Extended Flory-Huggins Model for Poly-(ethylene) Oxide Solutions

Extended Flory-Huggins model for poly-(ethylene) oxide solutions

M. I. Chaudhari, L. R. Pratt

Tulane University, New Orleans, LA 70118

            Classic Flory-Huggins model for polymer phase equilibria is based on a lattice model and widely used to understand thermodynamics of polymer solutions. Nevertheless, this model is only applicable to polymer solutions that are sufficiently similar components. Bae et.al. (1993) showed that this model is unsatisfactory for aqueous poly-(ethylene) oxide solutions (FIG 1). Composition independence of interaction parameter and assumption of zero volume of mixing is often inadequate for such systems. Here, we present extended Flory-Huggins model based on evaluation of the activity of the small molecular solvent (water).  Molecular simulations of poly-(ethylene) oxide chains molecules capped with small hydrophobic groups solvated with water are analyzed for generating solution structure information required for extended model. We will survey the structural information that is a basis for a recent proposal for high-angle neutron scattering for these solutions. Then we present the computed solvent activities, and the implied Flory-Huggins interaction parameter (X) are presented. Our aim is to validate this extended model by molecular dynamic simulation and understand thermodynamics of these PEO polymers in solutions.

FIG. 1: Composition dependence of the Flory-Huggins interaction parameter interred from the data of reference below for the PEG/water system for two temperatures. ¯₁ is the volume fraction of the small-molecule solvent. Assumption of composition independence of X₁₂ is less satisfactory for PEG/water system. The temperature dependence is is stronger, and thus clearer, for this system, and the interaction strength increases with increasing temperature, reflecting significant entropic contribution.

1. Bae, Y.C., Shim, J.J., Soane, D.S., and Prausnitz, J.M., J. App. Poly, Sci., 47, 1193-1206(1993)