Weak polyelectrolyte multilayers as tunable membranes for solvent resistant nanofiltration

• We present a first example of SRNF membranes based on weak polyelectrolyte multilayers.
• Within this approach SNRF membrane performance can be tuned by the pH of the multilayer coating solution.
• Membrane performance is studied in detail with the retention of 5 different organic dyes.
• Thin and dense membranes with mostly intrinsic charge compensation provide overall better performance.
• Long term stable membranes for troublesome organic aprotic solvents (THF, DMF and ACN).

This manuscript encompasses the investigation into the solvent resistant nanofiltration (SRNF) performance of multilayer membranes prepared from weak polyelectrolytes. These weak polyelectrolytes are unique in that the charge density is not fixed and depends on the coating pH, adding an extra variable as tuning parameter for SRNF performance. The weak polyelectrolyte based multilayers (PEMs) were prepared on a hydrolyzed PAN support membrane from poly(allylamine hydrochloride) (PAH) as polycation and poly(acrylic acid) (PAA) as polyanion. Detailed investigations on the role of the pH of the coating solution on the performance of the prepared SRNF-membranes were carried out with organic dyes of different size (~300–1000 Da) and charge. Variation in pH of the coating solutions was found to lead to a large degree of control over the separation performance of the prepared SRNF-membranes for the different dyes. The solvent permeabilities and the dye retentions were measured and correlated to variations in the PEM membrane structures, with more dye adsorption being found for membranes with more free acid and amine groups. The membranes were also found to be stable for long term-filtrations in solvents such as isopropyl alcohol (IPA), acetonitrile (ACN), tetrahydrofuran (THF) and in the challenging polar aprotic solvent, N,N-dimethylformamide (DMF). Results of this study clearly demonstrate the potential of using pH as tuning parameter for weak PEMs to prepare SRNF-membranes optimized for specific applications.