Compression behavior of high performance polymeric fibers and its dependence on crosslinking

The high performance polymeric fibers such as aramid (e.g. Kevlar{dollar}spcircler{dollar} 49), Poly(p-phenylene benzobisthiazole) (PBZT) and Poly(p-phenylene benzobisoxazole) (PBO) have excellent tensile and thermal properties. These fibers have limited applications in structural composites because of the poor compressive strength. Many approaches have been followed to overcome this drawback. These include control of molecular structure and fiber morphology. The control of molecular structure has chiefly relied on crosslinking individual molecules in such rigid-rod systems. In the present work, methyl pendent PBZT (MePBZT) fiber has been chosen as the model system to study the effect of heat treatment induced crosslinking on the compressive strength. Most of the previous studies have considered insolubility as a test for crosslinking. It has been shown that in PBZT type of rigid-rod systems, insolubility could also result from improvement of the crystalline order. The effect of heat treatments under tension as well as in unconstrained (free) condition has been studied. Using {dollar}sp{lcub}13{rcub}{dollar}C solid-state NMR, it has been shown that crosslinking in methyl pendent PBZT could be achieved via formation of methylene bridges upon heat treatment above 450{dollar}spcirc{dollar}C under free (unconstrained) condition. However, as a result transverse cracks are generated in the skin. It is concluded that crosslinking reactions must be facile in order to minimize the chain mobility and orientational changes during crosslinking. Kinking behavior and compressive failure mechanism in some of the rigid-rod polymeric fibers have also been studied.