Browsing by Subject "Polymers--Thermal properties"
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Item Development and study of high-Tc superconductor conductive polymer assemblies(2002) Schougaard, Steen Brian; McDevitt, John ThomasThis dissertation presents the development and study of organic polymeric conductor/High-Tc superconductor bilayers, for exploration of the superconductor proximity effect. A major obstacle to this research is the instability of the High-Tc superconductor towards H2O and CO2 as it is necessary to create hybrid structures where the two conductors have intimate contact. For this reason, a study of the corrosion characteristics of the RBa2Cu3O7 (R=Y, Eu, Nd) series was undertaken. In this study, R=Nd was shown to have enhanced corrosion resistance. In an effort to develop chemical processing methods that allow for the production of intimate contact between the cuprate superconductor and the conductive polymer systems, a study of alkyl amine adsorbed onto the surface of the superconductor is presented. In the electrochemical part of the study, alkyl amine ferrocene adsorbed onto the surface revealed that the amines are strongly adhered to the superconductor while still allowing electrons to pass from the superconductor to the ferrocene moiety. The self-assembled monolayer (SAM) structure of CF3(CF2)3(CH2)11NH2 atop the superconductor was elucidated by employing several techniques, including atomic scale atomic force microscopy (AFM). A novel multistep bilayer sample preparation protocol is presented involving thin film fabrication by pulsed laser deposition (PLD), patterning by shadow mask ablation, sample thinning by scanning probe and electropolymerization of the polymer. Scanning probe thinning yields a smooth superconductor surface, a known superconductor thickness, and if a blocking layer is employed prior to thinning, the localization of polymer growth. Studies of the response of a bilayer structure of Y0.6Ca0.4Ba1.6La0.4 Cu3O7/poly pyrrole to polymer redox cycling showed a semi-reversible suppression of transition temperature qualitatively consistent with a combination of corrosion and superconductor proximity effect. Quantitative analysis in the framework of the proximity effect shows evidence suggestive of an unexpected long superconductor coherence length. A qualitative model based on grain boundaries is postulated to explain this effect.Item Effects of confinement on the glass transition of polymer-based systems(2004) Pham, Joseph Quan Anh; Green, Peter F. (Peter Fitzroy)In recent years, considerable effort has been invested toward developing polymers for applications in which they serve as the active material component for devices such as transistors, light emitting diodes, and various sensors. Many of these applications require polymer thin films. Unfortunately, many of the properties of the thin films that impact device processing and performance are not well understood. Polymer films in this thickness range exhibit properties that are very different from the bulk. Properties such as the viscosity, the glass transition temperature, Tg, and phase transitions exhibit film thickness dependencies. This thesis examines three problems generally in the area of the glass transition temperature of polymer thin films. (1) The Tg of polymer-polymer mixtures: We examined the Tg of thin films of the miscible blend tetramethyl Bisphenyl-A polycarbonate (TMPC)/polystyrene (PS). Our results indicate that entropic, “chain packing,” effects and enthalpic effects associated with interactions between the dissimilar chain segments and the external interfaces (free surface and substrate) determine the Tg of these mixtures. (2) The glass transition of polymer-based nanocomposite films: We examined the film thickness dependencies of the glass transition temperatures of polystyrene based nanocomposite thin films containing small concentrations, 1-5 wt.%, of layered silicate clays and C60 fullerenes supported by silicon substrates, PS-LSi/Si and PS-C60/Si, respectively. Our results show that at these small concentrations the nanoscale particles can change Tg appreciably, particularly in films with thicknesses less than 45 nm. Shifts in Tg of up to 20 degrees were observed, regardless of the chemistry of the nanoparticles. (3) Polymer (polystyrene and polymethylmethacrylate) thin films in supercritical CO2: The effect of CO2 on the glass transition plays a central role in the processing of thin films for microelectronic applications. Our most significant finding is the phenomenon of retrograde vitrification, where the polymer in a CO2 environment exhibits a rubbery-to-glass transition as the temperature is decreased and surprisingly a glassy-to-rubbery transition with a further decrease in temperature. These findings have important implications on the processing of thin polymer films in CO2 environments.Item Gas transport properties of poly(n-alkyl acrylate) blends and modeling of modified atmosphere storage using selective and non-selective membranes(2007-12) Kirkland, Bertha Shontae, 1976-; Paul, Donald R.The gas transport properties of side-chain crystalline poly(n-alkyl acrylate) and poly(m-alkyl acrylate) blends are determined as a function of temperature for varying side-chain lengths, n and m, and blend compositions. The side chains of poly(n-alkyl acrylate)s crystallize independently of the main chain for n [is greater than or equal to] 10 which leads to an extraordinary increase in the permeability at the melting temperature of the crystallites. The compatibility of these polymers are examined and macroscopic homogeneity is observed for a small range of n and m when the difference /n - m/ is between 2 - 4 methylene units. Thermal analysis shows that the blend components crystallize independently of one another; at the same time, the crystallization of each component is hindered by the presence the other component. The permeation responses of these blends show two distinct permeation jumps as the crystallites from each component melt at their respective melting temperatures. Blends with continuous permeation responses are found to have higher effective activation energies than observed for common polymers. Thermal analysis proved to be a useful tool to help predict the permeation response for poly(alkyl acrylates); thus the thermal behavior of poly(n-alkyl acrylate) blended with n-aliphatic materials and random copolymers of poly(n-alkyl acrylates) are briefly examined. A bulk modified atmospheric storage design is proposed where produce is stored in a rigid chamber that is equipped with both selective and non-selective membrane modules that help regulate the oxygen entering and the carbon dioxide leaving the produce compartment. The design enables control of the atmosphere inside the chamber by modulating gas flow, i.e. the gas flow rate and composition, through the non-selective membrane by delivering fresh air upstream of the non-selective membrane. The model shows that the choice of materials for the selective and non-selective membranes dictate the range of concentrations achievable; however, the air flow rate allows the control between these ranges. The method to design a practical chamber from this model is also described.Item Thermally induced transitions in polymer thin films(2007) Arceo, Abraham, 1976-; Manthiram, Arumugam; Green, Peter F. (Peter Fitzroy)Polymers, by virtue of their chemical composition and molecular architecture, exhibit a diverse range of microstructural features and properties. As thin films, due primarily to effects associated with confinement and interfacial interactions, their properties may be film-thickness dependent. The significance of their thicknessdependent behavior is underscored by the fact that polymer films are of technological interest in areas that include, sensors, catalysts and organic electronics. One challenge associated with the use of thin film polymers is to understand the role of confinement and interfacial interactions on thermally induced transitions, such as vitrification and various morphological transitions. To this end, the work presented in this dissertation focuses on the behavior of thermally induced transitions in two thin film polymer-based systems: (1) an A-b-B diblock copolymer which can undergo a disorder-to-order transitions (ODT), wherein the ordered state exhibits varying geometrical symmetries, depending on the relative volume fractions of the A and B components; (2) an amorphous polymer filled with particles of nanoscale dimensions. The first of three problems examined is the influence of supercritical carbon dioxide (scCO₂) on the order-disorder transition of thin film symmetric A-b-B diblock copolymer systems. We show that the transition (xN)ODT, where x is the energetic A-B Flory-Huggins interaction parameter and N is the total degree of polymerization of the copolymer, of the thin film decreased ~ 20% compared to the bulk; the decrease was more significant in scCO₂ environments. The decrease of (xN)ODT in scCO₂ is contrary to observations in bulk copolymer-scCO₂ systems where the effective A-B interactions are weaker, hence the condition for the transition increases to higher (xN)ODT values. With regard to the second problem, we show for the first time experimentally that nanoparticles induced order into thin films of a symmetric A-b-B diblock copolymer at temperatures below the bulk ODT. Finally, we examine the influence of polystyrene (PS) grafted nanoparticles on the glass transition of PS films of varying molecular weight and thickness. We demonstrate that by controlling spatial distribution of nanoparticles, through driving forces of entropic origin, the glass transition temperature of the film can be changed drastically, as much as tens of degrees.