Magister Scientiae Dentium - MSc(Dent) / Light-emitting diode (LED) curing lights were introduced to the dental market promising a higher curing efficiency than halogen-based lights. The earlier generation curing lights, however, proved not to be as effective as halogen lights. As a result 3M ESPE introduced a new high-powered LED curing light, the Elipar FreeLight 2, that delivers a greater irradiance. and threfore greater energy density than its precursor. Due to these changes, the light's manufacturer claims that the FreeLight 2 can cure resin composites at half of their recommended curing time. The aim of this study was to compare the effectiveness of cure when a FreeLight 2 was used to sure composite samples at 100% and at 50% of the recommended curing time. / South Africa

I. Crack wake bridges were studied in glassy alumina s-DCB specimens at elevated temperatures. Direct observation of fracture surfaces in unmodified s-DCB specimens revealed the sizes and shapes of the high temperature bridges in the crack wake relative to the position of the crack front. Similar fracture surface observations made on s-DCB specimens modified to include a rear notch wedge, allowing the measurement of the high temperature bridging forces, were used to analyze bridge behavior in terms of viscous and power law creep models. The sizes and positions of the bridges were found to be highly stochastic but there was a clear trend toward decreased area bridged with increasing COD. Simple uniaxial viscous or creeping columns were inadequate to explain the load supported by the bridges. II. R-curve behavior and slow crack growth were studied in 99.5% alumina chevron notched short bar specimens at temperatures up to $1200\sp\circ\rm C.$ Constant loading rate tests measured toughness as a function of crack length at various loading rates. Constant load tests measured subcritical crack growth as a function of time. The intrinsic toughness decreased with increasing temperature while the bridging contribution to the R-curve remained independent of temperature. The effect of loading rate on R-curve tests was masked by experimental scatter at all temperatures except $1200\sp\circ\rm C.$ Subcritical crack growth tests showed high n-values at 700 and $1000\sp\circ\rm C,$ indicative of region III type behavior and an n-value indicative of region I at $1200\sp\circ\rm C.$ Predictions using the obtained subcritical crack growth parameters showed a small decrease in the level of the R-curve with decreasing loading rate at 700 and $1000\sp\circ\rm C$ that was within the range of the experimental scatter. A discrepancy between the R-curve predictions and the data at $1200\sp\circ\rm C$ was attributed to a rate dependence of the bridging behavior that was not considered in the analysis.

This dissertation will study the material properties of two types of novel polymer films (polyelectrolyte multilayer films and photolithographic polymer films). The formation of polylelectrolyte multilayer films onto functionalized aluminum oxide surfaces and functionalized poly(ethylene terephthaltate) (PET) were studied. Functionalization of the aluminum oxide surfaces was achieved via silane coupling. Functionalization of PET surfaces was achieved via hydrolysis and amidation. Surface characterization techniques such as X-ray photoelectron spectroscopy (XPS) and dynamic contact angle measurements were used to monitor the polyelectrolyte multilayer formation. Mechanical properties of the aluminum oxide supported polyelectrolyte multilayer films were tested using a simplified peel test. XPS was used to analyze the surfaces before and after peel. Single lap shear joint specimens were constructed to test the adhesive shear strength of the PET-supported polyelectrolyte multilayer film samples with the aid of a cyanoacrylate adhesive. The adhesive shear strength and its relation with the type of functionalization, number of polyelectrolyte layers, and the effect of polyelectrolyte conformation using added salt were explored. Also, characterization on the single lap joints after adhesive failure was carried out to determine the locus of failure within the multilayers by using XPS and SEM. Two types of photolithographic polymers were formulated and tested. These two polymers (photocrosslinkable polyacrylate (PUA), and a photocrosslinkable polyimide (HRP)) were used to investigate factors that would affect the structural integrity of these particular polymers under environmental variables such as processing (time, UV cure, pressure, and temperature) and ink exposure. Thermomechanical characterization was carried out to see the behavior of these two polymers under these environmental variables. Microscopic techniques were employed to study the morphological behavior of the two polymer systems. Also, unique in-house characterization methods such as the vibrational holographic interferometry to measure residual stress in these polymer coatings upon processing, and the environmental tensile tester (ETT) to measure ink diffusion and swelling stresses were used to further characterize these two polymers.

The overall objective of this Ph.D. thesis is to control the wetting behavior of surfaces by exploring the effects of topography on wettability, and ultimately make ultrahydrophobic surfaces. Three different approaches were taken in preparing rough surfaces with controlled wettability. The first approach involved the use of photolithography that resulted in a series of silicon surfaces with different post size, shape and separation (Chapter 2). The second approach was the surface modification of low density polyethylene (Chapter 3). The last one was to adsorb polystyrene colloids with different diameters onto polyelectrolyte multilayers (Chapter 4). The wettability of the patterned silicon surfaces prepared by photolithography and hydrophobized using reactive silane chemistry was explored. Surfaces containing square posts with X-Y dimensions of 2 μm-32 μm exhibited ultrahydrophobic behavior with high advancing and receding contact angles. The contact angles were independent of the post height and surface chemistry. Surfaces with larger posts were not ultrahydrophobic-water droplets pinned on these surfaces. Increasing the separation between the posts caused increases in receding contact angles up to the point that water intruded between the posts. Changing the shape of the posts also increased the receding contact angles due to the more contorted contact lines. The oxidative etching of low density polyethylene films followed by uniaxial or biaxial tension resulted in the formation of micron size fragments. 5 minutes oxidized films had smaller islands than the 15 minutes oxidized ones. The fragments became smaller and more distant from each other with increase in strain that affected the wettability of the surfaces. At 400%, the films exhibited ultrahydrophobic behavior. At a higher strain, the islands were very small and apart from each other, the receding contact angle dropped significantly. Submicron and micron scale rough surfaces were prepared by adsorbing polystyrene colloids onto polyelectrolyte multilayers. The negatively charged colloids were efficiently adsorbed onto the outermost cationic polyelectrolyte surface, showing no aggregation. The advancing water contact angle increased and the receding contact angle decreased as the surface coverage increased, resulting in a remarkable hysteresis of ∼122°. Thus, hydrophobic surfaces could not be achieved by making rough surfaces by colloidal adsorption.

Styrenic block copolymer thermoplastic elastomers are one of the most widely used thermoplastic elastomers (TPEs) today. The focus of this research is to fundamentally understand the structure-process-property relationships in these materials. Deformation behavior of the block copolymers with cylindrical and lamellar morphologies has been investigated in detail using unique techniques like deformation calorimetry, transmission electron microscopy (TEM), combined in-situ small angle x-ray and wide angle x-ray scattering (SAXS/WAXS). The research involves the study of structural changes that occur at different length scales along with the energetics involved upon deformation. The structural changes in the morphology of these systems on deformation have been investigated using combined SAXS/WAXS setup. Small angle x-ray scattering probed the changes at the nano-scale of polystyrene (PS) cylinders, while wide angle x-ray scattering probed the changes at molecular length scales of the amorphous/crystalline domains of the elastomeric mid-block in these systems. TEM analysis of the crosslinked elastomers (by UV curing) further confirms the interpretation of structural details as obtained from SAXS upon deformation. New structural features at both these length scales have been observed and incorporated into the overall deformation mechanisms of the material. Characteristic structural parameters have been correlated to differences in their mechanical response in the commercially relevant cylindrical block copolymers. Effect of various process conditions and thermal treatments has been investigated. The process conditions affect the structure at both micro-scopic (grain size) and nano-scopic (domain size) length scales. A correlation has been obtained between a mechanical property (elastic modulus) and an easily measurable structural parameter (d-spacing). Effect of various phase transitions such as order-to-order transition has been studied. Selective solvents can preferentially swell one phase of the block copolymer relative to other and thus bring a change in morphology. Such kinetically trapped structures when annealed at higher temperature try to achieve their thermodynamic equilibrium state. Such changes in morphology significantly affect their tensile and hysteretic response. In another work it has been shown that by carefully compounding these styrenic block copolymers having different morphologies, it is possible to completely disrupt the local scale order and remove the grain boundaries present in these materials. Finally, a new test technique has been developed, by modifying an existing Charpy device to test polymeric films at a high strain rate. A custom designed load-cell is used for force measurements which imposes harmonic oscillations on a monotonic loading signal. The data obtained from this device can be used to analyze visco-elastic response of polymeric films at frequencies much higher than the conventional dynamic mechanical analyzer (DMA).

This dissertation presents experimental research investigating the behavior of particles on two different types of anisotropically curved liquid interfaces: cylinders and catenoids. The results are compared to the behavior predicted by theoretical models. Several types of liquids and many types of particles were examined. The size scale of the surfaces ranges from microns to millimeters, with nanometer and micron sized particles. Semi-cylinders, a few hundred microns in diameter, were made by creating a line of liquid on a surface. Three different fluids were used to create the semi-cylinders: Gallium, ionic liquids, and molten polystyrene (PS). Particle behavior on semi-cylinder liquid interfaces made from these materials was observed. Scanning electron microscopy (SEM) and optical microscopy were used to determine the location and assembly (related to particle attraction) of the particles on the surfaces of the fluids. PS semi-cylinders with silica particles were found to be the most promising experimental route, as PS will flow when heated above its Tg and will solidify when cooled to room temperature. As a solid, the PS surface is easily analyzed. Scanning force microscopy (SFM) was used on the PS semi-cylinders to image the deformation to the interface surrounding the particle, and a quadropolar deformation was found. PS catenoids, a few microns tall, were also investigated. The catenoids were produced by placing thin PS films heated above their Tg between two electrodes, separated from the surface of the film by a small air gap. A voltage was applied across the electrodes to create an electric field that produced electrohydrodynamic instabilities on the surface of the film that led to the formation of catenoids of molten PS that spanned the electrode gap. Semi-catenoids, several mm long, were also made from an ionic liquid by using chemically patterned wafers. SEM and optical microscopy were used to determine the particle location on the catenoid surfaces. The PS catenoids were found to be the most promising experimental system, and particles were observed to locate preferentially along the edges of the catenoid, instead of around the center as predicted.

This thesis work deals with (a) Curing of reactive, hot-melt polyurethane adhesives and (b) Adsorption studies using different interactions. Research on polyurethanes involves characterization of polyurethane prepolymers and a novel mechanism to cure isocyanate-terminated polyurethane prepolymer by a "trigger" mechanism. Curing of isocyanate-terminated polyurethane prepolymers has been shown to be influenced by morphology and environmental conditions such as temperature and relative humidity. Although the initial composition, final morphology and curing kinetics are known, information regarding the intermediate prepolymer mixture is yet to be established. Polyurethane prepolymers prepared by the reaction of diisocyanates with the primary hydroxyls of polyester diol (PHMA) and secondary hydroxyls of polyether diol (PPG) were characterized. The morphology and crystallization kinetics of a polyurethane prepolymer was compared with a blend of PPG prepolymer (the product obtained by the reaction of PPG with diisocyanate) and a PHMA prepolymer (the product obtained by the reaction of PHMA with diisocyanate) to study the effect of copolymer formed in the polyurethane prepolymer on the above-mentioned properties. Although the morphology of the polyurethane prepolymer is determined in the first few minutes of application, the chemical curing of isocyanate-terminated prepolymer occurs over hours to days. In the literature, different techniques are described to follow the curing kinetics. But there is no established technique to control the curing of polyurethane prepolymer. To make the curing process independent of environmental factors, a novel approach using a trigger mechanism was designed and implemented by using ammonium salts as curing agents. Ammonium salts that are stable at room temperature but decompose on heating to yield active hydrogen-containing compounds, NH3 and H2O, were used as 'Trojan horses' to cure the prepolymer chemically. Research on adsorption studies involved making functionalized, thickness-controlled, wettability-controlled multilayers on hydrophobic substrates and the adsorption of carboxylic acid-terminated poly(styrene-b-isoprene) on alumina/silica substrates. Poly(vinyl alcohol) has been shown to adsorb onto hydrophobic surfaces irreversibly due to hydrophobic interactions. This thin semicrystalline coating is chemically modified using acid chlorides, butyl isocyanate and butanal to form thicker and hydrophobic coatings. The products of the modification reactions allow adsorption of a subsequent layer of poly(vinyl alcohol) that could subsequently be hydrophobized. This 2-step (adsorption/chemical modification) allows layer-by-layer deposition to prepare coatings with thickness, chemical structure and wettability control on any hydrophobic surface. Research on adsorption characteristics of carboxylic acid-terminated poly(styrene-b-isoprene) involved syntheses of block copolymers with the functional group present at specific ends. Comparative adsorption studies for carboxylic acid-terminated and hydrogen-terminated block copolymers was carried out on alumina and silica substrates.

Irradiation alters the local stoichiometry of oxides significantly. The resulting stoichiometric changes play a critical role in the dynamics of defects and microstructure evolution in oxides under irradiation. Stoichiometry in oxides is also sensitive to the surrounding oxygen environment. Motivated to study the equilibrium state of UO2, this thesis investigates a theoretical approach to model spatial distribution of defects and charge carriers. In general, the levels of point defects and electronic charge carriers in an oxide are sensitive to the oxygen partial pressure in contact with the oxide and temperature. The objective of this research is twofolded. First, a detailed point defect model based on density functional theory results is devised. The model takes into account multiple charge states for each defect type as well as all the dependencies of the formation thereof. Second, a space-charge analysis is used to find the effect of surface and environment on the spatial variation of concentrations at equilibrium. A surface charge is known to form on the surface as a result of its interaction with the environment. This interaction is explained by the theory of ionosorption. The resulting effect is found by coupling bulk concentrations from the point defect model with the surface charge through solving the electrochemical sytem. The results found by the point defect model are shown to match the experimental data on UO2±x . As for the space-charge analysis, the concentrations of individual defects showed an order of magnitude variation in the subsurface region. This implies the importance of the space-charge effect in any kinetic study of the system since it is controlled by the defect composition at the interface. / A Thesis submitted to the Materials Science and Engineering Program in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester, 2011. / November 7, 2011. / Includes bibliographical references. / Anter El-Azab, Professor Directing Thesis; Petru Andrei, Committee Member; Milen Kostov, Committee Member.

The development of robust thermal barrier coating (TBC) systems is crucial in many high-temperature applications. The performance of a TBC system is significantly limited by microstructural evolution mechanisms, such as sintering at elevated temperatures. Sintering reduces the porosity of TBC and makes it denser which eventually increases the thermal conductivity and reduces the strain compliance of TBC. Understanding how sintering proceeds in TBC systems is thus important in improving the design of such systems. An elaborate phase field model was developed in order to understand the sintering behavior of columnar TBC structure. The model takes into account different sintering mechanisms, such as volume diffusion, grain boundary diffusion, surface diffusion, and grain boundary migration, coupled with elastic strain arising from the thermal expansion mismatch in thermal barrier coating system. Direct relations between model parameters and material properties were established. Such relations facilitate quantitative studies of the sintering process in any material of interest. The model successfully demonstrates a strong dependence of the sintering process in TBC on the initial morphology and dimensions of coatings, strain, and temperature. / A Thesis submitted to the Graduate School in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester, 2011. / October 27, 2011. / Microstructure Evolution, Phase Field Model, Sintering, Thermal Barrier Coatings / Includes bibliographical references. / Anter El-Azab, Professor Directing Thesis; Anke Meyer-Baese, Committee Member; Sachin Shanbhag, Committee Member; Xiaoqiang Wang, Committee Member.

The improvement of electrical energy storage (EES) devices such as batteries and electrochemical capacitors (ECs) is crucial to the widespread adoption of electric drive vehicles and the increased mobility of portable electronics. This research takes a unique approach to the improvement of EES devices through the investigation of a novel nanocomposite system to improve the performance of particle based electrodes. The majority of commercially available batteries and ECs have electrodes fabricated from a powder of fine particles (typically with particle sizes on the order of several 'ms). There is a severe lack of options for transforming these powders into usable electrodes. The traditional electrode fabrication method is to mix the active material powder with a polymer binder to form a sheet or film, which can then be implemented into the device. However, reliance on and incorporation of the polymer binder introduces several disadvantages and performance limitations. In this research, porous networks of carbon nanotubes (CNTs) are investigated to replace the polymer binder in the fabrication of particle based electrodes for electrochemical devices. The multifunctional CNT networks provide the supporting structure and electron conduction pathways to create freestanding and flexible composite electrodes with high electrical conductivities (50 - 100+ S/cm). Two case studies were carried out to explore the properties and performance of the new electrode structure: 1) Activated carbon (aC) particle based electrodes for electrochemical capacitors and 2) Silicon (Si) particle based electrodes for lithium-ion batteries. Samples were fabricated and characterized with an emphasis on obtaining processing-structure-property relationships to guide further development of these unique nanocomposite materials. The aC-CNT electrodes showed specific capacitances of ~50 F/g (in 6M KOH) with less than 10% capacitance loss after 30,000 cycles; demonstrating the ability of the CNT networks to maintain structural integrity during operational conditions. Si-CNT electrodes had high coulombic efficiencies (> 90%) and initial reversible capacities of over 2000 mAh/g. Additionally, fundamental issues are addressed such as possible electrode failure mechanisms and the limits of particle weight fractions that are achievable. Knowledge of the maximum weight fraction of particles obtainable within the CNT networks is important to determine the feasibility of the electrodes for commercial use. A volume-fraction-limited phenomenon is proposed for the mechanism of the particle loading limit and discussed with supporting evidence. / A Thesis submitted to the The Graduate School in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester, 2010. / November 12, 2010. / Includes bibliographical references. / Zhiyong Liang, Professor Directing Thesis; Tao Liu, Committee Member; Hsu-Pin Wang, Committee Member; Jianping Zheng, Committee Member.