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The behavior of polymer—solvent systems has been studied as a function of hydrostatic pressure to 3000 atm. The swelling of crosslinked neoprene and crosslinked Hevea has been studied in several solvents, and the temperature of phase separation of polystyrene in cyclohexane has been determined. These measurements permit an indirect evaluation of the volume of mixing that is felt to have advantages over the direct method due to the small magnitude of the effect in polymer systems. In all of the cases studied by the swelling technique, the volume of mixing is negative and tends to be greater in magnitude the better the solvent. For the very poor solvent—polymer system studied by the precipitation technique, a small positive value is obtained. In addition, the compressibility of swollen polymers is compared with that expected from an assumption of additivity and found to be much smaller. These phenomena are presumably the result of a more tightly packed liquid structure within the polymer network. This explanation suggests that the descriptions of the volume of mixing in terms of the apparent specific volume, while certainly formally correct, may be misleading physically. An attempt to relate the observations on these systems to a theory involving a power series expansion in terms of the solvent fugacity was unsuccessful due to the slow convergence of the series.

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