On the determination of the average molecular weight, radius of gyration, and mass/length ratio of polydisperse solutions of polymerizing rod-like macromolecular monomers by multi-angle static light scattering

The angular dependence of the light scattered from polydisperse solutions of rod-like or worm-like linear polymers formed by the polymerization of rod-like macromolecular monomers was studied with the aid of computer simulations. Except at very low conversion degrees, these ensembles are characterized by curved Zimm-like plots. An appraisal of the use of polynomial fittings for the derivation of the weight-average molecular weight (MW) and of the z-average square radius of gyration (〈Rg2〉z) from such plots is presented and discussed. Depending on the average size and shape distribution, the use of polynomial fittings allows the applicability of the Zimm method well beyond the standard condition q2〈Rg2〉z ≪ 1, q being the scattering wavevector. In addition, the derivation of the w/z-average mass/length ratio ML of the polymers from only partially linear Casassa plots, from which large errors in the derived ML values can be made, is analyzed. By combining the Casassa method with the complementary Holtzer plots, it is usually possible to assess the reliability of the results and give an estimate of their accuracy. However, it was also found that apparently linear Casassa plots, allowing a good estimate of the ML value, may arise in particular situations even when the Holtzer plot would indicate otherwise.