High-temperature structures of poly(p-hydroxybenzoic acid)

The structures of poly(p-hydroxybenzoic acid) (PHBA) at the two high-temperature phase transitions (ca. 340 and 430°C) have been investigated by X-ray powder diffraction methods (X-ray tube focusing and synchrotron parallel beam), chain conformation/packing models, and dielectric measurements. The structure of PHBA between ca. 340 and 430°C, derived from the X-ray patterns and available electron diffraction results, is satisfactorily accounted for by the following model: orthorhombic cell, a = 9.24 angstrom, b = 5.28 angstrom, and c = 12.50 angstrom, with two successive phenylene planes along the c-axis rotated 60° from each other in the unit cell; the phenyl groups in the a-b plane are oriented in an ordered herringbone-type packing in each phenyl layer with their planes 60° and 120° with respect to the a-axis; and each chain segment (of two monomer repeats) in the unit cell is positioned randomly in the a-b projection among the two different sites involving a twofold axis of reorientation through an angle π.