Thin film assemblies of branched polyethylenimine (PEI) and poly(acrylic acid) (PAA), deposited using the layer-by-layer technique, were studied in an effort to produce all-polymer thin films with low oxygen permeability. Altering the pH of PEI and PAA results in large thickness variations (from 90 nm to 4.74 μm for 30-bilayer films). Cross-linking these films with glutaraldehyde (GA) create an inhibition of polymer interdiffusion, causing exponential film growth to be reset. AFM images show the surface morphology of PEI/PAA assemblies can be controlled by the pH and the final polymer layer deposited, with surface roughness ranging from 2.1 to 49.1 nm. Most pH combinations failed to produce a film with high oxygen barrier, but 8 bilayers of PEI at pH 10 and PAA at pH 4 produce a 305 nm thick film with an oxygen transmission rate below 0.005 cm 3 /(m 2 day). This unique thin film barrier ( P O 2 < 3.2 × 10 −21 cm 3 (STP) cm/(cm 2 s Pa)) is a promising alternative to current polymeric membranes, flexible electronics, and food packaging materials.
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Yang et al. (2011) studied this question.