Sequence segregation in the melt and viscoelasticity of thermotropic liquid crystalline polymer. A rheo-X-ray scattering study

Abstract

The influence of shear rate on the monomeric sequence segregation of molten thermotropic random copolyester was investigated in-situ using wide-angle X-ray scattering, and the microstructure was correlated with the viscoelastic properties. The X-ray pattern of the aligned macromolecules exhibited the 002 reflection in the melt, which confirmed periodicity along the polymer chains. The azimuthal spread of the 002 reflection is a measure of the degree of longitudinal register of neighboring chains. Increasing the shear rate increased the degree of macromolecular alignment, as measured by P‾2, the order parameter, and the azimuthal spread of the 002 reflection was reduced indicating the increase of the degree of longitudinal register. Small-strain oscillatory shear demonstrated that the monomeric sequence segregation appears to act as transient (physical) crosslinks, the liquid crystalline phase exhibited unusual elasticity and exhibited a well-defined rubber-like regime where G’>>G”. Hence, the van-Gurp-Palmen plot displayed a minimum. The presence of segregated sequences in the melt explains the unusual crystallinity in the random copolymer, the predominantly elastic behavior in the melt, and enables tuning the physical properties by controlling the microstructure.