Thermodynamics of polymer solutions. Part 1.—Natural rubber and benzene
Activities of rubber-benzene solutions have been determined at 10, 25 and 40°C over the range of concentration, expressed as the segment fraction φ2 of polymer, from 0.05 to 0.945, using high pressure osmometry and an improved vapour sorption method. Results are in general agreement with earlier work of Gee and co-workers, except that they yield lower values for the reduced partial molar excess enthalpy : χ=HE1/RTφ22≅ 0.1 ± 0.1. Excess volumes, also determined at 25°C, are small and positive, amounting to ca. 0.09 % at φ2= 0.5. These results, including the excess volumes, are compared with calculations according to recent theory which takes account of the equation-of-state characteristics of the pure components. Good agreement is established. The small experimental entropy of dilution compared with that calculated from the combinatorial contribution alone is well explained by equation-of-state contributions. The observed increase of the conventional free-energy parameter χ with concentration arises from the latter contribution and is unambiguously predicted by theory.