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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
121

Synthesis, characterization and thin film morphology of poly(styrene-block-methyl methacrylate) containing UV photolabile junction points

Goldbach, James T 01 January 2003 (has links)
Diblock copolymers are a class of polymers where two dissimilar polymer blocks are joined together at a common end. The dissimilarity of the blocks causes a phase separation to take place, however the connectivity of the blocks keeps the length scale of this phase separation on the order of the radius of gyration of each of the blocks. Throughout this thesis, the synthesis and morphology of diblock copolymers that have a specific, UV cleavable chemical moiety located at the junction point between the two blocks is investigated. The two junction points targeted to this end are the [4π + 4π] photodimer of anthracene and the 2-nitrobenzyl ester. The diblock copolymer containing the [4π + 4π] photodimer of anthracene exhibits thermal as well as UV lability upon heating above ∼130°C, or UV irradiation at 280 nm. Thin films of this polymer on neutrally-interacting substrates were annealed at 80°C under the presence of supercritical carbon dioxide to avoid thermal degradation that would occur if films were annealed above the glass transition temperature of both blocks (>110°C). The transition from microphase-separated to macrophase-separated morphology is thoroughly investigated by AFM upon thermal and UV cleavage to create blends of diblock and homopolymer in situ. In addition, the selective removal of each polymer block after cleavage by washing with selective solvents is demonstrated. The diblock copolymer containing the 2-nirobenzyl ester moiety is also synthesized and its morphology in thin films is investigated. This diblock, upon cleavage, leaves behind more useful functionality than the diblock with the [4π + 4π] photodimer of anthracene as junction point, as well as being thermally stable at temperatures that allows thermal annealing of thin films. The UV cleavage characteristics of this copolymer in solution are investigated by SEC. Also, irradiation of thin films at a wavelength that does not degrade or cross-link either polymer block causes selective junction point cleavage, as expected. The morphology of UV cleaved thin films on neutrally-interacting substrates is investigated by AFM.
122

Studies of liquid adsorption, condensation and surface conductivity in porous media

Qi, Hao 01 January 2003 (has links)
In the petroleum industry, accurate estimates of hydrocarbon reserve and its producibility are without a doubt among the most important issues. Quantitative estimates require the knowledge of three basic parameters of the rock formation: the porosity &phis;, the water saturation S w and the permeability k. Electrical conductivity is one of the most commonly made measurements used to deduce these quantities. Some empirical relationships used to make such estimates are quite well established and understood, however, many still lack a sound scientific foundation. Systematic laboratory investigation and theoretical understanding of the underlying petrophysics are much needed. This dissertation consists of three projects aimed at understanding both the surface conductivity observed in shaly sandstone, and the related phenomena of molecular adsorption on heterogeneous surfaces. In the first project, we carried out nitrogen adsorption experiments on three shale samples whose fractal dimensions had been previously characterized by small angle scattering (SANS). We found that analyzing the adsorption isotherm data according to the available theoretical predictions always resulted in D values that are lower than those obtained by SANS. The second project, a numerical simulation of adsorption on fractal surfaces, was designed to understand the origin of discrepancies revealed in the first project. We found that the interplay between van der Waals adsorption and capillary condensation always leads to a crossover between the two theoretical limits. The simulated isotherms exhibit the same general features we observed in our experimental data. The third project was aimed at understanding the surface conduction in porous media. We isolated the surface conductivity by growing water layers on the surface with water adsorption isotherm technique. Some of our results indicate that AC impedance measurement could let us determine the surface conductivity and separate it from that of the bulk, thereby more accurate estimates of oil saturation can be achieved in using the empirical Archie's relation. Our studies shed more light on the various aspects of the surface ionic conduction, including Debye - Hückle length, CPA phenomenon, and effects of saturation, cation species, temperature, and substrate.
123

Diblock-copolymer melts at patterned surfaces and dilute polymer solutions under shear

Petera, Dirk 01 January 1999 (has links)
Diblock-Copolymer Melts at patterned surfaces are investigated. Above the order-disorder transition (ODT) of the bulk, density-functional theory is used to calculate the density profile at the surface. Below the ODT self-consistent field theory (SCFT) is employed to extract numerically possible density variations across a thin film. The orientation of the diblocks and lamellar morphology at the surface depends on the ratio of natural lamellar bulk period to surface period [special characters omitted]. The diblocks can be either perpendicular or parallel to the surface, inducing parallel or perpendicular lamellar morphology. Individual polyelectrolytes and their counterions are studied with SCFT. Scaling behavior and density distribution are numerically calculated and compared to known results. Although the scaling roughly agrees with expected results, the density distribution and the end-to-end vector distribution do not. Apparently, fluctuations significantly contribute to angular averaged SCFT of an individual polymer. Dilute polymer solutions under shear are investigated with renormalization group theory of the Gaussian model and with Brownian dynamics simulations of the bead-rod model. The material functions derived from the first model using coupled Langevin equations agree qualitatively with the ones derived from the diffusion equation of the same model. They exhibit shear thickening for high shear rates due to the unrestricted extensibility of the chain. The excluded volume interaction is responsible for shear thinning at intermediate shear rates. The simulations yield shear thinning behavior for all shear rates, independent of the presence or absence of excluded volume or hydrodynamic interactions. A good agreement with experiments is obtained for the relative extension with increasing shear rate. In theta-solutions an unexpected shrinkage of the chain is observed for high shear rates.
124

First-principles model for photoreflectance spectra from strained quantum wells

Mayhew, Laurel Merwin 01 January 1999 (has links)
We present the most complete and accurate first-principles model of photoreflectance (PR) spectra from strained quantum wells currently in the field. We calculate the PR spectrum, [special characters omitted], by taking the difference between reflectivities as a function of quantum well absorption under electric fields representing the pump beam on and off. We calculate the band-to-band quantum well absorption using Fermi's Golden Rule, convolved with a Lorentzian lineshape. The Lorentzian broadened excitonic absorption includes variationally calculated binding energies. A 6 band Luttinger-Kohn k·p bandstructure calculation, including strain and valence-band mixing, provides dispersion relations, matrix elements, and wavefunctions. We compared the calculated PR spectrum for a 100 Å In0.18 Ga0.82As/GaAs single quantum well with the experimental spectrum with excellent agreement. Our results agreed very well with experimental photoluminescence and photoluminescence excitation spectroscopy data and with another group's empirical fit evaluation of the experimental PR spectrum from a similar sample. They identified one heavy-hole state in a particular energy region where we identified two closely spaced heavy-hole states. Our theoretical PR spectra compared favorably with the experimental spectra for two AlxGa 1-xAs/GaAs single quantum wells in the literature. Three other models exist for calculating the PR spectrum from quantum wells. The first PR model was a semi-empirical first derivative calculation. The second calculated the PR spectrum from an excitonic dielectric constant first derivative. Our model calculates both the excitonic and band-to-band absorption for two fields without assuming a derivative form. Our theoretical PR spectra matched these experimental spectra better than their theoretical models did. The third PR model used the band-to-band absorption derivative to calculate the spectrum for a superlattice, which our model was not designed to handle. The agreement between their experimental and theoretical spectra was poor. We determined sensitivity of the PR spectrum to various parameters. The excitonic contribution dominated, but the band-to-band contribution provided a significant background spectrum. The spectrum was fairly sensitive to quantum well width, composition, and conduction band offset. The spectrum was very sensitive to the high electric field (10 kV/cm). This first principles model provides a deeper understanding of the physics of photoreflectance spectroscopy.
125

Transport and phase equilibria of benzene in FAU type zeolites

Saravanan, Chandra 01 January 2000 (has links)
We have studied lattice models for self-diffusion of benzene in FAU type zeolites, to explore the effect of the thermodynamics of confined fluids on the transport properties of molecules in zeolites. Our model assumes that benzene molecules are located near Na+ ions in supercages, and in 12-ring windows separating adjacent supercages, respectively. The study was performed in three stages. First, to disentangle the effect of a vapor-liquid phase equilibria on diffusion in zeolites, the transport of benzene in Na-Y is modeled in the absence of attractive guest-guest interactions. The loading dependence of diffusion coefficient, D&thetas;, at a constant temperature, referred to as a diffusion isotherm, is modeled with site-blocking effects using a mean field theory (MFT) that yields,[special characters omitted] where a&thetas; ≅ 11 Å is the mean intercage jump length and 1/k&thetas; is the mean supercage residence time. A completely analytical expression is derived to calculate k&thetas;. The MFT is tested using a mean field approximation (MFA) where k&thetas; and a&thetas; are calculated from kinetic Monte Carlo simulations yielding excellent qualitative agreement. Further calculations are performed to test MFA by calculating “exact” diffusion coefficients from mean square displacement (MSD) calculations also yielding excellent qualitative agreement. Next, by including guest-guest attractive interactions, we have performed lattice grand canonical Monte Carlo simulations of benzene adsorption in Na-X zeolite to determine whether strongly confined benzene molecules exhibit subcritical properties. We observe a phase transition from low to high density of adsorbed benzene, analogous to vapor-liquid equilibrium, at temperatures as high as 300 K and above. By performing thermodynamic integration to construct the coexistence curve, we obtain a critical point for benzene in Na-X at Tc = 370 ± 20 K, &thetas;c = 0.45 ± 0.05 fractional coverage. We suggest that careful adsorption experiments should be performed to reveal this vapor-liquid transition. Finally, we study the influence of adsorbate coupling on the self-diffusion of benzene in Na-X and Na-Y zeolites. We propose a simple model for determining how adsorbate-adsorbate interactions modify activation energies of site-to-site jumps. We have calculated diffusion isotherms for a wide range of system parameters at different levels of theory viz., MFT, MFA and MSD, and segregated the resulting diffusion isotherms into supercritical and subcritical isotherms. The supercritical systems exhibit three characteristic diffusion isotherms, depending upon the degree of degeneracy of lattice sites, whereas the subcritical diffusion systems are dominated by cluster formation, exhibiting diffusion isotherms with broad regions of constant diffusivity. Our model for benzene in Na-X is in excellent qualitative agreement with pulsed field gradient NMR diffusivities, and in qualitative disagreement with tracer zero-length column (TZLC) data. We suggest that high temperature TZLC experiments should be performed, to test whether the coverage of maximum diffusivity decreases with increasing temperature.
126

Hierarchical organization in polymeric systems

Shin, Dongseok 01 January 2007 (has links)
Hierarchical assembly of materials has attracted significant interest, since it provides opportunities to fabricate novel materials. In this thesis, we investigated three different systems where polymer chains organize hierarchically. First, a semicrystalline triblock copolymer, poly(L-lactic acid- b-ethylene oxide-b-L-lactic acid) (PLLA- b-PEO-b-PLLA), was prepared and the effect of the block-wise construction on the sequential crystallization was investigated by comparison to the corresponding homopolymer blend. In the resultant spherulitic morphology, the crystallization of PEO occurred within the framework established by the PLLA crystals. The preformed PLLA crystals biased the PEO chain orientation and the effect was more significant in the block copolymer system, where PEO chains were covalently anchored to PLLA. Secondly, the influence of the microenvironment of multifunctional chains on their organization was studied. For this investigation, styrene-based linear polymers having two different pendant groups, a carboxylic acid and a neutral group, on every repeat unit were prepared. With alkyl (n-C 10H21-) groups as the neutral pendant, the linear macromolecules assembled into thermally reversible globular aggregates through non-covalent interaction with multifunctional tertiary amines. The aggregates had a structural hierarchy and remained stable without inter-particle crosslinking. In the absence of the alkyl pendant groups, control over the structure and properties of the aggregates was lost. In a third system, the coupled self-assembly of bionanoparticles and block copolymers was investigated. A simple way to incorporate bionanoparticles into a thin film of water-insoluble block copolymer was developed by combining the bionanoparticle adsorption on a polymer film and subsequent annealing under solvent vapor. Through the use of a block copolymer having a positively charged component, the loading of bionanoparticles increased significantly. When highly loaded, a hierarchical co-assembly of the block copolymer and the bionanoparticle was observed where the microphase separation of the block copolymer forced a segregation of bionanoparticles to the grain boundaries, forming a much larger scale structure.
127

STM study of lead and bismuth structures on copper(001)

Chen, Yun 01 January 1996 (has links)
This work studies the stable room temperature structures of metal on metal overlayers. The purpose of this study is twofold. First, the advent of the atomic imaging technique of Scanning Tunneling Microscopy (STM) makes it now possible to directly observe complicated surface structures. This was demonstrated by the success of unraveling the (7$\times$7)-Si(111) surface structure that had withstood the storm of investigation for over 20 years. We wanted to apply this technique to the study of metallic overlayer structures, a field known for its large variety of structures. The second purpose was to investigate the feasibility and to hopefully develop a simple intuitive approach to metallic overlayer structure prediction, an approach that incorporates explicitly the extended nature of the metallic electron system. The Pb and Bi systems have large numbers of valence electrons per atom. Therefore, the electronic effects are expected to be prominent. The difference between the numbers of valence electrons of the respective atoms is relatively small but significant. This provides an opportunity to observe a quantitative variation of the effects. This study constitutes the first STM investigation of the ordered Pb and Bi structures on Cu(001). Many structural features observed using this technique disagree with the existing models derived from the electron diffraction and other scattering experiments. The differences between our observation and the current structural models are qualitative ones, thus demanding a rethinking and supplementation of the basic ideas involved in the construction of the structural models. Observation of the great similarities between the adsorbed Pb and Bi systems, lead to the new modeling approach. In our discussion, the metal adsorbate layers are described as a simple quasi-two-dimensional metal weakly coupled to the host substrate. This is consistent with the familiar notion that all metals exhibits similar properties. Our detailed analysis also discusses the Brillouin zone mechanism as a new way of lowering the energy of a given structure. In the end it enabled us to justify the occurrence of all the ordered Pb and Bi structures on Cu(001). We propose this simple model as a general approach to predict stable structures of metallic overlayers.
128

Polymers on nanoperiodic, heterogeneous surfaces

Rockford, Lee David 01 January 2001 (has links)
Herein we establish a relationship between controlled nanoscale surface interactions and subsequent macromolecular ordering. Chemically heterogeneous striped surfaces of polar silicon oxide and non polar gold are generated over large areas, via glancing angle evaporation on facetted silicon substrates. The processing conditions required for generation of stripe widths comparable to the size of a polymer molecule are outlined. Substrates with 20–30 nm metal linewidths and 40–60 rim stripe periods are prepared. Spin and solution casting of incompatible polymer mixtures of polystyrene (PS) and polymethylmethacrylate (PMMA) on heterogeneous surfaces are found to generate films with unique, substrate directed morphologies dependant on the kinetics of the casting process. Spin cast films posses a surface adsorbed layer of blended composition due to rapid polymer adsorption from solution, while solution cast films phase separate at the substrate/polymer interface on a molecular level. Preferential adsorption of PS to the non polar gold stripes and PMMA to polar silicon oxide stripes is observed at the substrate beneath the macroscopically phase separated domains of the blend components. Preferential adsorption occurs over a large molecular weight range, with a molecular weight dependence on the morphology of the adsorbed polymer lines found. Solution cast films of the symmetric copolymer poly(styrene-block-methylmethacrylate), P(S-b-MMA), on heterogeneous surfaces show lamellar microdomain orientations perpendicular to the substrate plane, parallel to the striping. Commensurability of the block copolymer and substrate stripe periods is found to be essential for producing such a surface directed morphology. The commensurability window depends inversely on the degree of confinement of the morphology, with unconfined films requiring more stringent conditions for surface directed morphology reorientation. The distance over which the orientation of the microdomains persists in thick films is found to depend on the ordering kinetics, scaling with copolymer molecular weight. Confinement effects such as tension and compression and defects in the lateral long range orientation of surface directed lamellar morphologies are observed for slightly incommensurate morphologies, with the amount of strain and defect concentration found to increase with the loss of commensurability.
129

Monte Carlo studies of ionized impurity scattering in silicon and silicon-germanium alloys

Kay, Leonard Edward 01 January 1991 (has links)
An improved Monte Carlo model for ionized impurity scattering is developed and applied to transport problems in Si and the Si$\sb{1-x}$Ge$\sb x$ alloy system. The model includes scattering cross sections derived from phase-shift analysis, implementation of the Friedel Sum Rule, and a simple phenomological model for multiple-potential scattering. Using a single adjustable parameter, majority and minority electron mobilities are calculated for Si and fit to experimental data. Experimental results for Si of $\mu\sb n(N\sb A)/\mu\sb n(N\sb D) \approx$ 2 at 300 K are reproduced and a value of 3 $< \mu\sb n(N\sb A)/\mu\sb n(N\sb D) <$ 4 is predicted at 77K. Low-field mobilities are then calculated for both strained and unstrained Si$\sb{1-x}$Ge$\sb x$ over wide ranges of doping, Ge mole fraction, and electric field, at 300K and 77K. A significant improvement in mobility (up to 50%) is observed for transport perpendicular to the growth plane in strained Si$\sb{1-x}$Ge$\sb x$, especially at 77K. High field MC simulations show that some strained mobility enhancement remains even at an electric field of 100 kV/cm. The improved model is then used in both DDE and Monte Carlo simulations at 300K and 77K of two strained-layer n-p-n Si$\sb{1-x}$Ge$\sb x$ HBTs with basewidths of 1000 A and 650 A and maximum Ge contents of 15% and 10% respectively. We find that as a result of improved mobility in the base and collector and velocity overshoot in the high field region, $h\sb{fe}$ and $f\sb T$ are improved significantly for the strained 650 A basewidth HBT as compared to a similar unstrained structure, especially at 77K.
130

Direct imaging of deformation and disorder in extended-chain polymers

Martin, David Charles 01 January 1990 (has links)
The rigid-rod polymers poly(paraphenylne benzobisthiazole) (PBZT) and poly(paraphenylene benzobisoxazole) (PBZO) can be spun from lyotropic liquid-crystalline solutions into solid fibers with extraordinary mechanical properties. However, these fibers are comparatively weak in compression, with deformation occurring by strain localization into kink bands. Here, we examine the ultrastructure of PBZT and PBZO fibers as a function of processing condition. In particular, High Resolution Electron Microscopy (HREM) is used to directly image structural features such as grain boundaries, dislocations, and the molecular level details of deformation processes. HREM images of PBZT and PBZO enable the quantitative determination of crystallite size, shape, orientation, and internal perfection as a function of processing condition. The nature of the disorder present within and between PBZT and PBZO crystallites is analyzed, modeled, and compared to analyzed, modeled, and compared to experimental Wide Angle X-ray Scattering (WAXS) and Selected Area Electron Diffraction (SAED) data. HREM images in and around PBZT and PBZO kink bands reveal that local, sharp bending and/or breaking of covalently bonded molecules is involved in compressive failure. A model for kinking is proposed which involves the nucleation and growth of a region of shear deformation bounded by partial edge dislocations. A stress analysis using this model is used to correlate systematic morphological features of kinks with local material instabilities. A quantitative analysis of the linear density of kinks with a fiber as a function of applied plastic strain enables the energy required to form a kink to be determined. The geometry and energetics of grain boundaries in extended chain polymer solids is discussed. Possible mechanisms for grain boundary motion are presented. Comparisons between different grain boundary structural models and experimental HREM data are shown.

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