ESTIMATION OF NUMBER-AVERAGE MOLECULAR-WEIGHTS OF COPOLYMERS BY GEL-PERMEATION CHROMATOGRAPHY LIGHT-SCATTERING

The true number-average molecular weight, M(n), of copolymers is obtai ned by GPC coupled with a light-scattering detector even if the compos ition and therefore the refractive index increment varies with elution volume, provided slices taken are monodisperse with respect to molecu lar weight and composition. In contrast, only an apparent weight-avera ge molecular weight, M(w)(app), can be obtained by the conventional GP C-light scattering combination, even for a perfect chromatographic res olution. The errors in M(n) associated with nonhomogeneous slices are estimated. Experimental data with mixtures of linear polystyrene and p oly(methyl methacrylate) confirm these estimations and indicate that t he error in M(n) is less than 10% for the system under investigation.