Molecular Weight and Polydispersity of Rubber by Diffusion Measurements

Abstract

THE study of diffusion in solutions of natural rubber (light crepe) by Lamm's¹ method showed that even with a concentration of 0·1 per cent the normalized experimental diffusion curves diverge from the ideal Gaussian curve (Fig. 1), being characterized by a marked asymmetry and an excess of the maximal ordinate. It follows from an analysis of the experimental curves by the method of moments (up to moments of the fourth order) that they belong to Type IV Pearson curves, that is, to asymmetrical distribution curves with asymptotic branches. The determination of the perturbation multiplier enables us to calculate the course of the experimental curves with a fair degree of accuracy. The physical cause of asymmetry of the diffusion curves is the difference in the rate of diffusion to both sides of the interface (of the polymer into the solvent and back) due to a marked intermolecular interaction in the solution of the polymer at a given concentration. With a decrease of the concentration or of the molecular weight of the dissolved substance, the asymmetry of the diffusion curves becomes less pronounced. However, this asymmetry does not preclude the computation of the average diffusion coefficient D from the standard deviation of the curve. It can indeed be shown that the probable error does not exceed 1 per cent. The average value found for natural rubber in carbon tetrachloride is D_20° = 0·71 × 10^â7 cm.²/sec.