It is proposed that the overall free energy of a polymer solution has three fundamentally different types of contributions
i.e.the translational
the configurational and the thermal correction paris The second and the third pans have the additive properties for segments and can be calculated by path integral method with the introduction of a new concept of effective concentration.Taking the volume fraction as the effective concentration
one obtains the Li- Zhao theory of polymer solutions reposed previously.The results calculated by the present theory match the spinodals data of PS/cyelohexane systems with different molecular weights much better than those based on the Flory-Huggins theory.