Credit: © 2006 ACS

Polymers with continuous sections made from different materials (block copolymers) can self-assemble into a range of structures. This occurs through a phase-separation process in which the like sections from each chain aggregate with one another to form different domains. Researchers at Pohang University in South Korea have used this approach to make nanoporous films with impressive anti-reflection properties that may be useful for optical applications.

Jin Kon Kim and colleagues1 made a block copolymer in which a polystyrene (PS) chain is attached to the end of a poly(methyl methacrylate) (PMMA) one. After a thin film (100–200 nm thick) of this material was spin-coated on a glass surface, it was exposed to ultraviolet light. This treatment cross-links the PS domains and makes them stronger, whereas it breaks apart the PMMA chains. The PMMA fragments are washed away with acetic acid to leave a nanoporous film. The resulting pore size (10–50 nm) is much smaller than the wavelength of visible light and so light is not scattered by them. The pore volume is one of the most important factors for the reflectance properties of such films and this can be simply controlled by changing the ratio of PMMA to PS in the copolymer. A film made from a polymer with the optimum PMMA:PS ratio of 0.69 had a reflectance of less than 0.1%.

The work by Kim and co-workers allows better control of pore size and film thickness than previous approaches, and does not require high-temperature annealing of the polymer films.