Morphology and mechanical properties of a phase separated and a molecular composite 30% PBT/70% ABPBI triblock copolymer

The morphology of a triblock copolymer of 30% rigid-rod poly(p-phenylene benzobisthiazole) (PBT) and 70% semi-flexible coil poly(2,5(6)benzimidazole) (ABPBI) was examined by wide angle X-ray scattering and scanning and transmission electron microscopy. Samples that were vacuum cast from a solution formed a microphase separated film with 0.1 μm particles and platelets of well-oriented 10 nm PBT crystallites in a ductile ABPBI matrix. Fibres were dry-jet/wet-spun from an optically homogeneous solution into a water coagulation bath to inhibit large scale phase separation. Heat-treated fibre contained crystallites of PBT and ABPBI with lateral dimensions no larger than 3 nm, demonstrating that PBT molecular segments were well dispersed and that a rigid-rod, molecular level composite had been achieved. The molecular level dispersion and high orientation in the 'molecular composite' fibre resulted in excellent mechanical properties with a modulus of 100 GPa and a tensile strength of 1.7 GPa which were about an order of magnitude greater than for the vacuum cast copolymer film.