Experimental studies of fiber wet‐spinning and solution processing of films of molecular composites are presented. The rigid rod polymer was poly (p‐phenylenebenzobisthiazole) (PPBT) and the flexible polymers were poly (2,5(6′)‐benzimidazole) (ABPBI) and poly (2,5(6′)‐benzothiazole) (ABPBT). Effects of the flexible polymer molecular entanglements in solution on the processing are discussed. These fibers and films have very high modulus and strength, which improve upon heat treatment. The uniaxial modulus of highly oriented molecular composites follows the linear rule of mixtures.

A polybenzimidazole having the structural repeat unit and designated AB‐PBI was formed into films by casting or precipitating from dilute solutions. Selected films were swollen in a plasticizing medium arid drawn about 2X. All films were dried in a vacuum oven to aid in the removal of residual solvent and moisture. Wide angle X‐ray scattering (WAXS) was used to show the type of ordering found in this material and to indicate the extent of molecular orientation. Neither a glass transition nor a crystalline melting point has been observed for this polymer, A Perkin‐Elmer DSC was used to scan the temperature range from −150 to H‐400°C in search of secondary transitions. The rheovibron proved more sensitive and a transition was located in the vicinity of −60°C. An activation energy‐was determined. The morphology of this polymer is subject to speculation. The ability of the chain to adopt a linear conformation leads to the possibility of a liquid‐crystalline nature in solution. A model based on the data in hand is suggested. Stress‐strain data was used to compare the mechanical properties of films prepared by several methods as well as to demonstrate the effect of orientation on strength.

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A reliable semiempirical method for predicting glass transition tempertures of linear polymers, random copolymers, and selected crosslinked networks from knowledge of only their chemical structure is presented. For cases in which new moieties not in the database are encountered, a scaling technique of similar moiety contributions has proven successful. The basic database was composed of 178 linear homopolymers ranging from aliphatic to aromatic heterocyclic polymers and 12 random copolymers. The crosslinked networks investigated in this study involved only diacetylene end‐group reactions.

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Phase diagrams of two types of rigid rod polymer/flexible coil polymer/methanesulfonic acid (MSA) ternary systems were determined by polarized optical microscopy at ambient conditions. The rigid rod polymer is a wholly aromatic high temperature resistant (no measurable Tg) poly (p‐phenylenebenzobisthiazole) (PPBT). One of the flexible coil polymers is a wholly aromatic high temperature resistant poly (2,5′(6′) benzimidazole) (ABPBI), the other is a thermoplastic poly[2,2′ ‐(1–4‐phenylene)‐6,6′ ‐bis (3‐phenyl‐quinoxaline)] (PPQ) with Tg of 359°C. The solvent is methane‐sulfonic acid (MSA). The experimentally determined critical concentration points, Ccr, are in excellent agreement with Flory's recent theory. Total phase segregation between the polymer pair in ternary solution was predicted and observed at C > Ccr. Different decomposition mechanisms of phase separation were observed as a function of concentration.

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