ORDERED STRUCTURES OF POLY(P-HYDROXYBENZOIC ACID) .1. NONBONDED INTERACTIONS IN LAYERS OF PHENYL RINGS ORTHOGONAL TO THE CHAIN AXIS

An examination of the influence of the intermolecular energy in layers of phenyl rings perpendicular to the chain axis on the packing featur es and the feasibility of certain modes of motion is presented. It is a first step in an analysis of the packing of polymer chains in the st ructures of poly(p-hydroxybenzoic acid) (PHBA) that have so far been o bserved in experiments. Energy profiles and maps have been obtained ba sed on empirically justified potential functions. Increased rotational freedom for phenyl rings is observed in the smectic-like high tempera ture structures as compared to the low temperature crystal modificatio ns. The consideration of strongly correlated rotations assumed in the model of a perfect crystal shows the possibility of reorientational mo tion for crankshaft-like chain fragments in the high temperature modif ications. This rotation of crankshaft-like fragments is proposed as be ing the origin of the reorientational disorder observed experimentally in the smectic-E-like phase of PHBA.