Effect of pressure on the swelling and fluxes of dense PDMS membranes in nanofiltration: An experimental study

In Organic Solvent Nanofiltration (OSN), the exact effect of pressure (0.1–5 MPa range) on the membrane, mass transfer and membrane selectivity is still unclear despite the many investigations that have been reported in the last decade. The same questions continue to be raised: Does pressure contribute to both convective and diffusive flux? Does pressure induce polymer compaction? Does pressure induce flux limitations? The present work specifically focused on swelling experiments dedicated to highlighting the effect of pressure on thick polydimethylsiloxane (PDMS) films under conditions that are analogous to those in steady-state nanofiltration. On the other hand, a composite PDMS membrane has been used to determine the OSN fluxes. Thus this study features the particular case of a dense rubbery structure of crosslinked polydimethylsiloxane as the membrane and three solvents with very distinct physical and chemical properties that are able to induce a wide range of swelling, namely, ethanol, toluene and n-heptane. Two specific apparatuses were used to characterize the effect of pressure on the membrane under conditions similar to those upstream or downstream of the NF cell. The results obtained in this study indicate the applicability of the solution-diffusion theory to the modeling of this type of transport behavior. In addition, the results show very good agreement with those from previous studies performed by Paul and Ham, who were the first to use to attempt the role of pressure on swollen polymers and the transport of liquid through dense polymeric membranes. From the recorded OSN flux values, it was shown that a simple linear relationship exists versus either the applied solvent pressure or the specific swelling extent. In this latter case it was possible to calculate the diffusion coefficients (D) of the studied solvents using the solution diffusion approach. It was found that the D values were constant within the whole pressure range and well consistent with the solution-diffusion mechanism.