Self-Assembled Materials as Novel Nanotechnology-Enabled Ultrafiltration Membranes
Nanotechnology is being used to enhance conventional ceramic and polymeric water treatment membrane materials through various avenues. Among the numerous concepts proposed, the most promising to date include zeolitic and catalytic nanoparticle coated ceramic membranes, hybrid inorganic-organic nanocomposite membranes, and bio-inspired membranes such as hybrid protein-polymer biomimetic membranes, aligned nanotube membranes, and isoporous block copolymer membranes. Zeolitic and catalytic membranes appear reasonably far from commercial reality and offer small to moderate performance enhancements. Overall, bio-inspired membranes are farthest from commercial reality, but offer the most promise for performance enhancements; however, nanocomposite membranes offering significant performance enhancements are already commercially available.
Isoporous block copolymer membranes represent a fully-polymeric analog to the ordered structure associated with bio-inspired materials and are able to be produced through conventional fabrication methods. These membranes represent a possible route towards more precise particle and macromolecular separations, which are of interest across many industries. Herein, membranes with vertically-aligned nanopores are formed from a poly(isoprene-b-styrene-b-4 vinyl pyridine) triblock terpolymer via a hybrid self-assembly/nonsolvent induced phase separation process. Polymer concentration, solvent composition, and evaporation time in the fabrication process were varied to tailor ordering of the selective layer and produce enhanced water permeability. Water permeability was doubled through the optimization process, while maintaining the surface morphology and, thus, the resulting selectivity of the membrane. This was achieved by increasing volatile solvent concentration, thereby decreasing the evaporation period required for self-assembly. Fine-tuning was required, however, since overly-rapid evaporation did not yield the desired pore structure. Transport models, used to relate the in-situ structure to the performance of these materials, revealed narrowing of pores and blocking by the dense region below. It was shown that these vertically aligned nanoporous membranes compare favorably with commercial ultrafiltration membranes formed by conventional phase inversion and track-etching processes, which suggests there is practical value in further developing and optimizing these materials for specific industrial separations.