Effect of Polydispersity on the Cloud Point Curves of Polymer Mixtures.

A method is presented for the calculation of cloud point curves of polymer-polymer mixtures when the polymers involved are polydisperse. The method is based on the Flory-Huggins free energy of mixing with a concentration-independent chi parameter. Numerical results are given for the cases where the molecular weight distributions are represented by the Schulz-Flory type. When the two polymers have similar average molecular weights and polydispersities, the cloud point curves become flatter as the polydispersity increases. When the two polymers have similar average molecular weights but differ in their polydispersities, the cloud point curves become more skewed as the difference in the polydispersity increases. The results point out that, if the polydispersity effect is not properly accounted for, the value of chi deduced from experimental cloud points is liable to be in error, especially with regard to its temperature coefficient or its concentration dependence. Author