Improving oxygen barrier and reducing moisture sensitivity of weak polyelectrolyte multilayer thin films with crosslinking
Thin films of polyethylenimine (PEI) and poly(acrylic acid) (PAA), deposited using layer-by-layer assembly, were studied in an effort to understand the influence of various crosslinking methods on their oxygen and water vapor barrier. Glutaraldehyde (GA), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC) and thermal crosslinking were evaluated with respect to film thickness and barrier properties. AFM images show the surface morphology of PEI/PAA assemblies can be significantly altered by the method of crosslinking, with surface roughness ranging from 55 to 118 nm. The thickness of an 8 bilayer PEI/PAA assembly decreased from 640 to 551 nm after crosslinking with a 0.1M aqueous GA solution. This same crosslinking treatment reduced the oxygen transmission rate (OTR), measured at 23°C and 100% RH, from 0.61 to 0.09 (cc/m2 · day). Although not as impressive for water barrier, increasing the number of bilayers and heating time with thermal crosslinking reduces the water vapor transmission rate (WVTR) by as much as 46%. The impressive gas barrier of these nanocoatings make them a promising alternative to currently used materials for flexible electronics and food packaging.