Freezing point depression of polymer solutions and gels

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Abstract
The freezing point depression of polymer solutions was measured in the concentration range 5% to 30% (by volume of polymer), particular attention being paid to avoid excess supercooling. The polymer‐solvent combinations investigated were polystyrene‐benzene, polyvinyl acetate‐benzene, cellulose acetate‐acetic acid, and polyvinyl alcohol‐water. Through the proper adjustment of the rate of cooling, the freezing curves showed a sharp break, from which the freezing points were determined as a function of polymer concentration with sufficient accuracy to permit discussions of the thermodynamic properties of these polymer solutions. The discussion of these data was made in terms of the interaction parameter, μ, well known in the theory of polymer solutions. The new feature brought out in this work is the marked change of μ with polymer concentration. For all the polymer‐solvent systems studied, fairly rapid fall in μ was observed with increasing concentration. Such behavior in μ is important because it is in no way specific to these systems and leads to the expectation that, for polymer solutions in general, μ (or, at least μs, the entropy contribution to μ) should fall progressively as the polymer concentration is increased, with a corresponding increase in the degree of overlapping of the excluded volumes of polymer molecules. The earlier theories of polymer solutions require improvement in this direction. In the cases of the systems, cellulose acetate‐acetic acid and polyvinyl alcohol‐water, the orientation and strong heat of mixing effects must also be considered because of the polar nature of the components. In order to carry out further quantitative discussion on the results obtained, we must wait until more complete theories of polymer solutions will be developed. It has become apparent, nevertheless, that the freezing point determination furnishes our best source of information concerning the thermodynamic properties of polymer solutions. As for aqueous solutions of polyvinyl alcohol, possible effects of the intermolecular junction points which might be produced in the solution on cooling upon the results were examined, but negative results were obtained. Finally, a possibility was suggested for development of this method to permit extensions to lower and higher concentration ranges than those covered by the present work, the latter (higher concentration range) including gels produced by polymer networks and assemblages of moist fibers.

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