Solvent effects on the viscosity of moderately concentrated polymer solutions

Abstract

Viscosity η_o and osmotic pressure π were measured for polymer solutions at 25°C, for concentrations up to 30 g/dl. To elucidate the effects of polymer-solvent interactions of various types, the systems chosen were polystyrene in toluene (good solvent) and decalin (poor solvent), and poly(methylmethacrylate) in chlorobenzene (good) and m-xylene (poor). The surprising result was the generalization that η_o for poor-solvent systems increases with concentration faster than for good-solvent systems, so that eventually η_o becomes far greater with poor solvents. This behavior is enhanced with polar polymers, and is not explainable on the basis of entanglements in the usual sense. It is believed that polymer aggregation occurs in poor solvents, independent of molecular weight effects. Data were correlated with both the powerlaw and the Simha methods. Characteristic shapes of such curves were related to polymer-solvent effects. Theories for the Huggins constant were found to be inadequate, as was a model proposed earlier (by MCW) for concentrated solutions. The latter was modified by adopting an ad hoc friction constant, which proved successful with good solvents and could be correlated with p to incorporate the poor-solvent data as well.