AbstractViscosity measurements were carried out as a function of pressure and temperature with solutions of polystyrene in eight (endothermal) θ‐solvents at the respective critical composition by means of a Searle‐type apparatus. A rolling‐ball viscometer was used for the investigation of the pure solvents. In all cases the viscosity coefficient increases in a more or less exponential manner when the pressure is raised. For θ‐conditions, the volumes of activation of the solutions exceed that of the pure solvent by typically 10–15. The exact amount of this extra efffect stemming from the presence of the polymer and its variation with temperature can be qualitatively correlated with the heats of mixing. The ratio of the viscosity of the solution at 1000 and 1 bar, respectively, can be varied for a given solvent power (θ‐temperature) by the choice of the solvent from ca. 2 (cyclopentane) to 4 (trans‐decalin). Within a given system, the maximum effects that can be realized by a change of the solvent power via the variation of temperature ranges from ca. 3 to 6 (te
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