Morphology of a phase‐separated and a molecular composite PBT/ABPBI polymer blend

The structure and morphology of homopolymers and blends of rigid‐rod poly(p‐phenylene benzobisthiazole) (PBT) and semiflexible coil poly[2,5(6)benzimidazole] (ABPBI) were examined by wide‐angle x‐ray diffraction and scanning and transmission electron microscopy. When samples were processed from a solution where the total polymer concentration of 30% PBT/70% ABPBI blend was greater than a critical concentration, large‐scale phase separation occurred and 0.1–4 μm ellipsoidal particles were present in a ductile matrix. The ellipsoids were chiefly composed of aggregates of well‐oriented 10‐nm PBT crystallites, while the matrix material was chiefly ABPBI. When the concentration was less than a critical concentration, the solution was optically homogeneous. In processing of fiber and film samples from the homogeneous solution, large‐scale phase separation was inhibited by rapid coagulation in a water bath. After heat treatment, these samples were found to contain crystallites of both PBT and ABPBI with lateral dimensions of ordered regions no larger than 3 nm. The PBT homopolymer was dispersed in the matrix at the molecular level in ordered regions at a scale no larger than 3 nm, resulting in a rigid‐rod molecular composite. In the rigid‐rod molecular composite fiber both the molecular‐level dispersion and high orientation contributed to higher values of strength and modulus compared to the properties of a phase‐separated fiber. The strength and modulus of highly oriented fiber were only 25% higher than those of planar isotropically oriented film, suggesting that the level of dispersion of rod molecules is more important than orientation of the reinforcing phase in rigid‐rod molecular composites.