Molecular Layer-by-Layer Assembly of Model Membranes for RO and NF Applications

Reverse osmosis (RO) and nanofiltration (NF) membranes are comprised of a dense active layer bonded to a porous support.  The active layer in commercial membranes is a highly crosslinked polyamide that is formed through interfacial polymerization directly from the support structure.  Although effective, the rapid polymerization rate and reaction conditions produce films with rough surface structures and chemical heterogeneity.  Therefore, quantification of membrane structure-property relationships is difficult, since changes to synthesis conditions alter multiple properties and film roughness prevents depth profiling analysis techniques.  This need was identified by a breakout group from the 2011 World Material Summit focused on materials research opportunities in water, who concluded that “there is no quantitative understanding of the connections between the molecular structure of the membrane and water permeability.”¹  A similar position was expressed by Geise et al., who stated that “better ways are needed to control and analyze their (membranes) structure and to assess its impact on transport of water, solutes, and particulate matter.”²

We are developing quantitative measurement methods to correlate membrane structure and dynamics to performance (flux and selectivity), in response to this measurement gap.  An approach for synthesizing model crosslinked polyamide membranes based on molecular layer-by-layer (mLbL) deposition, which enables precise control over network structure, surface chemistry, residual charge, roughness, and film thickness as well as placement of specific functional groups at defined locations within the network, is developed to produce clear structure-property links.  These model membranes are then characterized via analytical techniques such as x-ray reflectivity, neutron scattering, x-ray photoelectron spectroscopy and atomic force microscopy to probe the surface and internal structure, which is then correlated to membrane performance.  This presentation will highlight our progress to-date in both the fabrication of the model membrane materials as well as the advanced measurements of their properties.

(1)     Society News, MRS Bulletin 2012, 37, 163.

(2)     Geise, G.M.; Lee, H.-S.; Miller, D.J.; Freeman, B.D.; McGrath, J.E.; Paul, D.R. J. Polym. Sci. Polym. Phys. 2010, 48, 1685.