Abstract
It is known that light-scattering data for a polymer in a mixed solvent together with the refractive-index increment of the polymer evaluated at osmotic equilibrium (i.e., at fixed chemical potentials of solvent components) permit unambiguous determination of the molecular weight of the polymer–irrespective of preferential interactions between polymer and solvent. However, the analysis presumes either of two conditions; the polymer is homogeneous in molecular weight or the “osmotic” refractive increment does not depend on molecular weight. When both conditions are violated, the apparent molecular weight obtained may differ from the true weight average. This problem is examined with the aid of parameters derived from light-scattering data of Dondos and Benoit. The error in typical cases is shown to be less than uncertainty in scattering measurements if polymer heterogeneity is not extreme (say Mw/Mn≤2); but the behavior of each scattering system must be assessed individually. The data also afford material for some observations on the dependence of preferential interactions on composition of a binary solvent.
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