Herman and George R. Brown Chair
Department of Chemical & Bimolecular Engineering
Tulane University

Concentration dependence of the Flory-Huggins interaction parameter in aqueous solutions of capped PEO chains

Abstract

Poly(ethylene oxide) (PEO) or poly(ethylene glycol) (PEG) chains are the most important synthetic water-soluble polymers.   "Pegylation" is a standard strategy in drug delivery designs.   Nevertheless, the standard theory of polymer solutions, the Flory-Huggins (FH) theory is clearly not accurate for aqueous solutions of PEO chains.  We show how the dependence on volume fraction of the Flory-Huggins interaction parameter describing the free energy of mixing of polymers in water is obtained by exploiting the connection to the chemical potential of the water, for which quasi-chemical theory is satisfactory. We test this theoretical approach with simulation data for aqueous solutions of capped PEO oligomers.  Consistent with experiment, this constant is determined to be strongly concentration dependent. These results predict phase separation that is supported by direct observation of the

coexistence of the two solutions on simulation time scales. This approach directly provides the osmotic pressures, and thus access to osmotic stress experiments. Reflecting repulsive interactions between the chains in the water, a good solvent for these chains, the osmotic second virial coefficient for the chains is positive .