Using Taylor dispersion profiles to characterize polymer molecular weight distributions

Abstract

Optical interferometry and NMR spectroscopy can be used to characterize polymer molecular weight distributions from diffusion measurements. The present work develops the relatively simple Taylor dispersion technique for this purpose. Small volumes of polymer solution are injected into laminar carrier streams of solvent flowing through a long capillary tube. The broadened distribution of the dispersed polymer sample is monitored at the tube outlet by a liquid-chromatography differential refractometer detector. Moments analysis of the refractive-index profiles generated by the dispersion of dilute nonionic i-mers with diffusion coefficients D_i shows that the profile height and first moment divided by the profile area provide the mass-average values of D_i^1/2 and D_i^−1/2, respectively. This information can be used to estimate molecular weight distributions if the form of the distribution (Poisson, log-normal etc.) is known. Alternatively, a linear combination ∑a_iP(M_i*) of a small basis set of dispersion profiles P(M_i*) for monodisperse solutes of molecular weight M_i* spanning the molecular weight range of the polydisperse sample can be fitted to measured profiles. The number-average and mass-average molecular weights (M_N and M_M) are evaluated using M_N = ∑a_i/(∑a_i/M_i*) and M_M = (∑a_iM_i*)/∑a_i. The proposed methods for studying molecular weight distributions are tested by measuring dispersion profiles for aqueous solutions of mixed glycols with accurately known weight distributions prepared by mixing ethylene glycol and pure ethylene glycol oligomers.