X-ray Analysis of the Kinetics of Transesterification in Blends of Wholly Aromatic Thermotropic Copolyesters

Abstract

The kinetics of the transesterification reaction occurring in blends of wholly aromatic copolyesters have been investigated by simulation of the changes that occur in the X-ray scattering data as a result of the interchange. The X-ray fiber diagrams of copolyesters prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) contain nonperiodic diffraction maxima along the chain axis direction that vary in position (scattering angle) with the monomer composition and are due to the structural correlations in chains of completely random comonomer sequence. Melt blends of two different compositions initially show the scattering maxima of both components, but these shift as the specimen is held in the melt, until they merge to give the diffraction data characteristic of the intermediate composition. We have simulated the changes in these data by modeling the changes in the correlation function that occur as a result of the transesterification reaction. We find that the data are best reproduced using a completely random interchange model for a completely compatible blend. Simulations based on preferential reactivities of one or other of the monomers or partial segregation of one phase gave significantly inferior agreement with the positions of the observed diffraction maxima. The statistical modeling allows us to derive kinetic parameters for the reaction, for which we obtain an activation energy of 142 kJ/mol and a rate constant of 2.3 × 10-4 s-1 at 315 °C. Compared to the equivalent data for isotropic polyesters, the activation energy compares very well, but the rate constant is 1 order of magnitude lower, perhaps due to the ordering in the thermotropic melt.