A method for studying optical anisotropy of polymers as a function of molar mass.

Optical properties of polymers are extremely important in many end-use applications, as is the ability of anisotropic polymers to depolarize incident radiation. To date, most light-scattering studies of the optical anisotropy of macromolecules have dealt with the bulk state or measured ensemble properties of dilute solutions. Here, we introduce a method to determine the optical anisotropy as a continuous function of molar mass. By direct, on-line coupling of size-exclusion chromatography and depolarization multiangle light scattering (SEC/D-MALS), molar mass averages, polydispersities, molar mass distributions, and the distribution of the optical anisotropy as a function of molar mass may all be determined. To quantify the anisotropy, it has been expressed in terms of the Cabannes factor, thus permitting the Rayleigh ratio necessary for light-scattering calculations to be corrected for anisotropy. The effects of tacticity, heavy atom substitution on the main chain, and chain helicity on the depolarization behavior of polymers have been studied using atactic and isotactic PMMA; atactic and brominated PS; and the semiflexible polypeptide PBLG, which maintains an extended structure in solution. An introduction to the theory of SEC/D-MALS is given.