REVERSIBLE CHANGES IN THE SOLID-STATE OF HBA HNA LIQUID-CRYSTALLINE COPOLYESTERS STUDIED BY X-RAY-DIFFRACTION

Real time X-ray diffraction patterns were recorded as a function of he ating and cooling cycles using a synchrotron radiation source for copo lyesters of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with monomer ratios 75/25, 58/42 and 30/70. The angular positio ns of the two main interchain X-ray diffraction reflections 110 and 20 0 were obtained at temperatures below the melting point with a powder diffractometer, and the unit cell dimensions of the copolymers were me asured. For the 30/70 and 58/42 compositions the cell dimensions vary linearly and reversibly with temperature. For the 75/25 copolymer the cell dimensions show a clear bend with temperature, which can be assoc iated with a transformation from an orthorhombic to a quasi-hexagonal phase. It is further shown that at higher temperature the dimensions o f the D110 and D200 coherently diffracting domains increase while the scattering intensity of the diffraction peaks decreases with temperatu re. These changes are reversible with temperature. Results are discuss ed in terms of partial melting of smaller crystallites contributing in a reversible manner to the occurrence of a mobile high temperature qu asi-hexagonal phase.