Mutual diffusion in polymer solutions: Spanning phase diagrams

Abstract This paper aims to discuss the trend of variation of mutual diffusion coefficient in polymer solutions exhibiting different phase behavior. An equation for mutual diffusion coefficient in polymer solutions was derived in the framework of classical irreversible thermodynamics using Onsager’s variational principle. The behavior of mutual diffusion coefficient in different phase diagrams was investigated by changing the Flory-Huggins interaction parameter to include concentration and temperature dependencies using Koningsveld interaction factor. The proposed diffusion coefficient satisfies the asymptotic behavior at infinite dilution, becomes zero on the spinodal line and changes sign at the onset of entering the unstable zone of phase diagrams. It was shown that the diffusion slows down near the critical point and in general by approaching the spinodal curve. Moreover, it was shown that the derived equation for mutual diffusion coefficient can be expressed in terms of osmotic bulk modulus which is consistent with the result of a variational approach using Landau-Ginsberg-Wilson Hamiltonian.