Thermally induced reorganization in LCP fibers: Molecular origin of mechanical strength

The molecular reorganization occurring in liquid crystalline polymer fiber during heat treatment is of great interest for many commercial reasons. Using thermal analysis techniques, WAXS and real time temperature dependent synchrotron SAXS, the structure and morphology of commercial LCP (liquid crystalline polymer), Vectran®, HBA/HNA (p-hydroxybenzoic acid/6-hydroxy-2-naphthoic acid), and its variant polymer fiber COTBP, HBA/HNA/BP/TA (BP-benzophenone, TA-terephthalic acid), have been examined. Both fibers have the typical liquid crystalline polymer structure, i.e., highly aligned with aperiodic sequencing along the fiber axis. There is a three-fold increase in strength in both fibers with heat treatment; however, the modulus is observed to increase significantly in COTBP but not in Vectran®. This paper reports on the changes and the differences on the structural and morphological behavior for both the as-spun and heat-treated LCP fibers. We propose an 'oriented entanglement' model to describe the differences between the two polymer fibers.