Layer-by-Layer Assembly of Clay-filled Polymer Nanocomposite Thin Films

A variety of functional thin films can be produced using the layer-by-layer assembly technique. In this work, assemblies of anionic clay and cationic polymer were studied with regard to film growth and gas barrier properties. A simple, yet flexible robotic dipping system, for the preparation of these thin films, was built. The robot alternately dips a substrate into aqueous mixtures with rinsing and drying in between. Thin films of sodium montmorillonite clay and cationic polymer were grown and studied on poly(ethylene terephthalate) film or a silicon wafer. After 30 clay polymer bilayers were deposited, the resulting transparent film had an oxygen transmission rate (OTR) below 0.005 cm3/m2/day/atm. This low OTR, which is unprecedented for a clay-filled polymer composite, is believed to be due to a ?brick wall? nanostructure comprised of completely exfoliated clay bricks in polymeric ?mortar?. The growth of polymer and clay assemblies is then shown to be controlled by altering the pH of polyethylenimine (PEI). Growth, oxygen permeability, and mechanical behavior of clay-PEI assemblies were studied as a function of pH in an effort to tailor the behavior of these thin films. Thicker deposition at high pH resulted in reduced oxygen permeability and lower modulus, which highlights the tailorability of this system.