Thin films of sodium montmorillonite clay and branched polyethylenimine (PEI) are deposited on various substrates using layer-by-layer assembly. Films with 40 polymer-clay layers contain more than 84 wt% clay, have hardness as high as 1 GPa, and are completely transparent. Oxygen transmission rates (OTR) through these films decrease as the pH of PEI increases. These pH-tailorable properties are the result of changing PEI charge density, which causes the polymer to deposit more thickly at high pH because of low charge density. After 70 PEI (at pH 10)-clay layers are deposited onto 179 poly(ethylene terephthalate) film, the resulting 231 nm assembly has an OTR below the detection limit of commercial instrumentation (<0.005 cc/(m 2 day atm)). When multiplied by thickness, the resulting oxygen permeability is found to be less than 0.002 10 -6 cc/(m day atm)), which is lower than values typically reported for SiOx. This is the lowest permeability ever reported for a polymer-clay composite and is believed to be due to a brick wall nanostructure created by the alternate adsorption of polymeric mortar and highly oriented, exfoliated clay platelets. Because of their high level of transparency and gas barrier, these films are good candidates for a variety of flexible electronics, food, and pharmaceutical packaging. 2010 American Chemical Society.
