Light Scattering from Solutions of Polymers in Mixed Solvents

Abstract

A distribution‐function theory of light scattering in multicomponent systems is presented, taking into account the angular distribution of the scattered light. The general theory is applied to solutions of one flexible homopolymer in two solvents. An apparent polymer molecular weight M 2,ap and an apparent second virial coefficient are expressed as functions of the concentration of a second solvent, which is assumed to be poor for the polymer, in terms of molecular distribution functions to discuss the procedure of determining the true second virial coefficient and the molecular dimension in the mixed‐solvent system. In the absence of angular dissymmetry, the result is equivalent to the fluctuation theory of Kirkwood, Goldberg, and Stockmayer. It is pointed out that the second virial coefficient can be determined at low concentrations of the second solvent without applying the dialysis technique of Casassa and Eisenberg. Further, it is shown that, even if the refractive‐index increment with respect to the second solvent does not vanish completely, the molecular dimension can be determined under the condition, 0.8<M 2,ap/M 2<1.2, where M 2 is the true polymer molecular weight. The determination of the theta point in the ternary system at which the unperturbed dimension is measured is also discussed.