Crosslinking PDADMAC/PSS polyelectrolyte multilayer membranes for stability at high salinity

Polyelectrolyte multilayer (PEM) nanofiltration (NF) membranes based on PDADMAC (poly(diallyldimethylammoniumchloride)) and PSS (poly(sodium 4-styrenesulfonate)) are known for their high physical and chemical stability. However, under high salinity conditions, the stability of these membranes is compromised due to weakened electrostatic interactions, leading to increased permeability and decreased retention. This study addresses this challenge by crosslinking PDADMAC/PSS multilayers with the photosensitive, negatively charged crosslinker DAS (disodium 4,4’-diazidostilbene-2,2’-disulfonate tetrahydrate). Initially, this crosslinking is studied on model surfaces, demonstrating full stability against desorption by surfactants at high enough DAS concentrations (1 g·L^-1) and at long enough UV exposure (10 minutes). Experiments on PEM membranes demonstrate that DAS crosslinking significantly enhanced the stability of PDADMAC/PSS membranes at high salinity, with no permeability increase or loss of selectivity observed up to 1.5 M NaCl, in contrast to non-crosslinked membranes showing a reversible 61% permeability increase and an irreversible loss in MgSO₄ retention of 15%. At 4 M NaCl, the permeability of non-crosslinked membranes increased by 300% versus 90% for crosslinked membranes, again indicating the improved stability of the latter. Crosslinking with DAS further allows tuning of the membrane properties, denser membranes are formed with a lower molecular weight cut-off (MWCO), from around 861 Da of non-crosslinked membranes to around 354 Da of membranes crosslinked with a low DAS concentration (1 g·L^-1). DAS introduces negative charges (sulfonic acid groups) into the PEMs, changing the membrane charge from positive to highly negative, as evidenced by the high Na₂SO₄ retention (∼95%) and low CaCl₂ retention (∼7%) of crosslinked membranes. This study demonstrates the potential of crosslinking with DAS to produce stable PDADMAC/PSS NF membranes with tunable selectivity for challenging separation processes in high-salinity environments.