Reversible changes in the solid state of HBA/HNA liquid crystalline copolyesters studied by X-ray diffraction

Abstract

Real time X-ray diffraction patterns were recorded as a function of heating and cooling cycles using a synchrotron radiation source for copolyesters of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) with monomer ratios 75 25 , 58 42 and 30 70 . The angular positions of the two main interchain X-ray diffraction reflections 110 and 200 were obtained at temperatures below the melting point with a powder diffractometer, and the unit cell dimensions of the copolymers were measured. 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 associated with a transformation from an orthorhombic to a quasi-hexagonal phase. It is further shown that at higher temperature the dimensions of the D 110 and D 200 coherently diffracting domains increase while the scattering intensity of the diffraction peaks decreases with temperature. These changes are reversible with temperature. Results are discussed in terms of partial melting of smaller crystallites contributing in a reversible manner to the occurrence of a mobile high temperature quasi-hexagonal phase.