Comparison of the accuracy of curve-fitting methods for the determination of gas permeability parameters of sheet polymer samples

Accuracy of the gas permeability parameters (GPPs), i.e. solubility, diffusivity and permeability deduced from permeation measurements, is investigated for the case of homogeneous polymer sheet samples. The widely used time-lag method (TLM) and the recently introduced full curve-fitting method (FCFM) are compared on simulated and on measured permeation curves artificially distorted in various ways in order to mimic potential deficiencies of permeation measurements. Accuracy of the methods is defined as the relative deviation of the calculated from the real GPPs, i.e. those which are deduced from the distorted and the original, non-distorted curves, respectively. The following distortions have been applied: temporal truncation of the permeation curves, increasing the noise level of the measurement and shifting the permeation curve either along the concentration or the time axis. (The latter two transformations correspond to an unnoticed background shift in the readings of the concentration detection unit and an uncertainty in the actual inception of the permeation process, respectively). While all these distortions mimic realistic deficiencies of permeation measurements, the last one is relevant only in case of fast permeation processes through highly permeable membranes. For all but the last transformation, FCFM has been found to yield more accurate GPPs than TLM.