pubs.acs.org/NanoLett
Super Gas Barrier of Transparent Polymer-Clay Multilayer Ultrathin Films Morgan A. Priolo,†,‡ Daniel Gamboa,‡ Kevin M. Holder,§ and Jaime C. Grunlan*,†,‡,| †
Materials Science & Engineering Program, Texas A&M University, College Station, Texas 77843-3003, United States, Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123, United States, § Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States, and | Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, United States ‡
ABSTRACT Flexible and transparent polymeric “superbarrier” packaging materials have become increasingly important in recent years. Layer-by-layer assembly offers a facile technique for the fabrication of layered, polymer-clay superbarrier thin films. At only 51 nm thick, these nanocomposite thin films, comprised of 12 polymer and 4 clay layers, exhibit an oxygen permeability orders of magnitude lower than EVOH and SiOx. Coupling high flexibility, transparency, and barrier protection, these films are good candidates for a variety packaging applications. KEYWORDS Layer-by-layer assembly, transmission electron microscopy, thin films, oxygen barrier, clays, composites
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hin layers with high barrier to oxygen and other gases/vapors are a key component to a variety of applications, such as flexible electronics and food packaging.1,2 In fact, lack of a reasonable barrier layer is the primary hurdle faced by the flexible electronics industry.3 It has been estimated that transparent films with an oxygen transmission rate (OTR) below 10-5 cm3/(m2·day·atm) are needed for reliable, flexible organic light emitting diodes (FOLEDs) to achieve sufficient performance and lifetime requirements.4 Vapor deposited thin films, such as SiOx or Al2O3, provide significant gas barrier, but are prone to cracking when flexed, require special (i.e., nonambient) processing environments (e.g., vacuum), and can involve complex fabrication when layered with polymers.5,6 Alternatively, flake-filled polymers, with highly aligned nanoparticles, have been shown to enhance barrier due to the high aspect ratio flakes causing a permeating molecule to travel an extensive diffusion path.7 This increased diffusion length, often referred to as a tortuous pathway, is the key to high barrier flake-filled systems. Individual clay platelets are approximately 1 nm thick disk-shaped nanoparticles, with aspect ratios ranging from 20 to several thousand, that can act as gas impermeable flakes.8-10 The ability to incorporate clay into a polymeric matrix with a high level of exfoliation and orientation is one of the most important challenges in the fabrication of polymer-clay nanocomposites with enhanced properties.11 Experimentally, the addition of clay into polymers has been shown to enhance the barrier properties relative to the neat polymer;12 however, these composites are subject to clay aggregation at high loadings,
which leads to increased opacity and random platelet alignment that ultimately reduce the barrier enhancement.13-17 Layer-by-layer (LbL) assembly is a method of building multifunctional thin films through alternating exposure of a substrate to aqueous cationic and anionic mixtures.18 LbL assemblies can exhibit a wide array of properties, including electrical conductivity,19,20 superhydrophobicity,21,22 and antimicrobial.23,24 In many cases, one or more of the deposition ingredients are charged nanoparticles. Negatively charged clay nanoplatelets were shown to form highly structured LbL films when combined with a positively charged polymer.25 LbL films containing clay have since been studied for their high strength,26 antiflammabilty,27 and oxygen barrier properties,28 while remaining highly transparent. In the case of a barrier, a correlation between oxygen permeability and clay spacing was discovered, where increasing the spacing between deposited clay layers dramatically improved barrier properties at film thicknesses less than 250 nm.29 It was suggested that the spreading of clay layers allowed permeating molecules the opportunity to get trapped within the polymer-filled channels, traveling perpendicular to the diffusion direction, as modeled by Cussler.7 LbL assembly of a three-component system was used in this study to further increase the space between clay layers as the film is deposited. The resulting polymer nanocomposite thin films have unprecedented barrier performance, with oxygen permeability below that of SiOx (
