STRUCTURE AND LINEAR VISCOELASTIC BEHAVIOR OF MAIN-CHAIN THERMOTROPICLIQUID-CRYSTALLINE POLYMERS

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 a romatic copolyesters based on random units of 75 mol % 1,4-hydroxybenz oic acid (B) and 25 mol% 2,6-hydroxynaphthoic acid (N). The thermal ch aracterization, performed between -50 degrees C and 360 degrees C, sho wed that annealing below 290 degrees C gives rise to a secondary endot herm. However, only one endotherm is observed when annealing is carrie d out above 290 degrees C, showing that all transitions are completed by 310 degrees C. Hot-stage wide-angle x-ray scattering demonstrates t hat the second endotherm is associated with a solid-solid transformati on from pseudohexagonal to orthorhombic. The orthorhombic phase, found on slow heating or annealing below the melting point, has a higher me lting point than the pseudohexagonal phase. The increase in melting po int which can result from such thermal treatments is avoided in all th e experiments reported here by comparatively rapid heating of the spec imens into the melt. In situ optical microscopy and x-ray scattering m easurements show that the ''as-molded'' samples for rheological experi ments exhibit preferred orientation, which is associated with their me chanical history. However, holding the sample in the molten state over periods of time leads to a relaxation of the degree of orientation, u ntil a macroscopically unoriented (textured) state is obtained. This r eduction of degree of orientation is correlated with an increase of th e complex viscosity, where a plateau value is reached in the final uno riented 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, i t is also found that this linearity extends only up to strains of abou t 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 b ehave like lightly cross-linked materials, adding thus support to the elastic network hypothesis. (C) 1997 The Society of Rheology.