title: Controlling effective aspect ratio and packing of clay with pH for improved gas barrier in nanobrick wall thin films.
abstract: Polymer-clay thin films constructed via layer-by-layer (LbL) assembly, with a nanobrick wall structure (i.e., clay nanoplatelets as bricks surrounded by a polyelectrolyte mortar), are known to exhibit a high oxygen barrier. Further barrier improvement can be achieved by lowering the pH of the clay suspension in the polyethylenimine (PEI) and montmorillonite (MMT) system. In this case, the charge of the deposited PEI layer is increased in the clay suspension environment, which causes more clay to be deposited. At pH 4, MMT platelets deposit with near perfect ordering, observed with transmission electron microscopy, enabling a 5× improvement in the gas barrier for a 10 PEI/MMT bilayer thin film (85 nm) relative to the same film made with pH 10 MMT. This improved gas barrier approaches that achieved with much higher aspect ratio vermiculite clay. In essence, lower pH is generating a higher effective aspect ratio for MMT due to greater induced surface charge in the PEI layers, which causes heavier clay deposition. These flexible, transparent nanocoatings have a wide range of possible applications, from food and electronics packaging to pressurized bladders.
authors.affiliation: Department of Mechanical Engineering, Texas A&M University , College Station, Texas 77843, United States.
authors.forename: David A
authors.initials: DA
authors.lastname: Hagen
authors.forename: Lauren
authors.initials: L
authors.lastname: Saucier
authors.forename: Jaime C
authors.initials: JC
authors.lastname: Grunlan
mesh_terms: D000538:Aluminum Silicates / Q000737:chemistry*; D000077215:Clay; D003460:Crystallization / Q000379:methods; D004058:Diffusion; D005740:Gases / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008422:Materials Testing; D008567:Membranes, Artificial; D053758:Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D010100:Oxygen / Q000737:chemistry*; D010539:Permeability; D016062:Porosity
chemical_list: D000538:Aluminum Silicates; D005740:Gases; D008567:Membranes, Artificial; D010100:Oxygen; D000077215:Clay
keywords: clay; layer-by-layer assembly; nanocoatings; oxygen barrier; polyelectrolyte multilayer; transmission electron microscopy