Refractive index increments of polymers in solution: 2. Refractive index increments and light-scattering in polydisperse systems of low molecular weight

Abstract

Theoretical equations are deduced which predict that the specific refractive index of a polymer in solution is v = v ∞ + v 0 M ̄ n , where M ̄ n is the number average molecular weight, and v ∞ and v 0 depend on the refractive index, n 1, of the solvent, on the specific refractivity of the pure polymer, and on the specific volume of the polymer in solution. These equations are used to show that the true weight average molecular weight and second virial coefficient, as obtained from light scattering, can be obtained from the observed values of these quantities only if M ̄ n and the dependence of v on M ̄ n are known. The difference between the true and observed values can be very pronounced for values of M ̄ n less than about 10 3. Mixtures of two polymers whose refractive increments depend on molecular weight are also considered. Methods of distinguishing the effects of polydispersity of composition from effects due to the dependence of the average v on the average molecular weight are discussed for mixtures of components whose individual properties are unknown.