Nanoporous polymer films demonstrate high optical transmission
A low refractive index is essential if antireflection coatings are to limit light reflection off glass surfaces and increase the quality of optical lens systems. Obtaining this low refractive index has usually required a sequence of coating layers creating destructive interference, which maximizes light transmission. Recently, researchers at the Universität Konstanp (Konstanz, Germany) developed a single homogeneous layer comprising a nanoporous polymer film that achieves this same goal. The research team led by Jürgen Mlynek demonstrated the concept by spin-casting a binary polymer mix of polystyrene and polymethylmethacrylate (PPMA) onto glass, then exposing the mix to the solvent cyclohexane, which selectively dissolved polystyrene and left a porous film of PMMA behind. Atomic force microscopy verified the absence of lateral structures larger than 100 nm and revealed an irregular surface rather than individual pores.
An index of refraction of 1.255 [PC1]was obtained, along with optical transmission through the glass as high as 99.3%, which is comparable to or better than the industrial standard--magnesium fluoride. The researchers say the major advantages of their approach are the ability to fine-tune the refractive index of the film and, with multiple layers, further reduce reflected light. More wear-resistant polymers could be used or organic-inorganic hybrid materials could produce mineralized coatings with low refractive indices. See www.uni-konstanz.de/ FuF/Physik/Mlynek/Steiner/.