The free energy of a polmyer solution is derived by a consideration of the monomer density fluctuations and incorporating threehyphen;body interactions. Explicit interpolation formulas are obtained for the concentration dependence of the correlation length for arbitrary strengths of twohyphen; and threehyphen;body interactions within the random phase approximation. When the ternary interactions are important, as is the case under the conditions of phase separation in polymer solutions, the derived free energy leads to new correspondinghyphen;states equations for the spinodals. The critical volume fraction fgr;c, and Verbar;fgr;minus;fgr;cVerbar;/fgr;care found to be proportional tonminus;1/3andn1/9, respectively, wherenis the degree of polymerization of the polymer and fgr; is the coexistent polymer volume fraction. A comparison is made between the predictions and the experimental results reported in the literature.
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