Commercial thin film composite membranes for reverse osmosis are prepared via interfacial polymerization of trimesoyl chloride (TMC) and m-phenylenediamine (MPD), to form a ~100 nm thick crosslinked polyamide (PA) active layer atop a porous support. The resulting PA has an extremely rough ridge-and-valley structure and exhibits complicated depth heterogeneities in density and distribution of polar functional groups. As such, it is difficult to interpret the results of membrane performance testing – measurements of solution flux and solute rejection – in such a way that the influence of intrinsic material properties can be cleanly disentangled from the influence of large-scale morphological irregularities arising from the synthesis method. To simplify the geometry of the system, we investigate a series of PA films prepared by molecular layer-by-layer (mLbL) assembly. These exhibit low surface roughness and can be tuned to a well-defined thickness, from &lt;10 nm to &gt;100 nm, by controlling the number of monomer deposition cycles. The films can then be transferred to a macroporous polyacrylonitrile support layer without introducing defects, which are then amenable to common performance testing techniques. The chemistry of the PA films is varied to interrogate the effects of network crosslink density and membrane hydrophilicity. In commercial TMC/MPD membranes, crosslink density and carboxylic acid content are coupled; reducing the extent of TMC reaction by controlling processing conditions simultaneously lowers the concentration of junction units and raises the concentration of residual acid groups. We extend the TMC/MPD chemistry to include formulations containing isophthaloyl chloride (IPC) as a di-functional acid chloride and diaminobenzoic acid (DBA) as an acid-bearing diamine. Substitution of the pure tri-functional acid chloride TMC for a mixture of di- and tri-functional IPC and TMC lowers the crosslink density without introducing additional carboxylic acid. Conversely, replacement of MPD with DBA raises the carboxylic acid content without altering the crosslink density. In each case, membrane permeability is described via a multi-layer solution-diffusion model. This model captures the observed structure of mLbL PA membranes, which tends to consist of a thin layer of lightly-crosslinked material atop a dense underside, and enables the extraction of material parameters of interest as a function of membrane chemistry.

Tailoring the performance of molecular layer-by-layer assembled reverse osmosis membranes via polyamide chemistry

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.