Tailorability of moisture barrier and mechanical performance in clay-based multilayer nanocomposite thin films

Abstract

Clay-based nanocomposites have gained significant interest in the last few years due to their biocompatibility and potential applications in food packaging, wearables, and flame-retardant coatings. Among existing manufacturing methods, layer-by-layer (LbL) deposition of multilayer films with nanoclays has shown the ability to produce extremely well aligned platelets with controllable nanoscale thickness, and has also been shown to improve gas barrier performance. In contrast, the equally important moisture barrier performance and mechanical properties have not been thoroughly explored. This study focuses on the performance of three types of LbL films consisting of polyethyleneimine (PEI), polyacrylic acid (PAA), and montmorillonite (MMT) or vermiculite (VMT) nanoclays, resulting in PEI/VMT bilayer (BL) films, PEI/VMT/PEI/PAA quadlayer (QL) films, and PEI/MMT/PEI/PAA QL films on a polyethylene terephthalate substrate. Surface roughness as low as 26.2 ± 4.9 nm and inorganic platelet content as high as 96.7 ± 1.1 wt% are achieved with the VMT BL films. This highly aligned, densely packed, high aspect ratio platelet structure enables a water vapor transmission rate (WVTR), at 50% relative humidity and 37 °C, as low as 720.9 mg m⁻² day⁻¹ for a 500 nm thin VMT BL. High stiffness values are estimated for all coatings: 162.47 GPa, 70.94 GPa, and 37.06 GPa for 500 nm VMT BL, VMT QL, and MMT QL films, respectively. A high elastic limit is observed in MMT QL cracking with crack onset strains exceeding 5%, attributed to the higher organic content compared to the BL and the smaller aspect ratio of MMT platelets compared to VMT. These trends observed in moisture barrier and mechanical durability performances across BL vs. QL and VMT vs. MMT highlight the importance of tailored design criteria for optimizing coating applications.