Solvent-induced osmotic stresses can produce a regular series of small explosions (collective osmotic shock or COS) within a material that create nanoporosity, report researchers from Cambridge University, Cambridge, UK. Such nanoporous materials may improve the performance of industrial nano- and ultrafiltration, and suit a variety of other applications, notes Easan Sivaniah of the university's Cavendish Laboratory and lead author of a recent paper on the technology in Nature Materials.
"In terms of the filtration market, what our technology essentially can do is transform a cheap, low-efficiency membrane into a high-performance nanoporous membrane via a few controllable steps," he explains, citing potential applications for water purification as well as gas and organic separation, especially in the pharmaceuticals area. "There has been interest in the use of these nanoporous materials for catalysts and applications in electrode-containing devices (fuel cells, photovoltaics, batteries)." The structures created feature stratified multilayers that open up possibilities in photonics, sensors, and as shock-absorbing materials, he adds.
"If we can attract the right sort of commercial interest, then we can move into pilot-scale filtration applications fairly quickly (1–2 years). The technology has the advantage of being scalable, and so can be 'plugged' into existing membrane preparation lines."
Basically, the approach involves using a solvent to swell trapped minor components in a structure. The built-up osmotic pressure causes COS. "In our experiments, we essentially show how this works in materials with these trapped minor components, leading to a series of bursts that connect together and to the outside, releasing the trapped components and leaving an open porous material."
The researchers demonstrated the technique using thin films of a commercially available spherical block copolymer of polymethyl methacrylate (PMMA) and polystyrene (PS). Immersion in acetic acid (a solvent for PMMA oligomers) initiates COS and results in a perforated multilayer structure (Figure 1).
A key challenge at this point is developing a predictive model for the structure-formation process, says Sivaniah. Starting with materials with controlled dimensions leads to very controllable nanoporosity. There's no particular limit in the thickness of material that can undergo COS, he adds.