Nanoporous Membranes

Materials engineered with well-defined porosity and surface properties are essential as they find use in applications such as separation catalysis, water purification, and fuel cell technology. Block copolymer mesophases, such as hexagonally packed cylinders and bicontinuous structures, provide selective transport channels of defined size. Diblock copolymers which comprise a nondegradable block (matrix) such as polystyrene (PS) and a chemically-etchable block such as poly(lactic acid) (PLA) can be used to generate nanoporous materials upon removal of the sacrificial component. Upon chemical etching, provided that the PLA assemble into cylinders within a continuous PS phase, the final structures will have high porosity with the potential to align the cylinders/pores using an external field. Furthermore, the nanochannels formed by the PLA can be also be functionalized with specific groups either by mixing the PS-PLA copolymers with a fully inert copolymer such as PS-PEO or even more efficiently by forming a ABC copolymer where the C block is the sacrificial component. By doing so, the surface chemistry of the channel can be effectively controlled by the B block.

A thin film membrane made of a poly(vinylidene fluoride) (PVDF) backbone and poly(oxyethylene methacrylate) (POEM) side chains. These copolymers assemble into bicontinuous phases that can selectively separate small molecules. However, effective separation demands materials with few imperfections in the assembly, and often, time-consuming steps are required in order to achieve sufficient long-range order. Recently, the combination of industrially well-established membrane formation methods with the self-assembly of a block copolymer has led to the development of well-organized asymmetric membranes exploiting the natural de-wetting of copolymers at the air-liquid interface^.

More examples of block copolymer membranes are finding applications in fuel technology^. Fuel cells require the use of membranes with high and low permeability to protons and water, respectively. The potential ability to modify block copolymer membranes almost infinitely to give the desired characteristics makes them especially suitable for this use.