Crystal structure of the trigonal modification (alpha form) of syndiotactic polystyrene

A model of packing for the ordered modification of the alpha 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 Angstrom, c = 5.04 Angstrom, according to a statistical rhombohedral symmetry for the disordered alpha' modification and a trigonal, quasi-rhombohedral, symmetry for the ordered alpha '' modification. The glide plane symmetry of the isolated chain is not maintained in the lattice; we suggest that the space group is R (3) over bar for the statistically disordered alpha' modification; it is P3 for the ordered alpha '' modification (instead of R (3) over bar c and P3c1 previously proposed). A rotation of nearly 7 degrees of the triplets of chains with respect to the symmetric situation corresponding to the space group P3c1 for the alpha '' modification, can account for the observed differences between the intensities of the hkl and khl reflections in the electron diffraction patterns of syndiotactic polystyrene.