The thickness dependence of Matrimid films in water vapor permeation

Composite or asymmetric membranes with thin active layers are used in industrial spiral-wound and hollow fiber modules to maximize productivity. These thin active layers are more susceptible to plasticization and undergo physical aging more rapidly than thick films. In this work, the sorption and transport properties of poly(3,3′-4, 4′-benzophenone tetracarboxylic-dianhydride diaminophenylindane) (Matrimid® 5218) as membranes of thickness 1–58 μm were studied with mixed gas/water vapor feed streams (CH4, CO2 and H2O) at 7.5 bar and 35 °C. The fractional free volume (FFV) of the films decreased significantly with decreasing film thickness for films below 10–15 μm. Water vapor solubility was independent of thickness, but water permeability reduced by an order of magnitude as film thickness decreased. This loss in permeability was related to a decrease in the diffusion coefficient, which was in turn related to the loss of fractional free volume. The net effect of competitive sorption between water vapor and gas components and CO2 induced plasticization effect resulted in a reduction in both carbon dioxide and methane permeabilities and CO2/CH4 selectivities as vapor activity increased.

► The impact of water vapor on CO2 and CH4 permeation through thin Matrimid films is studied. ► Water vapor permeability decreased by an order of magnitude as film thickness decreased. ► The permeability loss was related to accelerated physical aging in the thinner films. ► CO2 and CH4 permeabilities and selectivities all declined with film thickness. ► These results reflect both plasticisation by CO2 and competitive sorption with water.