From Brush to Barrier: Fine-Tuning Donnan Exclusion in Polyelectrolyte Multilayer Membranes with Polymer Brushes

Polyelectrolyte multilayer (PEM) membranes are widely used in nanofiltration applications. Selective PEM layers are constructed by alternate adsorption of polycations and polyanions to a porous support membrane. A vital aspect of this process is charge overcompensation, which facilitates the adsorption of the next polyelectrolyte layer. However, as a result of charge overcompensation, PEM membranes always have a charged surface, which contributes to Donnan exclusion of ions. High ion retentions can be undesirable in some applications as it complicates concentrate treatment and requires remineralization of the permeate. Therefore, we propose using diblock copolymers (DBCs) as the terminating layer of the PEM coating to diminish Donnan exclusion. DBCs can limit charge overcompensation by preventing further adsorption through steric hindrance. Poly(2-(dimethylamino)ethyl methacrylate)-block-poly(ethylene glycol) (PDMAEMA-b-PEG) DBCs were used as model DBCs. The length of the PDMAEMA block was systematically changed to study the effect of the anchor block length on membrane properties. We demonstrate that DBCs can indeed be used to tune the salt retention in Donnan-dominated poly(diallyldimethylammonium chloride) (PDADMAC)/poly(sodium 4-styrenesulfonate) (PSS) membranes. For DBCs containing an anchor block length of 114 monomers, even a nearly neutral membrane surface could be achieved. Importantly, this tunability of the ion retention can be achieved without diminishing the membrane's permeability or molecular weight cutoff. The DBC coating does not affect the salt retention in dielectrically dominated poly(allyl amine) hydrochloride/PSS membranes. This confirms the hypothesis that DBCs can be used to specifically tune Donnan exclusion in PEM membranes. The DBC approach is particularly promising when designing membranes for applications with a high scaling risk.