A soluble, rigid-rod polyimide has been synthesized from 3,6-diphenylpyromellitic dianhydride (DPPMDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB). Hot, isotropic solutions of the polymer in m-cresol undergo two major transitions during cooling for concentrations greater than 1 %. A mechanical sol/gel transition occurs first possibly through a nucleation-free, liquid-liquid phase separation. This is followed by the development of lyotropic, liquid-crystalline spherulites through a nucleation and growth mechanism. Both processes are thermally reversible. On heating however, the gel/sol transition and the liquid crystal/isotropic solution transition appear to occur almost simultaneously. The latter process is endothermic, typical of an order/disorder transition. Isothermal annealing studies show that although the rate of spherulite formation is faster at lower temperatures, the structures formed are less ordered than those formed at higher temperatures. An increase in polymer concentration increases the rate of spherulitic growth at a given temperature but does not affect the structure developed. Scanning electron microscopy observations of gels from which the solvent had been extracted reveal that the rigid-rod chains pack in a microfibrillar network.
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