Morphology and properties of rigid-rod poly(p-phenylene benzobisoxazole) (PBO) and stiff-chain poly(2,5(6)-benzoxazole) (ABPBO) fibres

The morphology of two new high performance polymer fibres, rigid-rod poly(p-phenylene benzobisoxazole) (PBO) and stiff-chain poly(2,5(6)-benzoxazole) (ABPBO), has been examined by wide angle X-ray scattering and scanning and transmission electron microscopy. Heat treatment of as-spun PBO and ABPBO fibres produces transversely broadened crystallites, which are unlike uniaxially elongated crystallites in heat treated fibres of rigid-rod poly(p-phenylene benzobisthiazole) (PBT) or stiff-chain Kevlar, i.e. poly(p-phenylene terephthalamide) (PPTA). Heat treated fibres of PBO have greater three-dimensional crystallinity and long range crystallite ordering than axially disordered PBT, but much less than well ordered PPTA. Fibres of both as-spun and heat treated PBO have a crystalline c-axis with high orientation and low paracrystalline disorder, but heat treatment of as-spun fibres of ABPBO significantly increases c-axis orientation and decreases c-axis paracrystallinity, due to additional extension of the stiff-chain ABPBO molecules. Eventual improvements in orientation and ordering of PBO fibre through improvements in synthesis and processing may, based upon theoretically predicted values, result in up to triple the strength and modulus of commercial PPTA fibre.