The system methylcyclohexane/polystyrene. Experimental critical curves, cloud‐point and spinodal isopleths, and their description with a semi‐phenomenological treatment

Abstract Cloud‐point and spinodal pressures up to 900 bar were investigated for the system methylcyclohexane/polystyrene using a pressure‐pulse‐induced scattering technique based on the theory of Debye. The molar‐mass range of the polymer samples is 9–50 kg/mol. The values determined isothermally in a large concentration range for a polymer sample with weight‐average molar mass M w = 17,5 kg/mol lead to isopleths which allow the construction of phase diagrams displaying cloud‐point and spinodal curves (in terms of temperature or pressure against concentration). Critical isopleths have been determined for three further samples with M w = 9,0, M w = 28,5 and M w = 50,0 kg/mol. A semi‐phenomenological treatment of the data is presented which allows an accurate description of experimental phase diagrams and isopleths for one single sample (PS 17,5). A prediction of the critical P(T) curves for the other samples, of different average molar mass, then fails completely. Fitting the model parameters to the critical data of all four samples improves the description of the critical curves but produces phase diagrams showing considerable deviations from the data. One reason for the failure of the model may be sought in its inherent incapability to deal with dilute polymer solutions in which the individual coils are separeated by regions of pure solvent. A first rough attempt to deal with this effect already brings a marked improvement in the agreement between experiment and model calculation for the critical curves, albeit at the cost of introduction of a number of adaptable parameters.