Molecular weight and concentration effects on gel/sol transitions in a segmented rigid‐rod polyimide solution

Gel/sol and order/disorder transitions can be observed in a segmented rigid‐rod polyimide solution at different concentrations and molecular weights. This polyimide was synthesized from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and 2,2′‐bis(trifluoromethyl)‐4,4′‐diaminobiphenyl in refluxing 3‐cresol in a one‐step process. Above about 140°C, the polymers are completely soluble in 3‐cresol in a concentration range up to 12(w/w)%. During cooling, the hot solutions undergo a mechanical sol/gel transition to form thermally reversible gels. In the gel state, an ordered structure develops. Both the gel/sol and order/disorder transitions are dependent upon concentration, temperature, and molecular weight, but the two transitions exhibit different kinetics. Possible mechanisms of these transitions are discussed.