Estimation of gas transport coefficients from differential permeation, integral permeation, and sorption rate data

Experimental methods for studying the transport of gases in polymers may be divided into three categories: integral permeation rate measurement, in which the cumulative amount of a penetrant that has passed through a membrane is determined; differential permeation rate measurement, in which the rate of penetration through a membrane is measured directly; and sorption rate measurement, or determination of the cumulative amount of a penetrant absorbed in a polymer sample. This paper reviews commonly used techniques for estimating diffusion coefficients from transport data of all three types. Several new estimation formulas are presented, and the relative merits of different measurement and estimation methods are discussed. A general relationship between the traditional time lag method for integral rate data analysis and a recently developed moment method for differential rate data analysis is established, extending the applicability of the moment approach to the analysis of non-ideal transport in membranes of arbitrary geometry and composition.