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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 Hw = 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,,, = 9,0, M, = 28,5 and M, = 50,O 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 separated 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.

With a new type of high-pressure cell the angular dependence of light scattering intensities at scattering angles B between 30" and 150" can be measured as a function of pressure P . The linear extrapolation of (v)-I as a function of sin2 (8/2) to B = 0 does not show an angular dependence of the scattering intensities. For this result the term including sin' (8/2) in Debye's equation for the critical opalescence vanishes. The reason for_this is probably due to the low molar mass of the polystyrene (number-average molar mass M, = 1 OOO g h o l ) . This means that the range of molecular forces of a polymer/solvent system is too small to be detected by light scattering. A second linear extrapolation of (r)-I to B = 0 as a function of P yields th: spinodal points of the system polystyrene/pentane in the temperature-pressure-concentration diagram.

No abstract

The temperature changes caused by adiabatic pressure changes were studied in a polymersolvent system at high pressures. It is shown that the temperature determination by means of light scattering measurements is in good agreement with thermodynamic calculations of the temperature change for adiabatic pressure changes. The directly measured temperature changes by use of a thermocouple are found to be far too small. New York 1970 P. Debye, H. Coll, D. Woermann, J. Chem. Phys. 32, 939 ( I W )