Analysis of some extrapolation methods to derive Kθ for polymers in mixed solvents

Abstract

Unperturbed dimensions of a polymer have been computed from several graphical methods all based on approximate expressions for the expansion factor. These include Stockmayer–Fixman (SF) and Kurata–Stockmayer (KS) plots. However, deviations from linearity have been noticed when viscosity data for polymer–good solvent systems at large molar mass are available. These deviations are more emphasized in polymer–mixed solvent systems. Thus, a modification of the former equations has been proposed accounting for the dependence of the viscosimetric interaction parameter, B(computed from the slope), on the molar mass through the second virial coefficient. Second virial coefficients have been calculated from experimental data on intrinsic viscosities. Experimental data on linear polymers in mixed solvents, namely polystyrene, poly(methyl methacrylate) and poly(dimethylsiloxane), have been used to test the validity of the modified equations. Good quantitative agreement is found between reported and experimental unperturbed dimension parameters, and a ‘numerical factor’, C, is shown to be important. Modified SF and KS plots show good linear correlation, even at large polymer molar masses in good solvents.