Compaction of Pressure-Driven Polymer Membranes: Measurements, Theory, and Mechanisms.

Membrane compaction is inherent in pressure-driven membrane processes, resulting in a decrease in porosity and pore size of polymeric membranes as solvent flow compresses the porous structure of the polymer. The compaction of pores reduces solvent permeability and significantly impacts separation performance. Despite the importance of membrane compaction, its fundamental mechanisms have not been well studied. In this study, we combine well-controlled experiments and theory to analyze the relationship between pressure and porosity profiles within membranes. Specifically, we stack six porous films in a customized dead-end cell and measure the solvent content of each film immediately after pressure-driven solvent permeation tests. We show that, when a viscous solvent permeates through the stacked membranes, membrane porosity continuously decreases from the feed side to the permeate side. Compaction results in ∼25% reduction in solvent content (or porosity) at the membrane permeate side under 10 bar applied pressure and up to ∼50% reduction under 40 bar. We further analyze the stress-strain behavior of solvent-swollen films under mechanical compression and compare it to the compacted porosity in the solvent permeation tests. Our analysis reveals that the compression pressure in the permeation tests corresponds to the reduction in hydrostatic pressure within the membrane. In addition, we investigate the compaction behavior when membranes with varying pore sizes are stacked. Our results show that solvent permeating from tight-to-loose films induces significantly greater compaction than flow in the loose-to-tight direction. The compaction in tight-to-loose membrane stacking causes a 27% greater loss in membrane solvent content compared to the reverse arrangement. Notably, this study demonstrates that porosity gradients resulting from compaction cannot be interpreted as concentration gradients that drive diffusive solvent transport. Overall, our findings can inform the design of membranes with improved resistance to compaction by guiding material selection, processing techniques, and structural optimization.