NANOSTRUCTURE INVESTIGATION OF POLYMER SOLUTIONS, POLYMER GELS, AND POLYMER THIN FILMS

This thesis discusses two systems. One is structured hydrogels which are hydrogel systems based on crosslinked poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) containing micelles which form nanoscale pores within the PDMAEMA hydrogel. The other is nanoporous block copolymer thin films where solvent selectivity is exploited to create nanopores in PS-b-P4VP thin films. Both of these are multicomponent polymer systems which have nanoscale porous structures. 1. Small angle neutron scattering of micellization of anionic surfactants in water, polymer solutions and hydrogels Nanoporous materials have been broadly investigated due to the potential for a wide range of applications, including nano-reactors, low-K materials, and membranes. Among those, molecularly imprinted polymers (MIP) have attracted a large amount of interest because these materials resemble the "lock and key" paradigm of enzymes. MIPs are created by crosslinking either polymers or monomers in the presence of template molecules, usually in water. Initially, functional groups on the polymer or the monomer are bound either covalently or noncovalently to the template, and crosslinking results in a highly crosslinked hydrogel. The MIPs containing templates are immersed in a solvent (usually water), and the large difference in the osmotic pressure between the hydrogel and solvent removes the template molecules from the MIP, leaving pores in the polymer network containing functionalized groups. A broad range of different templates have been used ranging from molecules to nanoscale structures inclucing stereoisomers, virus, and micelles. When micelles are used as templates, the size and shape before and after crosslinking is an important variable as micelles are thermodynamic objects whose structure depends on the surfactant concentration of the solution, temperature, electrolyte concentration and polymer concentration. In our research, the first goal is to understand the micellization of anionic surfactants in polymer solutions and the corresponding hydrogels using small angle neutron scattering (SANS). SANS has been widely used to investigate structures ranging from sub-nanometer to sub-micrometer. Since the scattering lengths of H and D atoms are quite different, the scattering contrast can be enhanced (and varied) through isotopic labeling. It is possible to investigate the structure of micelles in polymer solutions and hydrogels using H/D contrast matching methods with SANS. For this aim, water-soluble and chemically crosslinkgable poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) was synthesized using group transfer polymerization. In order to control the size and shape of micelles, the degree of quaternization of the polymer was also controlled through the reaction of PDMAEMA with methyl iodide. The micellization of deuterated sodium dodecylsulfate (d-SDS) in (quaternized) PDMAEMA solutions and the corresponding hydrogels was then observed using SANS and the size and shape of d-SDS micelles was obtained by modeling. 2. Nanopatterning using block copolymer/homopolymer blends Block copolymers are well-known to self-assemble into meso- and nano-scale structures. The use of block copolymers for nanostructured patterns has attracted increasing attention due to their potential use as templates and scaffolds for the fabrication of functional nanostructures. In order to realize the potential of these materials, it is necessary to be able to control the orientation of the nanoscale pattern in a precise manner. Numerous methods such as manipulation of the interfacial surface energies, use of electric fields, and controlling the rate of solvent evaporation have developed to control orientation. In addition, it has been shown that nanopores within cylindrical domains oriented normal to the substrate can be generated by several methods. For example, one component can be degraded by UV exposure, or the homopolymer in a block copolymer/homopolymer blend can be extracted in a selective solvent. In our work, polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP)/poly(4-vinylpyridine) (P4VP) films on silicon substrates were prepared using spincoating. The homopolymer was then extracted in ethanol generating pores perpendicular to the substrate. It is noted that the pore size and density were readily controlled by the amount of P4VP homopolymer in the PS-b-P4VP/P4VP solutions, giving simple control of the film structure. It was also possible to make pores more uniform and ordered by annealing in solvent vapor before extracting the homopolymer

Analysis of Stefan Problem with Level Set Method

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76235/1/AIAA-2002-2874-847.pd

Spinor representation in isotropic 3-space via Laguerre geometry

We give a detailed description of the geometry of isotropic space, in parallel to those of Euclidean space within the realm of Laguerre geometry. After developing basic surface theory in isotropic space, we define spin transformations, directly leading to the spinor representation of conformal surfaces in isotropic space. As an application, we obtain the Weierstrass-type representation for zero mean curvature surfaces, and the Kenmotsu-type representation for constant mean curvature surfaces, allowing us to construct many explicit examples.Comment: 30 pages, 9 figure

Essays on Product Recalls, New Product Preannouncements, and Shareholder Value

Product recalls are widely recognized as a manufacturer’s worst nightmare. They put the value generated by product innovation at risk. In my dissertation, I investigate the factors that determine the effects of product recalls on firm value and the contingent influence of product recalls on the effects of new product preannouncements on firm value. In the first essay, I examine the determinants of the short- and long-term effects of product recalls on firm value. The findings offer important insights. First, while brand advertising has a significantly negative effect on short-term abnormal returns to product recall announcement, but not in the long run, especially when the recall involves a large number of vehicles. Second, advertising is positively associated with long-term shareholder value to product recall announcements. Third, diligent response to the recall (post-recall preparation) also has a positive effect on long-term shareholder value. Finally, although the voluntary recall initiation strategy has a significantly negative effect on short-term abnormal returns to product recall announcement, it has significantly positive effect on long-term firm value. Thus, the results suggest that managers should advertise judiciously, prepare post-recall response diligently, and initiate recalls to mitigate the negative impact of the product-harm crises. In the second essay, I focus on the determinants of the effect of new product preannouncements on short-term shareholder value changes in an environment characterized by frequent product recalls. The findings indicate that product recalls reduce the significant short-term abnormal financial returns from new product preannouncements. The results show that the product recalls can dampen the effect of new product innovativeness on the short-term abnormal returns to new product preannouncements. However, advertising spending during product recalls can attenuate the negative effects of product recall volume on short-term returns to new product preannouncements. The findings offer managers clear guidelines on when to preannounce new products and on how to manage advertising amid product recalls to realize greater financial value from new product preannouncements

Effect of double-side welding on the microstructural characteristics and mechanical performance of dissimilar AA6061-AA5052 aluminium alloys

In this study, cold metal transfer (CMT) based double-side welding process was employed to weld AA5052-H32 and AA6061-T6 plates having a thickness of 6 mm. The microstructure and mechanical integrity of the weldment was examined systematically. Symmetric and defect free joint with full penetration was achieved. The multiple heating and cooling cycles during CMT welding did not affect the weldment while the microstructure comprised of columnar and equiaxed dendrites and the heat affected zone (HAZ) width was < 50 µm at all interfaces. Precipitates such as Al3Mg2, Mg2Si, and α-Al(FeMn)Si were observed in the fusion zone. Intergranular Al-Si eutectic structure along with few microliquefaction cracks were noticed in the α-Al matrix. The maximum tensile properties for double-side welded joint were 214 MPa and 12.30 % with a joint efficiency of 70.68 %. Tensile specimens underwent ductile fracture in the weaker AA5052-H32 side with confined plastic deformation

Investigation of switching mechanism in Ta2_{2}O5_{5}-based ReRAM devices

Redox-based Restive Random Access Memory (ReRAM) has recently receivedstrong attention due to its potential payout toward high density, low-cost,low-energy NVMs. Development and understanding of Ta$_{2}$O$_{5}$ based ReRAM devices in this research work have been made under following experiments, (i)Ta$_{2}$O$_{5}$ switching layer, (ii) ohmic electrode, (iii) Forming-free ReRAM devices, (iv)ReRAM and MOSFET integration, and (v) implementation of modular arithmetic function. In order to optimize the Ta$_{2}$O$_{5}$ switching layer, various approaches such as the effects of RF sputtering power in Ta$_{2}$O$_{5}$ deposition, the thickness effect of Ta$_{2}$O$_{5}$ switching layer, and the Bi-layer (Ta$_{2}$O$_{5}$ /TaO$_{x}$) structure have been made. The optimized 7 nm-thick Ta$_{2}$O$_{5}$ ReRAM device shows lower VFORM (1.8 V), reasonable V$_{S}$ET (0.8 V) with large memory window (R$_{OFF}$ /R$_{ON}$ >300 at V$_{RESET−STOP}$ =-2.0 V), stable endurance up to 106 cycles (@1.0 μs) and goodretention at 125 $^{\circ}$C for 10$^{4}$ seconds. Further, defect density in the switching oxide can also affect the switching properties of ReRAM devices and a modulation of defect density is possible by deposition rate variation. The layer deposition rate changes depending on RF sputtering power of Ta$_{2}$O$_{5}$ layer. The best RF power condition (236W) at given layer thickness (7 nm) was found in terms of memory window (R$_{OFF}$ /R$_{ON}$ > 800 at V$_{RESET−STOP}$ = -2.2 V) with high reliability (retention and endurance) performance. By introducing optimal Bi-layer (Ta$_{2}$O$_{5}$ /TaO$_{x}$) stack in Ta$_{2}$O$_{5}$ ReRAM device, the R$_{OFF}$ performance further improves with 7.0 nm-thick Ta$_{2}$O$_{5}$ / 20 nm-thick TaO$_{x}$ at low I$_{CC}$ level (50 μA) due to a better control of defects in the Ta$_{2}$O$_{5}$ switching layer. However, the V$_{FORM}$ of the Bi-layer increases from 1.8 V to 3.8 V in spite of highly conductive nature of TaO$_{x}$ layer. [...

Bilateral information asymmetry and irreversible practice adoption through agri-environmental policy: an application to peat land retirement in Norway

Bilateral information asymmetry and irreversible practice adoption through agri-environmental policy: an application to peat land retirement in Norwa