Molecular Layer-by-Layer Deposition of Highly Crosslinked Polyamide Membranes for Reverse Osmosis Applications
Peter M. Johnson, Joonsung (. Yoon, Jennifer Y. Kelly, John A. Howarter, Christopher M. Stafford
In membranes for reverse osmosis and nanofiltration, the discriminating layers consist of an interfacially polymerized polyamide film on a porous support. For reverse osmosis membranes, the dominant commercial interfacial film is a crosslinked network created from a reaction of trimesoyl chloride (TMC) and m-phenylene diamine (MPD). The reaction rapidly occurs at an organic-water interface, but poor reaction control results in a rough surface structure with chemical heterogeneity. The polyamide layer is exposed to feed water during filtration, so scaling, biofilm formation, and/or chlorine exposure will degrade pristine surfaces, reducing selectivity and overall water flux. Changes in synthesis routes to produce improved membranes alter surface roughness, convoluting multiple parameters and making it difficult to determine structure-property relationships. A large number of analysis techniques also require smooth surfaces for ideal depth profiling. An ideal film to quantify membrane structure-property relationships would have a smooth surface and a controllable chemical structure. In this work, we demonstrate a solvent-based molecular layer-by-layer deposition (mLbL) technique to produce a polyamide film with reduced surface roughness and homogeneous network structure. Sequential reaction processes, like mLbL, have been performed to create conformal inorganic or organic layers on a variety of substrates. The mLbL technique builds a crosslinked polyamide network from successive exposures to TMC and MPD, preventing uncontrolled polymerization by limiting reaction sites to surface bound moieties. Films could be grown on any substrate that presents a high density of chemical groups reactive to the acyl chloride functionality in TMC. Grown films exhibited an order of magnitude reduction in the surface roughness while keeping the high crosslink density associated with commercial films.