Nanopores Created by Osmosis
Pore structures that could be used as filters, for instance, can be created in block copolymers through the action of UV radiation and solvents. This is the result of investigations into the phenomenon of "collective osmotic shock" conducted at the Cavendish Laboratory of Cambridge University in the United Kingdom.
The experiments used film containing spherical regions of PMMA surrounded by a PS matrix. The film was irradiated with UV light, which caused the PS to cross-link and the PMMA to break down into low-molecular-weight oligomers. Then, the researchers immersed the film in acetic acid, a good solvent for oligomeric PMMA. Since the PS matrix acts as a semi-permeable membrane, the solvent penetrates into the PMMA oligomers, generating the so-called "collective osmotic shock" - in this case, this means that cracks and fractures suddenly appear in the PS matrix as the PMMA swells in the solvent and the PS matrix expands. Perforated layers with pores in the nanometer range are left behind after removal of the PMMA.
To trigger the collective osmotic shock, the solvent used must meet three conditions: it must dissolve the oligomeric material selectively, and it must not act as a solvent for the matrix material. The third condition is that it must penetrate the microscopic barriers located between the individual layers. The researchers report that it is also possible to create layered nanopore structures in block copolymers of PS and polybutadiene. Separating small particles from liquids such as drinking water could be a future application. Use of a surface layer of porous copolymers might, in this way, considerably increase the effectiveness of membranes for ultrafiltration.