Interfacial tensions of phase-separated polymer solutions

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Abstract
Measurements of the capillary length in phase‐separated solutions of polymethylmethacrylate in 3‐octanone (PMMA/3‐OCT) for each of five different molecular weights of PMMA, and in phase‐separated solutions of polystyrene in methylcyclohexane (PS/MCH) for a single molecular weight of polymer, are reported. Measurements on PMMA/3‐OCT were by the sessile‐drop method; those on PS/MCH were by both the sessile‐drop and capillary‐rise methods. Interfacial tensions are estimated from estimated density differences and the measured capillary lengths. A theoretically predicted scaling law for the dependence of capillary length on polymer molecular weight and temperature is tested and at least semiquantitatively verified. As by‐products of the sample preparation, the molecular‐weight dependences of the critical composition and critical solution temperature of PMMA/3‐OCT were also determined. The volume fraction of polymer at the critical point, φc, is found to be proportional to the −0.37±0.01 power of the molecular weight, in agreement with values of this exponent found by others for polystyrene solutions.