Accelerated plasticization of thin-film composite membranes used in gas separation

Permeation experiments with He, N-2, O-2 and CO2, have been carried out wit h double layer composite membranes consisting of a silicone rubber support layer and a thin polyimide layer determining the permeation properties, The estimated thickness of the polyimide layer in the composite membranes was between 1.5 and 4 microns. This paper describes the phenomenon of accelerat ed plasticization of such thin polyimide layers used in gas separation memb ranes. The pressure-normalized fluxes were determined at 5 bars for N-2, O- 2 and in the range of 1-8 bars in the case of He and CO2. Helium permeation decreased with increasing feed pressure and no hysteresis behavior was fou nd for successive increasing and decreasing feed pressure steps. For CO2 th e pressure-normalized flux did not follow the typical behavior of glassy po lymers but increased continuously with increasing feed pressure. Also, CO2 permeation showed a clear hysteresis effects resulting in an increasing per meability with time and an elevated magnitude in successive decreasing feed pressure steps. Hence. stronger plasticization effects in the composite me mbranes must be concluded in comparison with thick single film membranes. T he plasticization effects are significantly pronounced as the polyimide con centration in the solution from which the membranes are cast become more di lute. The latter is interpreted as accelerated plasticization with decreasi ng film thickness. (C) 2001 Elsevier Science B.V. All rights reserved.