Processing and Gas Barrier Behavior of Multilayer Thin Nanocomposite Films

Thin films with the ability to impart oxygen and other types of gas barrier are crucial to commercial packaging applications. Commodity polymers, such as polyethylene (PE), polycarbonate (PC) and polyethylene terephthalate (PET), have insufficient barrier for goods requiring long shelf life. Current gas barrier technologies like plasma-enhanced vapor deposition (PECVD) often create high barrier metal oxide films, which are prone to cracking when flexed. Bulk composites composed of polymer and impermeable nanoparticles show improved barrier, but particle aggregation limits their practical utility for applications requiring high barrier and transparency. Layer-by-layer (LbL) assemblies allow polymers and nanoparticles to be mixed with high particle loadings, creating super gas barrier thin films on substrates normally exhibiting high gas permeability.

Branched polyethylenimine (PEI) and poly (acrylic acid) (PAA) were deposited using LbL to create gas barrier films with varying pH combinations. Film thickness and mass fraction of each component was controlled by their combined charge. With lower charge density (PEI at pH 10 and PAA at pH 4), PEI/PAA assemblies exhibit the best oxygen barrier relative to other pH combinations. An 8 BL PEI/PAA film, with a thickness of 451 nm, has an oxygen permeability lower than 4.8 x 10^-21 cm^3 * cm/cm^2 * s * Pa, which is comparable to a 100 nm SiOx nanocoating. Crosslinking these films with glutaraldehyde (GA), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC) or heating forms covalent bonds between PEI and/or PAA. Oxygen transmission rates (OTR) of 8 BL films crosslinked with 0.1M GA or 0.01M EDC show the best oxygen barrier at 100% RH.

Graphene oxide (GO) sheets and PEI were deposited via LbL with varying GO concentration. The resulting thin films have an average bilayer thickness from 4.3 to 5.0 nm and a GO mass fraction from 88 to 91wt%. Transmission electron microscopy and atomic force microscopy images reveal a highly-oriented nanobrick wall structure. A 10 BL PEI/GO film that is 91 nm thick, made with a 0.2 wt% GO suspension, exhibits an oxygen permeability of 2.5 x 10^-20 cm^3 * cm/cm^2 * s * Pa.

Finally, the influence of deposition time on thin film assembly was examined by depositing montmorillonite (MMT) or laponite (LAP) clays paired with PEI. Film growth and microstructure suggests that smaller aspect ratio LAP clay is more dip-time dependent than MMT and larger aspect ratio MMT has better oxygen barrier. A 30 BL PEI/MMT film made with 10 second dips in PEI has the same undetectable OTR as a film with 5 minute dips (with dips in MMT held at 5 minutes in both cases), indicating LbL gas barrier can be made more quickly than initially thought. These high barrier recipes, with simple and efficient processing conditions, are good candidates for a variety of packaging applications.