Rigid-rod polymers with enhanced lateral interactions

Poly{(benzo-[1,2-d4,5-d']-bisthiazole-2,6-diyl)-1,4-phenylene} (PBT) and poly{(benzo-[1,2-d:5,4-d']-bisoxazole-2,6-diyl)-1,4-phenylene} (PBO) are rigid-rod molecules with extended chain conformation. Fibers and films processed from these polymers have very high tensile strength and tensile modulus, but their performance under compression has been disappointing. Much work has been done in various ways to correct this problem. Most workers have focused on approaches that enhance lateral interaction of these polymers to increase the compressive strength of the fibers and films. The methods included cross-linking by coupling of free radicals generated by heat-treatment at above and below PET and PBO decomposition temperature and through o-quinodimethane intermediate from benzocyclobutene, imbedding sol-gel glass or a thermoset matrix, synthesis of two-dimensional PBO, and introduction of hydrogen bonding in PET fiber. Despite the apparent achievement of crosslinking and intercalation, modest changes in fiber compressive strength have been reported, often at the expense of tensile strength. On the contrary, results on improving PET film compressive strength by inclusion of sol-gel glass and PBO film delimination resistance by imbedding a thermoset matrix were very encouraging. Recent reports on a new rigid-rod fiber claimed to have outstanding compressive strength are also included in this review. (C) 2000 Elsevier Science Ltd. All. rights reserved.