UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Photochemistry of Exoplanet Atmospheres: Modelling alien chemistry accurately and self-consistently(2023) Teal, Dillon James; Kempton, Eliza; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Exoplanets offer unique physical and chemical laboratories experiencing entirely alien environments compared to the Solar System planets. Their atmospheres, governed by the same laws of physics, display remarkable diversity and complexity. They serve as the most complex planetary phenomena we can directly observe, coupled to the planet's interior processes, formation environment, the properties of the host star, and complex chemical ecosystems. The art of modelling these systems is a rich field of study, and in this work I study the nature of photochemical models and what understanding they can provide for us based on the quality and breadth of their inputs. By characterizing the implicit uncertainty chemical models have without a well-characterized host star, I quantify the importance of host star characterization to chemical modelling, showing their sensitivity under different reaction schemes and microphysical models. I then apply this to recent observations of known exoplanet host stars LHS 3844 and AU Microscopii. Finally, I cover work to model sub-Neptune atmospheres across a wide parameter space aimed at understanding the influence of a planet's environment and unknowns on haze formation and observational prevalence in emission and transmission spectroscopy.Item LIGHT CONTROL OF CHEMICAL SYSTEMS: PHOTOCHEMICAL ELECTRON TRANSFER METHODS FOR RELEASING CALCIUM IONS AND THE PHOTOISOMERIZATION OF ALKENES TO MODULATE RHEOLOGICAL CHANGES.(2021) Heymann Loor, Romina R; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Our research combines organic photochemistry with the engineering principles of rheology through the study of photorheological fluids (PR). The two photochemical systems researched show changes in the rheological properties brought about by the addition of light. The investigated systems are the photoisomerization of cinnamic acid derivatives in the surfactant, Cetyltrimethylammonium bromide (CTAB), and calcium release through degradation ethylenediaminetetraacetic acid (EDTA) caused by an electron transfer mechanism. The CTAB system shows how a change in molecular conformation can cause significant changes in the bulk property of a solution. The calcium EDTA system employs targeted electron transfer to cause calcium release, which gels the biopolymer alginate with inexpensive, readily available materials. Chapter 2 details how the orientational binding, intermolecular interactions, and molecular geometry of cinnamic acid derivatives contribute to the rheological changes in CTAB. 1H NMR titration studies in CTAB identified binding patterns of the additives in CTAB. From those studies orientational binding models were developed for trans-ortho-methoxycinnamic acid (tOMCA), cis-ortho-methoxycinnamic acid (cOMCA), meta-methoxycinnamic acid (mMCA), para-methoxycinnamic (pMCA), ortho-hydroxycinnamic acid (oCoum), meta-hydroxycinnamic acid (mCoum), and para-hydroxycinnamic acid (pCoum). 1H-1H 2D NOESY spectra identified through space intermolecular interactions occurring within the micelle. Preliminary data into possible π-anion interaction between tOMCA molecules within the micelle is presented. Photolysis confirmed the creation of cis isomers for all additives but also identified coumarin by-products for oCoum. B3LYP calculations indicated out-of-plane geometry for all the cis isomers and possible intramolecular hydrogen bonding of oCoum. Finally, a model of binding interactions that lead to changes in the packing parameter of the surfactant and, therefore, a change into wormlike micelles for tOMCA versus cOMCA is introduced. In chapters three and four, we investigated calcium release using sensitizers that promote photoinduced electron transfer. Anthraquinones derivatives were shown in Chapter 3 to release calcium in stoichiometry amounts with UV light irradiation. In Chapter 4, flavins produced 1000-fold calcium release to sensitizer concentration in the visible light spectrum. In both chapters, there are detailed calcium release studies, degradation studies, and alginate experiments. We present calcium release studies at acidic and neutral pH, quantum yields, degradation of EDTA, sensitizer reoxidation studies, sensitizer degradation data, fluorescence, and transient spectra. While enough calcium was released to produce alginate gels, none were made in vitro at neutral and acidic pH.Item Characterization of Organic Compounds in Hydraulic Fracturing Fluid(2017) Luek, Jenna Lynn; Gonsior, Michael; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past decade, hydraulic fracturing combined with horizontal drilling has become the dominant technique for extracting shale gas in the US, and is increasingly important globally. A complex mixture of chemicals is used in hydraulic fracturing fluid to stimulate natural gas production, but federal regulations have exempted operators from reporting the specific chemicals used in any given well. Recent state-by-state regulations and voluntary disclosures have increased our understanding of these fluids, but knowing this list of chemicals going into a well is just the tip of the iceberg. Once these chemicals are injected, they mix with fluids and naturally occurring chemicals originating in the shale formation and can undergo physical, chemical, and biological transformations. In Chapter 2, I reviewed the literature to date regarding the characterization of organic compounds in injected hydraulic fracturing fluids and waste fluids returning to the surface. I identified a substantial knowledge gap in our understanding of organic compounds in these fluids, particularly non-volatile compounds, and potential transformations within the organic compound pool over the lifetime of the well. I analyzed a number of different shale gas wastewaters using ultrahigh resolution mass spectrometry and identified halogenated organic compounds in these fluids (Ch. 3), suggesting that these compounds were transformation products. Using a time series of shale gas fluids (Ch. 4), I was able to track changes in halogenated organic compounds and find evidence for both biological and chemical transformation pathways. Hierarchical cluster analyses helped identify sulfur-containing transformation products (Ch. 4), and I then determined that sulfur-containing molecules may be useful tracers of shale gas wastewaters in the environment (Ch. 6). In Chapter 5, I used toxicological tests and photoirradiation experiments to track the fate of organic compounds in shale gas wastewaters. Using a primarily non-targeted approach, I have been able to identify a number of organic compounds that are indicative of biological and chemical transformations occurring within hydraulic fracturing fluids and wastewaters. Understanding how these fluids change within the well and during storage and disposal provides critical information for engineering the safe and effective operation of wells, wastewater treatment techniques, and environmental impacts.Item RAPID Lithography: Photopolymerization Characterization and Initiation Kinetics(2012) Stocker, Michael Paul; Fourkas, John T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In order to improve upon the resolution of photolithography, a technique that is used to produce features for today's micro and nanodevices, techniques must move beyond e-beam and deep-UV sources. Multiphoton absorption polymerization (MAP) uses near-infrared light for the creation of complex, three-dimensional features on the sub-100 nm scale. The resolution of MAP can be enhanced further using a two-beam technique called resolution augmentation through photo-induced deactivation (RAPID) to the reach feature sizes as small as 40 nm. The mechanism and kinetics of photo-induced deactivation are not well understood. To better understand these processes, studies of different photoinitiators have been performed. We find that some photoinitiators are so efficient at deactivation that they are capable of undergoing self-deactivation by addition of another photon from the excitation source. This phenomenon is manifested in a polymerization trend in which feature size has a proportional velocity (PROVE) dependence, the opposite of the conventional velocity dependence. We also demonstrate that the velocity dependence can also be tuned between PROVE and conventional dependences. Kinetic models have been formulated to account for the observed deactivation. By reconciling experimental data for some sample photoinitiators with the kinetic model through the use of simulations, kinetic rate constants are determined. The self-deactivation efficiency of each photoinitiator was determined. The lifetimes of intermediates in the radical photopolymerization process were also determined. The kinetic rate constants associated with photoinitiators should allow for the customization of photoinitiators for specific applications and make RAPID a more efficient process capable of reaching resolution on the level of 30 nm and below.Item THE PHOTOCHEMISTRY OF POLYENYL RADICALS AND ITS APPLICATION TO UHMWPE FOR USE IN ARTIFICIAL CARTILAGE(2009) Kasser, Michael Jacob; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of UV light as an alternative to thermal treatments above the melting point (150 °C) to remove free radicals in irradiated UHMWPE was explored. It was found that, in contrast to the allyl free radical which is converted by 258 nm light to alkyl free radicals, polyenyl radicals are not converted to alkyl radicals by UV light. None-the-less, by sandwiching UV light treatments between low temperature thermal anneals (100 °C), it was possible to reduce free radical concentrations by 30%. This reduction was achievable for depths up to one millimeter. However, this reduction did not have a significant effect on oxidation due to an increase in oxidation susceptibility because of the concurrent increase in concentration of easily abstracted allylic hydrogens. By photoirradiating for the optimal amount of time, it was possible, for the first time, to synthesize a polyethylene sample whose residual free radicals consisted of almost entirely dienyl free radicals. This allowed unambiguous identification and simulation of dienyl free radical's EPR spectra to be a singlet containing nine peaks separated by 9 G hyperfine separation. Detailed studies of photoirradiation of UHMWPE containing free radicals revealed that photoirradiation with a continuous spectrum above 200 nm causes the decay of diene unsaturations and allyl free radicals, a reduction in the overall amount of free radicals, and an increase in the degree of unsaturation of polyenyl free radicals. Upon longer photoirradiation times, polyenyl radicals were converted from lower to higher degrees of unsaturation. This effect was identical in the presence and absence of oxygen, but was suppressed by hydrogen gas. These results showed that the conversion does not occur by a linear alkyl radical addition mechanism wherein alkyl radicals migrate to stable polyene unsaturations and polyenyl radicals thereby increasing their order, as previously suggested. The valid mechanism appears to be the direct photoconversion of diene unsaturations to dienyl radicals and lower order polyenyl radicals to higher order polyenyl radicals.