Feasibility of the light scattering method to characterize composition heterogeneities of copolymers

Feasibility of light scattering as a method to characterize the composition heterogeneities of copolymers is examined with special reference to those measurements which are made in an isorefractive solvent, i.e., a solvent giving zero refractive index incrementvfor the copolymer. It is established theoretically and experimentally that the concentration dependence in the Zimm plot for an isorefractive system is almost absent, irrespective of the thermodynamic properties of the solvent employed. In addition, the error to which determination of the heterogeneity parameterQis subject due to inaccuracy invis minimum whenvis zero. By virtue of these facts,Qmay be determined with higher precision in an isorefractive solvent than in any other solvents. Under optimum conditions, it seems possible to determineQwith sufficient precision, ifQ⪆ 1·103andQ/Mw⪆ 2·10−3. These values are about an order of magnitude lower than those generally believed to be the applicability limits of the light scattering method.