Browsing by Subject "oxygen barrier"
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Item Layer-by-layer Assembly of Nanobrick Wall Ultrathin Transparent Gas Barrier Films(2012-07-16) Priolo, Morgan AlexanderThin layers with high barrier to oxygen and other gases are a key component to many packaging applications, such as flexible electronics, food, and pharmaceuticals. Vapor deposited thin films provide significant gas barrier, but are prone to cracking when flexed, require special, non-ambient processing environments, and can involve complex fabrication when layered with polymers. The addition of clay into polymers can enhance barrier properties relative to the neat polymer; however, these composites are subject to clay aggregation at high loadings, which leads to increased opacity and random platelet alignment that ultimately reduce barrier improvement. Layer-by-layer (LbL) assembly is capable of producing thin films that exhibit super gas barrier properties, while remaining flexible and completely transparent. Montmorillonite (MMT) clay and branched polyethylenimine (PEI) were deposited via LbL assembly to create gas barrier films that can be tailored by altering the pH of the PEI deposition solution or the concentration of the MMT suspension. Films grow linearly as a function of layers deposited, where increasing PEI pH increases spacing between clay layers and increasing MMT concentration increases thin film clay content. An oxygen transmission rate (OTR) below the detection limit of commercial instrumentation (< 0.005 cm3/m2?day?atm) is observed after 70 layers of 0.2 wt % MMT or 24 layers of 2 wt % MMT are deposited with pH 10 PEI onto 179 ?m thick poly(ethylene terephthalate) (PET) film. Three-component films of PEI, poly(acrylic acid) (PAA), and MMT grow exponentially as a function of PEI/PAA/PEI/MMT quadlayers deposited. A transparent, ultrathin film of only four quadlayers deposited onto PET exhibits the lowest oxygen permeability ever reported for any thin film material, at only 51 nm thick. Finally, the first example of LbL assembly using large aspect ratio vermiculite (VMT) clay was performed. PEI/VMT films grow linearly as a function of layers deposited and exhibit 95 % light transmission with 97 wt % VMT. The barrier of these films is due to the highly aligned nanobrick wall structure that creates a tortuous path for permeating molecules. Coupling high flexibility, transparency, and barrier, these coatings are good candidates for a variety of packaging applications.Item Layer-by-Layer Nanocoatings with Flame Retardant and Oxygen Barrier Properties: Moving Toward Renewable Systems(2012-10-23) Laufer, Galina 1985-Numerous studies have focused on enhancing the flame retardant behavior of cotton and polyurethane foam. Some of the most commonly used treatments (e.g., brominated compounds) have raised concerns with regard to toxicity and environmental persistence. These concerns have led to significant research into the use of alternative approaches, including polymer nanocomposites prepared from more environmentally benign nanoparticles. These particles migrate to the surface from the bulk during fire exposure to form a barrier on the surface that protects the underlying polymer. This theory of fire suppression in bulk nanocomposites inspired the use of layer-by-layer (LbL) assembly to create nanocoatings in an effort to produce more effective and environmentally-benign flame retardant treatments. Negatively charged silica nanoparticles of two different sizes were paired with either positively charged silica or cationic polyethylenimine (PEI) to create thin film assemblies. When applying these films to cotton fabric, all coated fabrics retained their weave structure after being exposed to a vertical flame test, while uncoated cotton was completely destroyed. Micro combustion calorimetry confirmed that coated fabrics exhibited a reduced peak heat release rate, by as much as 20% relative to the uncoated control. Even so, this treatment would not pass the standard UL94 vertical flame test, necessitating a more effective treatment. Positively- charged chitosan (CH) was paired with montmorillonite (MMT) clay to create a renewable flame retardant nanocoating for polyurethane foam. This coating system completely stops the melting of a flexible polyurethane foam when exposed to direct flame from a butane torch, with just 10 bilayers (~ 30 nm thick). The same coated foam exhibited a reduced peak heat release rate, by as much as 52%, relative to the uncoated control. This same nanobrick wall coating is able to impart gas barrier to permeate plastic film. Multilayered thin films were assembled with "green" food contact approved materials (i.e., chitosan, polyacrylic acid (PAA) and montmorillonite clay). Only ten CH-PAA-CH-MMT quadlayers (~90 nm thick) cause polylactic acid (PLA) film to behave like PET in terms of oxygen barrier. A thirty bilayer CH-MMT assembly (~100 nm thick) on PLA exhibits an oxygen transmission rate (OTR) below the detection limit of commercial instrumentation (<= 0.005 cm^3/(m^2*day*atm)). This is the same recipe used to impart flame retardant behavior to foam, but it did not provide effective FR to cotton fabric, so a very different recipe was used. Thin films of fully renewable electrolytes, chitosan and phytic acid (PA), were deposited on cotton fabric in an effort to reduce flammability through an intumescent effect. Altering the pH of aqueous deposition solutions modifies the composition of the final nanocoating. Fabrics coated with highest PA content multilayers completely extinguished the flame and reduced peak heat release (pkHRR) and total heat release of 60% and 76%, respectively. This superior performance is believed to be due to high phosphorus content that enhances the intumescent behavior of these nanocoatings.Item Process Improvements for Gas Barrier Thin Films Deposited Via Layer-By-Layer Assembly(2015-05-04) Hagen, David AustinThin layers of aluminum have provided good oxygen barrier for food packaging for many years, but aluminum coatings can easily crack, are completely opaque, and are not environmentally friendly. One gas barrier solution for food, to flexible electronics, and pressurized bladders is to create polymer nanocomposite thin-films using layer-by-layer (LbL) assembly. These non-metal, water-based thin films contain a tortuous path through which a gas molecule must navigate. The work in this dissertation focuses on improving the process of creating these thin films to optimize their performance and achieve lower transmission rates with fewer layers. Excellent gas barrier was achieved in a layer-by-layer thin film with fewer layers by optimizing deposition time of cationic polyethylenimine (PEI) and anionic poly(acrylic acid) [PAA]. Substantial deposition occurs with short deposition times for the first four PEI/PAA bilayers, while thicker deposition occurs with longer deposition times beyond 4 bilayers. Eight bilayers (650 nm) were required to achieve an undetectable oxygen transmission rate (<0.005 cm^3/(m^2?day)) using 1 min deposition steps, but this barrier was obtained with only 6 BL (552 nm) using 1s deposition of the first four bilayers, reducing total deposition time by 73%. Polymer?clay bilayer films show good oxygen barrier properties due to a nanobrick wall structure consisting of clay nanoplatelets within polymeric mortar. Super oxygen barrier trilayer thin films have been deposited using two successive anionic layers of montmorillonite (MMT) clay and polymer (PAA) following every cationic polymer (PEI) layer during layer-by-layer assembly. It is shown here that adding an anionic polymer layer reduces free volume of the film by filling in gaps of the similarly charged clay layer, which increases the barrier performance by at least one order of magnitude. Barrier improvement can also be achieved by reducing the pH of the clay suspension in the PEI/MMT system. The charge of the deposited PEI layer increases in the clay suspension environment as the pH decreases, attracting more clay. This enables a 5? improvement in the gas barrier for a 10 PEI/MMT bilayer thin film (85 nm) made with pH 4 MMT, relative to the same film made with pH 10 MMT (57 nm).