Organic vapour transport in glassy perfluoropolymer membranes: A simple semi-quantitative approach to analyze clustering phenomena by time lag measurements

In this paper the gas and vapour transport through four different high fractional free volume amorphous glassy perfluoropolymers is studied, namely Teflon (R) AF and Hyflon (R) AD. The paper describes a new approach to evaluate the anomalous diffusion of alcohol vapours in these materials. Permanent gases and the vapours of aprotic solvents like acetone and dichloromethane exhibit normal behaviour. For these species the transient in the permeation curve can be described by the classical penetration theory with a single diffusion coefficient. In contrast, aliphatic alcohols exhibit an unusually long transient. Careful analysis of the curve shape by stepwise fitting with a nonlinear least squares optimization procedure demonstrates that such behaviour can be described in terms of simultaneous diffusion of species with different diffusion coefficients. This can be attributed to the co-existence of single penetrant molecules and larger clusters, the latter diffusing more than one order of magnitude slower than the single molecules. The proposed semi-quantitative method offers the possibility to estimate the average cluster size. The cluster size of methanol and ethanol in Teflon AF2400 is very close to that reported in the literature. The average cluster size is relatively independent of the polymer type and free volume but it decreases with increasing size of the alcohol. A critical analysis of the potential and limitations of the method is made. Application of the ENSIC model on independent sorption data gave excellent agreement with the experimental data and confirmed the clustering. A preliminary discussion on the possible evaluation of the transport properties in terms of concentration dependent diffusion is also provided. (C) 2010 Elsevier B.V.. All rights reserved.