The molecular weight of polyacrylonitrile

Abstract

Abstract There is a surprising lack of agreement between the values which have been reported for both the number‐average and weight‐average molecular weights of polyacrylonitrile. We have undertaken an investigation of both whole polymers and polymer fractions to discover and eliminate the sources of systematic error which would lead to these differences, and thereby to develop procedures yielding reliable molecular weight values for this polymer. Our experience indicates that the procedure which must be followed to obtain reliable osmotic data depends upon the conditions under which the polymer was prepared, whether the sample is a whole polymer or a fraction, and the purity of the solvent. Designating solution polymers by S and those prepared in an aqueous system with a persulfate‐bisulfite initiator by A, we conclude that reliable M̄ n values can be obtained for S polymers by using dimethylformaide (DMF) which has been purified by deionization or distillation, for fractions of A polymers by using deionized DMF, and for whole A polymers by performing the measurements upon DMF solutions which have been deionized by passage through a column of Amberlite MB‐3 resin. Fictitiously low M̄ n values are observed for A polymers dissolved in distilled DMF. The value of the secon virial coefficient is very ensitive to the presence of traces of water. Turning to weight‐average molecular weights, a 60% discrepancy exists between the values obtained from light scattering by Stockmayer and Casassa and by Bisschops from sedimentationdiffusion. We believe the correct M̄ w values lie midway between these. A part of the 60% difference is due to the omission of a correction to the sedimentation data for the effect of hydrostatic pressure. This correction was evaluated from sedimentation data by the procedure of Oth and Desreux and increases the molecular weight values deduced from sedimentation by 30%. The remainder of the difference appears to be due to the effects of depolarization and fluorescence upon the light scattering data. Thus, our data for two fractions which exhibited a limiting depolarization ρ u = 0.09 yielded high M̄ w values standing in agreement with the relation of Stockmayer and Casassa, while the result obtained for a whole polymer having a negligible limiting depolarization agrees with those obtained from the corrected sedimentation data. The latter polymer also exhibited a more normal scattering behavior. A comparison of the weight and number‐average values indicates that the M̄ w /M̄ n ratio is 2.1 ± 0.1 for the whole S polymers and 2.8 ± 0.3 for the whole A polymers. The broader distribution in the latter case suggests that termination occurs by both coupling and disproportionation in the aqueous polymerization.