Processing and Functionality Improvements of Layer-By-Layer Assembled Multilayer Super Gas Barrier Nanocoatings
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High oxygen barrier materials are becoming increasingly important for food packaging, pressurized systems, and flexible electronics protection. The widely used traditional aluminized plastics and inorganic gas barrier coatings (SiOx and AlxOy) exhibit low transparency and/or flexibility. Layer-by-layer assembly (LbL) provides a costeffective and environmental-friendly alternative, with lower oxygen permeability and greater transparency. This dissertation focuses on improvements of the coating process and development of new functionalities. Four studies are discussed in detail on this topic and two future research directions are also introduced. Layer-by-layer assembled multilayer nanocoatings have been shown to provide excellent oxygen barrier to poly(ethylene terephthalate) (PET) film, which is commonly used for encapsulation and packaging. Polymer-clay (polyethylenimine (PEI)/vermiculite (VMT)) and all-polymer ((PEI)/poly(acrylic acid) (PAA)) multilayer systems are shown to be equally beneficial as barrier coatings for polyolefin substrates (e.g., polyethylene and polypropylene), which suffer from a high oxygen transmission rate. A 30 bilayer PEI/VMT nanocoating reduces the OTR of biaxially oriented polypropylene (BOPP) by more than 160X, rivaling most inorganic coatings. WVTR is simultaneously reduced by 43%. In addition to VMT, montmorillonite (MMT) clay has been widely used to prepare gas barrier nanocoatings. In an effort to produce high oxygen barrier with fewer deposition steps, pH of the MMT aqueous suspension was reduced. In a PEI/PAA/PEI/MMT iii quadlayer system, the reduced pH of MMT causes the preceding PEI layer to be more charged, which results in more clay deposited. A compromise between high polyelectrolyte diffusion (high pH) and high clay deposition (low pH) was found at pH 6, where the best oxygen barrier is obtained. A 179 μm thick PET substrate, coated with just three quadlayers (44 nm thick), exhibits an undetectable oxygen transmission rate (< 0.005 cm3/(m2 day atm)). In another attempt to achieve better oxygen barrier of the PEI/MMT bilayer (BL) system, a “salty clay” strategy was carried out, where an “indifferent” electrolyte (NaCl) was introduced into the MMT suspension. NaCl alters the ionic strength of the clay suspension and subsequently changes the rheological behavior of clay platelets. As a result, a multilayer film with a more tortuous path and better clay coverage (and alignment) is produced. Lower OTR is achieved as a result of this structural engineering. An 8 BL PEI/MMT film, assembled with a clay suspension containing 5 mM NaCl, exhibits an order of magnitude reduction in OTR relative to its no-salt counterpart. The ability to self-repair after damaging (e.g., cracking) and restore original properties is highly desired for polymers and composites. The PEI/PAA nanocoating has the ability to self-heal after being cracked by stretching. Cracks in the film lead to loss of oxygen barrier. High humidity is employed as an external healing stimulus to repair these cracks. The OTR of a PEI/PAA multilayer nanocoating remains below detection after 10 stretching-healing cycles, which shows this healing process to be highly robust.
Song, Yixuan (2018). Processing and Functionality Improvements of Layer-By-Layer Assembled Multilayer Super Gas Barrier Nanocoatings. Doctoral dissertation, Texas A & M University. Available electronically from