From Brush to Barrier:\nFine-Tuning Donnan Exclusion\nin Polyelectrolyte Multilayer Membranes with Polymer Brushes

Polyelectrolyte multilayer (PEM) membranes are widely\nused in nanofiltration\napplications. Selective PEM layers are constructed by alternate adsorption\nof polycations and polyanions to a porous support membrane. A vital\naspect of this process is charge overcompensation, which facilitates\nthe adsorption of the next polyelectrolyte layer. However, as a result\nof charge overcompensation, PEM membranes always have a charged surface,\nwhich contributes to Donnan exclusion of ions. High ion retentions\ncan be undesirable in some applications as it complicates concentrate\ntreatment and requires remineralization of the permeate. Therefore,\nwe propose using diblock copolymers (DBCs) as the terminating layer\nof the PEM coating to diminish Donnan exclusion. DBCs can limit charge\novercompensation by preventing further adsorption through steric hindrance.\nPoly(2-(dimethylamino)ethyl methacrylate)-<i>block</i>-poly(ethylene\nglycol) (PDMAEMA-<i>b</i>-PEG) DBCs were used as model DBCs.\nThe length of the PDMAEMA block was systematically changed to study\nthe effect of the anchor block length on membrane properties. We demonstrate\nthat DBCs can indeed be used to tune the salt retention in Donnan-dominated\npoly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium 4-styrenesulfonate)\n(PSS) membranes. For DBCs containing an anchor block length of 114\nmonomers, even a nearly neutral membrane surface could be achieved.\nImportantly, this tunability of the ion retention can be achieved\nwithout diminishing the membrane’s permeability or molecular\nweight cutoff. The DBC coating does not affect the salt retention\nin dielectrically dominated poly(allyl amine) hydrochloride/PSS membranes.\nThis confirms the hypothesis that DBCs can be used to specifically\ntune Donnan exclusion in PEM membranes. The DBC approach is particularly\npromising when designing membranes for applications with a high scaling\nrisk.