Elastic response of layer-by-layer self-assembly nanofiltration membranes to hydraulic pressure

Membrane compaction has been often observed in pressure-driven membrane processes, resulting in deformation of membrane morphology and reduction in permeation and separation performance. Current studies on the compaction of nanofiltration (NF) membranes exclusively focused on thin-film composite (TFC) polyamide membranes prepared by interfacial polymerization (IP). The compaction of layer-by-layer (LBL) self-assembled NF membranes under high pressure has not yet been discussed. In this work, we explored the response of a flat-sheet LBL (PSS/PAH)2.5 membrane to hydraulic pressure in terms of the overall separation performance. The polysulfone (PSF) substrate of LBL membranes demonstrated an irreversible compaction similar to commercial acid-resistant KH membranes prepared by IP technology, but the polyelectrolyte (PE) selective layer surprisingly showed full recovery in the pore size and separation performance in the range of 10–40 bars. At 40 bar, the pore size of LBL membranes was enlarged, resulting in decreased rejection of MgCl2. However, in a 15 % phosphoric acid feed, the LBL membranes showed slight changes in the pore size, due to stronger binding of PSS and PAH at low pH. The response of the LBL layer to pressure was related to the substrate pore size, coating layers and the binding strength of the PE pairs. For commercial acid-resistant TFC KH membranes, the decrease in substrate porosity at high pressure caused a significant reduction in the permeance but minor changes in the separation layer; the separation layer of KH membrane remained intact in water but swelled in acid with slightly increased pore size but stable salt rejection. The sharp contrast of LBL and TFC NF membranes highlighted the unique “elastic” response of LBL active layer and provided a new dimension for the design and utility of LBL membranes in different applications.