Browsing by Subject "Layer-by-layer"
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Item Antimicrobial Activity of Cationic Antiseptics in Layer-by-Layer Thin Film Assemblies(2010-07-14) Dvoracek, Charlene M.Layer-by-layer (LbL) assembly has proven to be a powerful technique for assembling thin films with a variety of properties including electrochromic, molecular sensing, oxygen barrier, and antimicrobial. LbL involves the deposition of alternating cationic and anionic ingredients from solution, utilizing the electrostatic charges to develop multilayer films. The present work incorporates cationic antimicrobial agents into the positively-charged layers of LbL assemblies. When these thin films are exposed to a humid environment, the antimicrobial molecules readily diffuse out and prevent bacterial growth. The influence of exposure time, testing temperature, secondary ingredients and number of bilayers on antimicrobial efficacy is evaluated here. Additionally, film growth and microstructure are analyzed to better understand the behavior of these films. The antimicrobial used here is a positively-charged quaternary ammonium molecule (e.g. cetyltrimethylammonium bromide [CTAB]) that allow assemblies to be made with or without an additional polycation like polydiallyldimethylamine. While films without this additional polymer are effective, they do not have the longevity or uniformity of films prepared with its addition. All of the recipes studied show linear growth as a function of the number of bilayers deposited and this growth is relatively thick (i.e. > 100 nm per bilayer). In general, 10-bilayer films prepared with CTAB and poly(acrylic acid) are able to achieve a 2.3 mm zone of inhibition against S. aureus bacteria and 1.3 mm against E. coli when test are conducted at body temperature (i.e. 37oC). Fewer bilayers reduces efficacy, but lower test temperatures improve zones of inhibition. As long as they are stored in a dry atmosphere, antimicrobial efficacy was found to persist even when films were used four weeks after being prepared. The best films remain effective (i.e. antimicrobially active) for 4-6 days of constant exposure to bacteria-swabbed plates. This technology holds promise for use in transparent wound bandages and temporary surface sterilization.Item Layer-by-layer assembly of electrically conductive polymer thin films(Texas A&M University, 2007-09-17) Jan, Chien Sy JasonLayer-by-layer (LbL) assembly was used to produce highly conductive thin films with carbon black (CB) and polyelectrolytes. The effects of sonication and pHadjustment of the deposition mixtures on the conductivity and transparency of deposited films were studied. Drying temperature was also evaluated with regard to thin film resistance. Sonication and oven drying at 70oC produced films with the lowest sheet resistance (~ 1500 ????/sq), which corresponds to a bulk resistivity of 0.2 ??????????cm for a 14- bilayer film that is 1.3 ????m thick. Increasing the pH of the PAA-stabilized mixture and decreasing the pH of the PEI-stabilized mixture resulted in films with 70% transparency due to thinner deposition from increased polymer charge density. Varying the number of bilayers allows both sheet resistance and optical transparency to be tailored over a broad range. Variation of deposition mixture composition led to further reduction of sheet resistance per bilayer. A 14 bilayer film, made from mixtures of 0.25wt% carbon black in 0.05wt% PAA and plain 0.1wt% PEI, was found to have a sheet resistance of approximately 325 ????/sq. Bulk resistivity was not improved due to the film being 8 ????m thick, but this combination of small thickness and low resistance is an order of magnitude better than carbon black filled composites made via traditional melt or solution processing. Applications for this technology lie in the areas of flexible electronics, electrostatic charge dissipation, and electromagnetic interference shielding.Item Processing and Gas Barrier Behavior of Multilayer Thin Nanocomposite Films(2012-10-19) Yang, You-HaoThin films with the ability to impart oxygen and other types of gas barrier are crucial to commercial packaging applications. Commodity polymers, such as polyethylene (PE), polycarbonate (PC) and polyethylene terephthalate (PET), have insufficient barrier for goods requiring long shelf life. Current gas barrier technologies like plasma-enhanced vapor deposition (PECVD) often create high barrier metal oxide films, which are prone to cracking when flexed. Bulk composites composed of polymer and impermeable nanoparticles show improved barrier, but particle aggregation limits their practical utility for applications requiring high barrier and transparency. Layer-by-layer (LbL) assemblies allow polymers and nanoparticles to be mixed with high particle loadings, creating super gas barrier thin films on substrates normally exhibiting high gas permeability. Branched polyethylenimine (PEI) and poly (acrylic acid) (PAA) were deposited using LbL to create gas barrier films with varying pH combinations. Film thickness and mass fraction of each component was controlled by their combined charge. With lower charge density (PEI at pH 10 and PAA at pH 4), PEI/PAA assemblies exhibit the best oxygen barrier relative to other pH combinations. An 8 BL PEI/PAA film, with a thickness of 451 nm, has an oxygen permeability lower than 4.8 x 10^-21 cm^3 * cm/cm^2 * s * Pa, which is comparable to a 100 nm SiOx nanocoating. Crosslinking these films with glutaraldehyde (GA), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC) or heating forms covalent bonds between PEI and/or PAA. Oxygen transmission rates (OTR) of 8 BL films crosslinked with 0.1M GA or 0.01M EDC show the best oxygen barrier at 100% RH. Graphene oxide (GO) sheets and PEI were deposited via LbL with varying GO concentration. The resulting thin films have an average bilayer thickness from 4.3 to 5.0 nm and a GO mass fraction from 88 to 91wt%. Transmission electron microscopy and atomic force microscopy images reveal a highly-oriented nanobrick wall structure. A 10 BL PEI/GO film that is 91 nm thick, made with a 0.2 wt% GO suspension, exhibits an oxygen permeability of 2.5 x 10^-20 cm^3 * cm/cm^2 * s * Pa. Finally, the influence of deposition time on thin film assembly was examined by depositing montmorillonite (MMT) or laponite (LAP) clays paired with PEI. Film growth and microstructure suggests that smaller aspect ratio LAP clay is more dip-time dependent than MMT and larger aspect ratio MMT has better oxygen barrier. A 30 BL PEI/MMT film made with 10 second dips in PEI has the same undetectable OTR as a film with 5 minute dips (with dips in MMT held at 5 minutes in both cases), indicating LbL gas barrier can be made more quickly than initially thought. These high barrier recipes, with simple and efficient processing conditions, are good candidates for a variety of packaging applications.Item Thermal Transitions in Layer-By-Layer Assemblies(2014-10-13) Sung, ChoonghyunThermal transitions in layer-by-layer (LbL) assemblies were investigated under dry and hydrated conditions. In the dry state, the effects of film thickness and the film deposition method on the glass transition temperature (Tg) were studied. In aqueous conditions, the thermal behavior of microtubes and ultrathin films were studied. The effect of film thickness on the Tg for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PMAA) hydrogen-bonded LbL assemblies was investigated in the dry state. The Tg of PEO/PAA LbL films increased by 9 ?C as the thickness decreased from 150 nm to 30 nm. In contrast, the Tg of thermally crosslinked PEO/PMAA LbL films was not influenced by film thickness. The thermal properties of dry PEO/PAA and PEO/PMAA LbL assemblies prepared using dip- and spray-assisted methods were compared. For both LbL assemblies, Tg?s were similar regardless of deposition method. However, the breadth of glass transition was larger for the spray-assisted films. While PEO/PAA LbL assemblies showed smooth surface morphologies regardless of the deposition method, PEO/PMAA LbL assemblies showed different surface morphologies depending on the deposition method. The temperature-triggered transformation of LbL microtubes consisting of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) was studied in aqueous media. LbL microtubes were assembled from sacrificial porous membranes. Released microtubes became spherical and ellipsoidal upon high temperature incubation. In comparison, unreleased microtubes exhibited periodic perforations. The thermal transition of LbL assemblies of poly(diallyldimethylammonium chloride) (PDAC) and poly(styrene sulfonate) (PSS) was probed using electrochemical impedance spectroscopy (EIS). Two thermal transition temperatures were obtained from the plot of film resistance (Ttr,Rf) and the charge transfer resistance (Ttr,Rct) as a function of temperature. Below 20 layers, PSS-capped LbL films showed higher Ttr,Rct values compared to PDAC-capped films. In contrast, Ttr,Rf was largely uninfluenced by assembly parameters. In conclusion, thermal transitions in dry LbL assemblies were influenced by the film thickness and film deposition method. Thermal behavior of LbL microtubes were linked to the thermal transitions in hydrated state. EIS study on thermal transitions provided unique information on the structure of LbL assemblies.