‘ modification; it is P3 for the ordered α‘‘ modification (instead of R3̄c and P3c1 previously proposed). A rotation of nearly 7° of the triplets of chains with respect to the symmetric situation corresponding to the space group P3c1 for the α‘‘ modification, can account for the observed differences between the intensities of the hkl and khl reflections in the electron diffraction patterns of syndiotactic polystyrene.