Humidity-responsive gas barrier of hydrogen-bonded polymer-clay multilayer thin films
It is well-known that gas barrier behavior in most polymer and composite materials degrades at elevated humidity. In an effort to reduce this trend, the influence of relative humidity (RH) on the gas barrier of thin films comprising montmorillonite clay and polyvinylpyrrolidone, created via layerby-layer assembly, was investigated. These hydrogen-bonded thin films approximately doubled in thickness when RH was increased to 100% but returned to within 1% of the original thickness when RH was decreased to 0%, with minimal swelling/deswelling hysteresis. Transmission electron microscopy reveals a highly aligned nanobrick wall structure, which has a clay concentration of 74 wt % and greater than 95% visible light transmission. The oxygen transmission rate (OTR) through these films, deposited on 179 μm poly(ethylene terephthalate) film, remarkably decreases as a function of RH. A 40-BL film has an OTR of 3.9 (cc/(m2 ·day·atm)) at 0% RH, while exposure to 100% RH decreased this value by 11%. In this case, greater spacing between clay layers and maintenance of tight packing within the layers (due to relatively weak H-bonding between polymer and clay) combine to create a more tortuous path at high humidity. This study marks the first polymer−clay assembly that exhibits improved gas barrier at high humidity, which is important for various packaging applications (e.g., food and flexible electronics).