Tunable-Porosity Membranes For Water Treatment By
Discrete Nanoparticle Assembly
Patrizia Marchetti 1, Martin Mechelhoff 2, Andrew G. Livingston 1, *
1Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
2Lanxess Deutschland GmbH ,Group Function Innovation & Technology, 51369 Leverkusen, Germany
Thin film composite (TFC) membranes are an important class of membranes, mainly used for reverse osmosis and nanofiltration (NF) . They consist of a thin (submicron) separating “barrier” on top of a chemically different porous support. For aqueous applications, they are usually prepared by interfacial polymerization of polyamide on a support-layer consisting of, for example, polysulfone. The two main challenges with this procedure are the need for organic solvents, such as n-hexane, and the difficulty in a precise control of the pore size at nanometer scale. Colloidal assembly of polymeric nanoparticles represents an interesting alternative to conventional fabrication procedures for TFC membranes . This approach permits the organization of nanomaterials, acting as pre-structured building blocks, to create larger entities with new properties. The production process is also more environmentally friendly, as crosslinked organic nanoparticles can be easily handled in an aqueous solution without the need for an organic solvent.
In this work, a novel NF membrane was prepared by a facile single-step coating procedure of highly crosslinked poly(styrene-co-butadiene) polymer nanoparticles on top of a hydrophilic ultrafiltration support. The nanoparticles employed were characterised by high chemical and mechanical stability in extreme conditions, such as concentrated acid and base solutions and a wide range of organic solvents. During membrane fabrication, the nanoparticle surface loading was optimized, in order to avoid cracking during the drying step and obtain a thin, defect-free membrane separation layer of 144 nm (Figure 1).
Figure 1. Thin Film Nanocomposite membrane by nanoparticle assembly on top of polyacrylonitrile ultrafiltration support
The membrane exhibited a significant charge rejection mechanism and low fouling tendency, due to the high density of hydroxyl group functionalization on the nanoparticle surface. Interestingly, upon heating above the glass transition temperature (or minimum film formation temperature) the particles coalesced, forming a rubber-like dense film. This was a consequence of deformation and interpenetration of the nanoparticles, which in turn comprise a highly crosslinked particle core and a more flexible and less dense outer shell. The novel membrane formed by nanoparticle annealing showed a molecular weight cut-off below 500 g mol-1 and a viable water permeance of about 40 L m-2 h-1 bar-1, unlike membranes formed from conventional linear rubber-like polymers, which usually have insignificant water permeance of less than 0.4 L m-2 h-1 bar-1. The permeation pathways were found to be proportional to, and therefore tunable with, the annealing temperature.
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