Structure and linear viscoelastic behavior of main-chain thermotropic liquid crystalline polymers

Abstract

The influence of molecular weight on the ordering processes and linear viscoelastic properties of a series of thermotropic main-chain liquid crystalline polymers has been investigated. The polymers are wholly aromatic copolyesters based on random units of 75 mol % 1,4-hydroxybenzoic acid (B) and 25 mol% 2,6-hydroxynaphthoic acid (N). The thermal characterization, performed between −50 °C and 360 °C, showed that annealing below 290 °C gives rise to a secondary endotherm. However, only one endotherm is observed when annealing is carried out above 290 °C, showing that all transitions are completed by 310 °C. Hot-stage wide-angle x-ray scattering demonstrates that the second endotherm is associated with a solid-solid transformation from pseudohexagonal to orthorhombic. The orthorhombic phase, found on slow heating or annealing below the melting point, has a higher melting point than the pseudohexagonal phase. The increase in melting point which can result from such thermal treatments is avoided in all the experiments reported here by comparatively rapid heating of the specimens into the melt. In situ optical microscopy and x-ray scattering measurements show that the “as-molded” samples for rheological experiments exhibit preferred orientation, which is associated with their mechanical history. However, holding the sample in the molten state over periods of time leads to a relaxation of the degree of orientation, until a macroscopically unoriented (textured) state is obtained. This reduction of degree of orientation is correlated with an increase of the complex viscosity, where a plateau value is reached in the final unoriented state. The rheological characterization, on textured samples, show that B–N copolyesters exhibit a linear viscoelastic (LVE) regime similar to that observed in common flexible chain polymers. However, it is also found that this linearity extends only up to strains of about 10%, and is independent of the molecular weight. Dynamic oscillatory experiments in the LVE regime reveal a rubberlike region (minimum in delta loss angle), suggesting that the B–N thermotropic copolyesters behave like lightly cross-linked materials, adding thus support to the elastic network hypothesis.