Nano Focus: Graded index nanoporous Teflon coatings make transparent polymers invisible
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Graded index nanoporous Teflon coatings make transparent polymers invisible
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lastics used for lenses and windows are optically transparent but reflect light at their surface due to the refractive-index mismatch between the plastic (typically 1.5) and air (1). Such reflection is undesirable and requires an antireflective (AR) coating for mitigation. Unfortunately, currently used inorganic methods leading to brittle layers are not suitable on flexible plastics. Similarly, the microstructuring of polymeric surfaces, another approach explored, is not easily scalable or robust. To address this issue, the research group of Chris Giebink from The Pennsylvania State University uses coevaporation of Teflon with smaller molecules to build organic multilayers with a refractive-index gradient, as reported in a recent issue of Nano Letters (doi:10.1021/ acs.nanolett.8b03886). Coevaporation is a thermal method where the polymeric chains are volatilized into smaller fragments and deposited on a substrate where they are repolymerized. It is not conventionally used for most polymeric materials, but it works well for Teflon. Coevaporating Teflon with a smaller molecule called N,N′-bis(3-methylphenyl)N,N′-diphenylbenzidine (NDP) produces a Teflon layer with small inclusions of NDPs. After removal of the NDP with a solvent, nanopores are left behind, and the nanoporous Teflon shows a graded refractive index that decreases from the surface down to 1.07 as the concentration of NPD increases. Coating plexiglass with three layers of nanoporous Teflon with thickness from 35 nm to 112 nm and decreasing refractive indices of 1.41, 1.31, and 1.15, respectively, from the surface down to the plexiglass, the researchers showed that the reflectance in the visible spectrum could be reduced by roughly a factor of 40 in comparison with bare plexiglass (Figure a). Looking straight or with an angle below 60°, the coated plexiglas almost disappears (Figure b). The experimental data agree well with simulation models, such that the design
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can be employed for a a 10 wide range of antireBare (meas.) flection applications (Figure a). However, 1 Coated (meas.) Coated (sim.) according to Giebink, “one of the challenges 0.1 is to accurately control the layer thick0.01 ness when the layers get very porous (i.e., 0.001 very low index). This seems mainly to be 400 450 500 550 600 650 700 related to the pore Wavelength (nm) collapse from the extraction solvent b and can be improved by using a solvent with a lower surface Coated Uncoated tension.” These multilayered Teflon coatings are embodied with many advantages: hydrophobicity; resistance to chemi2 cm cal, mechanical, and environmental harsh conditions; and the (a) Comparison between the reflectivity spectra over the visible process itself is scalspectrum, between uncoated plexiglass (black), and coated plexiglass able and can coat as simulated (red) and as measured (blue). (b) Photograph showing a coated substrate next to an uncoated plexiglass substrate. With the surfaces that are three-layers Teflon-g
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