The molecular weight dependence of lower and upper critical solution temperatures

The molecular weight dependence of the Lower Critical Solution Temperature in polymer solutions has been derived using the Prigogine smoothed potential cell model. Satisfactory agreement is obtained with experimental results for a number of systems. It is found that the new structural term (or equation of state term) in χ introduced by the Prigogine theory accounts for the LCST. Any difference in chemical nature between the polymer and solvent molecules may be largely ignored. The structural term also has an effect on the molecular weight dependence of the familiar Upper Critical Solution Temperature. It is predicted that fractionation of a polymer could be improved by choosing a solvent for which the UCST and LCST were separated by only a small temperature interval. Fractionation at the LCST should not be more efficient than at the UCST.