Condensation of paper presented at Gibson Island Conference on Organic High Molecular Weight Compounds, June 8, 194o. (Continued from page 4o2.)
Indirect Methods for Determining the Molecular Weights of Macromolecules.--The various indirect methods for determining macromolecular weights all require calibration by means of one of the absolute methods already discussed, using the specific materials to which the indirect method is to be applied. By "specific materials" we mean macromolecules not only of the same chemical composition, but also prepared in the same manner. As will appear below, disregard of this fact could lead to errors of several hundred per cent. in the determination of relative molecular weights of polystyrenes by the viscosity method, and the same situation doubtless exists for other synthetic polymers.
I. The most important indirect method for macromolecular weights is Staudinger's viscosity method, which is applicable to the members of any polymer-homologous series of elongated or non-spherical molecules when the molecular shape is well-defined and "analogous" throughout the series. For linear and some branched molecules containing substantially only one kind of polymer-unit, Staudinger's viscosity rule may be expressed by the equation P = K([-7] --a) where P is the degree of polymerization, ET] is the intrinsic viscosity, and K and a are constants specific for the particular polymer-homologous series. (K = I/IOKm, where K,n is Staudinger's Viscosity constant). According to recent work from Staudinger's laboratory, the relationship betewen P and [7] is not quite linear for some branched chain molecules, as the polymeric vinyl derivatives. In any event, for large values of [7], the effect of a becomes negligible, and, in fact, this constant is usually ignored. K not only varies with macromolecules of different chemical composition, but may