Super oxygen barrier of polymer - Clay nano brick wall thin films
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Thin films of sodium montmorillonite (MMT) clay, poly(acrylic acid) (PAA), and branched polyethylenimine (PEI) were deposited on various substrates using layer-by-layer assembly in an effort to reduce oxygen transmission. After 70 polymer-clay layers have been deposited, the resulting transparent film exhibits an oxygen transmission rate below 0.005 cm 3 m -2 day-1 when the pH of the PEI solution is 10. This low permeability is believed to be due to a nano brick wall microstructure comprised of completely separated clay bricks in polymeric mortar. This brick wall creates an extremely tortuous path at thicknesses below 200 nm and clay concentration of approximately 80 wt%. Lowering PEI pH results in thinner films with increased oxygen permeability and higher modulus due to greater polymer charge density. When multiplying OTR by film thickness to achieve oxygen permeability, we find that these films are a better barrier than SiOx. Similar oxygen barrier can be achieved with just 10 clay layers in a quad-layer (QL) assembly made by building up PEI-PAA-PEI-MMT layers. With optical transparency greater than 86% and the ability to be microwaved, these thin film composites are good candidates for flexible electronics packaging and foil replacement for food.
