Separation into Phases of Polymer Mixtures in Solution

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Abstract
When three different polymers are dissolved in a common solvent and the total polymer concentration is above a certain value, three liquid layers are obtained because of the limited compatibility of the polymer components with one another. Tests have been carried out at room temperature on solutions of polystyrene t polyvinyl acetate + polyvinyl chloride in tetrahydrofuran, and polystyrene t polyvinyl acetate t atactic polypropylene in benzene. Analysis of the individual layers yielded the following results: each polymer is always contained in each layer; the mixing ratios of the polymers in the three layers are generally different from one another and differ also from the mixing ratio of the starting mixture; each of the three layers is distinguished from the others by the fact that a specific polymer has been accumulated therein in preference to the other polymers. Concerning the quantitative distribution of the polymers among the layers, some laws could be found which can be explained qualitatively on the grounds of molecular-physical considerations. A solution of polystyrene t polyvinyl acetate t polyvinyl chloride t atactic polypropylene in tetrahydrofuran (equal amounts of polymers, total concentration about 10 g/100 ml) forms four layers, the composition of which was also determined by analysis. When solutions of polymer mixtures are evaporated, separation into phases takes place because the polumers become incompatible with one another when the concentration exceeds a certain value. As a result, strongly inhomogeneous films are obtained after evaporation of the solvent. If the inhomogeneity of the films is not taken into consideration, misleading results may be obtained in the analysis of the films. It is shown that in the system polystyrene + polyvinyl acetate, with tetrahydrofuran as solvent, surprising phenomena appear on the addition in stages of cyclohexane. The phenomena are due to the overlapping of separating and dissolving processes which have, in part, opposite actions.

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