ORDERED STRUCTURES OF POLY(P-HYDROXYBENZOIC ACID) .2. LOW-ENERGY CHAIN-PACKING STRUCTURES

A detailed analysis of the packing of polymer chains of p-hydroxybenzo ic acid in the low- and high-temperature range is presented. Chain con formations and the orientation of the chains in the unit cell have bee n determined by a molecular mechanics approach, based on a two-step pr ocedure consisting of grid-scan calculations and a Monte-Carlo-type st udy. Intramolecular and intermolecular degrees of freedom were conside red simultaneously in the optimization process. The low-temperature st ructures of polymer samples (phases I and II) were found to be more st able than the structures observed in oligomer samples. In phase I, the cell parameters that are perpendicular to the chain axis tend to be s maller in the calculations than in experiments. Furthermore, the calcu lations suggest the existence of polymorphism at a microscopic level d ue to the coexistence of several packing possibilities with comparable stabilities.