Characterization of Molecular Weight Distributions by a Viscometric Method

Abstract

AN indication of the dispersion of the molecular weight distribution in a given polymer sample may be obtained from values of ratios between the different types of average molecular weight as determined by different experimental techniques1. For example, the ratio of weight-average to number-average molecular weights (M w /M n) is frequently used for this purpose, M w being obtained from light-scattering and M n from osmotic measurements. The utility of such methods is somewhat restricted because of the uncertainties associated with determinations of the individual molecular weight averages in certain regions of the molecular weight range. Osmometry, in particular, is less accurate for both low and very high molecular weights. Since the percentage error in the derived ratio is approximately the sum of percentage errors of the individual determinations, it is evident that quoted values of M w /M n in the region of 1.05 (typical of a sharp polymer fraction) must be accepted with reserve, because such a figure implies a greater accuracy in the individual methods than their current state of development would appear to warrant. If similar ratios could be obtained by the use of a single technique part of the uncertainty as to the magnitude of possible error would be eliminated. It is the purpose of this communication to propose a method for characterizing the dispersion of a molecular weight distribution based on the measurement of limiting viscosity numbers alone.