Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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 give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
20 results
Search Results
Item DEVELOPMENT OF GLYCOSAMINOGLYCAN MIMICKING NANOGEL TECHNOLOGIES FOR CONTROLLED RELEASE OF THERAPEUTICS TO TREAT RETINAL DISEASES IN DIFFERENT AGE GROUPS(2024) Kim, Sangyoon; Lowe, Tao L.; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Retinal diseases, such as diabetic retinopathy, glaucoma, macular degeneration, and retinoblastoma, affect around 13 million people worldwide, with projections indicating a rise to 20 million by 2030. These conditions lead to irreversible vision loss and significant impairment in both adults and children, with an annual economic burden of $139 billion in the United States alone. Aging significantly increases the risk of certain retinal conditions, and with improvements in healthcare leading to increased life expectancy, these conditions are becoming more prevalent due to the natural aging process and associated physiological changes in the eye. Current treatments are either destructive or have low efficacy and are not optimized for the younger population. While therapeutics including small molecular drugs, proteins and antibodies show promise in treating these diseases by reducing inflammation and neuronal apoptosis, their effectiveness is hindered by short half-lives and inability to cross the blood-retinal barrier (BRB). Nanoparticles offer a potential solution by improving drug delivery across biological barriers, yet no nanoparticles have been developed to effectively transport intact proteins or small molecules across the BRB to the retina without toxicity, slow clearance and stability. Therefore, there is an unmet need to evaluate the physical and physiological property changes of the eye along development and develop nanoparticle systems that can control and sustain the release of therapeutics across the blood retinal barrier (BRB) to treat the retinal diseases. In this project, the thickness, rheological property, permeability and morphological property changes of ocular barriers including sclera, cornea and vitreous humor in the developing eye from preterm to adult were evaluated using porcine ex vivo model. Two glycosaminoglycan mimicking nanogel systems, poly(NIPAAm-co-DEXcaprolactoneHEMA) nanogels with and without positive or negative charges and β-cyclodextrin based poly(β-amino ester) (CD-p-AE) nanogels were developed for sustained release of intact proteins including insulin and anti-TNFα, and small hydrophobic drugs, respectively across the ex vivo porcine sclera and in vitro BRB models: human fetal retinal pigment epithelial (hfRPE), adult retinal pigment epithelial (ARPE-19) and human cerebral microvascular endothelial (hCMEC/D3) cell monolayers. Completion of this project will have a significant impact on developing novel personalized nanotherapeutics to treat retinal diseases in different age groups.Item Radiation-Induced Modification of Aramid Fibers: Optimizing Crosslinking Reactions and Indirect Grafting of Nanocellulose for Body Armor Applications(2022) Gonzalez Lopez, Lorelis; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of this dissertation was to design, synthesize, and analyze novel aramid fibers by covalently grafting nanocellulose through electron beam irradiation. These nanocellulose functionalized fibers showed enhanced strength and larger surface areas, which improves their performance and applicability in fiber-reinforced composites. Unmodified aramid fibers have smooth and chemically inert surfaces, which results in poor adhesion to many types of resins. Nanocellulose was chosen as the ideal filler to functionalize the fibers due to its reactive surface and high strength-to-weight ratio. Aramid fibers were further modified by radiation-induced crosslinking reactions as a means to avoid scission of the polymeric backbone and to further increase the fiber strength.An indirect radiation-induced grafting approach was used for synthesizing these novel nanocellulose-grafted aramid fibers while avoiding the irradiation of nanocellulose. The fibers were irradiated using the e-beam linear accelerator (LINAC) at the Medical Industrial Radiation Facility (MIRF) at the National Institute of Standards and Technology (NIST). After the irradiation, the fibers were kept in an inert atmosphere and then mixed with a nanocellulose solution for grafting. The grafted fibers were evaluated by gravimetric analysis, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) spectroscopy. The mechanical properties of the synthesized fibers were studied by single fiber tensile tests. Aramid fibers were also irradiated at the MIRF in the presence of acetylene gas and triacrylate solution as a means to induce crosslinking reactions. These fibers were irradiated at both low doses and high dose rates at room temperature. A mechanism for the crosslinking of aramid fibers was proposed in this dissertation. Mechanical testing of the fibers after crosslinking showed an increase in the strength of the fibers of up to 15%. Ultra-high molecular weight polyethylene (UHMWPE) fibers were also studied, but due to an issue of entanglement of the fibers during the grafting process, their mechanical properties could not be analyzed. Future work will focus on using a better set up to avoid entanglement of these fibers. To complete the study of the radiation effects on polymers, this thesis explored the radiation-induced degradation of aromatic polyester-based resins. The composition of the resins studied included phenyl groups and epoxies, which complicate radiation-induced grafting and crosslinking reactions. Unlike aramid and polyethylene fibers, polyester-based resins have a C-O-C bond that is susceptible to degradation. The resins were irradiated at high doses in the presence of oxygen. The scission of the polymeric backbone of the polymers was studied using Electron Paramagnetic Resonance (EPR) analysis. EPR showed the formation of alkoxyl radicals and C-centered radicals as the primary intermediate products of the C-O-C scissions. The degradation mechanisms of the resins in the presence of different solvents were proposed. Changes in the Tg of the polymers after irradiation, as an indication of degradation, were studied by Dynamic Mechanical Analysis (DMA). The results obtained from this work show that irradiation of these resins results in continuous free radical-chain reactions that lead to the formation of recyclable oligomers.Item The Hydrogel Reimagined: Gel-Derived Sponges and Sheets as Absorbents for Water, Blood, and Oil(2022) Choudhary, Hema; Raghavan, Srinivasa R.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer hydrogels, i.e., crosslinked networks of polymer chains swollen in water, are well-studied materials. Superabsorbent polymer (SAP) gels that can absorb more than 100x their dry weight in water are widely used in personal hygiene products – but only in the form of microscale beads. If dry SAP gels were larger, they would either take too long to swell or would be brittle solids. This dissertation seeks to reimagine polymer gels in very different physical forms: as soft sponges or foldable, fabric-like sheets. We want these macroscale dry materials to retain the ability to absorb large amounts of liquid, either aqueous or organic. In short, we would like to make polymer gels in convenient, usable forms similar to everyday absorbents like towels and sponges. The key to making gels as macroscale absorbents is to make the gels porous. In our first study, we devised a way to create porous gels by foam-templating. The approach involves in situ foaming of a monomer solution followed by fast polymerization. We generate the foams using a double-barrelled syringe that has acid and base in its two barrels. Gas (CO2) is formed at the mixing tip of the syringe by the acid-base reaction, and gas bubbles are stabilized by an amphiphilic polymer in one of the barrels. The monomers are then polymerized by ultraviolet (UV) light to form the gel around the bubbles, and the material is dried under ambient conditions to give a porous solid. We show that this dry, porous gel absorbs water at a rate of 20g/s until equilibrium is reached at ~ 300x of its weight. This is the fastest swelling and expansion ever achieved by a hydrogel. We convert the chemical potential energy from gel expansion into mechanical work: the gel is able to lift weights against gravity, with a power-density of 260 mW/kg. Next, we synthesize porous gels in the form of large sheets that resemble cloth or paper towels. For this, we polymerize thin films of the foams and ambient-dry the films after plasticization. Our gel sheets are flexible, foldable, and can be cut with scissors like fabrics. At the same time, the sheets absorb more than 30x of their dry weight in various aqueous fluids (water, blood, polymer solutions). Remarkably, these gel sheets expand as they absorb water, unlike any commercial towels. The expanded sheets retain absorbed fluid when lifted upright whereas fluid drips out of commercial absorbent sheets. Because of these superior properties, our gel sheets could be used to absorb aqueous liquids in various settings such as homes, labs and hospitals. Lastly, we design oleo-sheets, which are counterparts to the above that can absorb oils, i.e., non-polar liquids. We synthesize oleo-sheets by templating foams in which the continuous phase is non-aqueous and contains hydrophobic monomers. The oleo-sheets are hydrophobic and can selectively absorb oil from water. They show a high absorption capacity (> 50 g/g) for a range of organic solvents. The sheets can also be made magnetically responsive and an oil-soaked oleo-sheet can be lifted up by a magnet. We also fabricate a ‘Janus omni-absorbent sheet’ that has two sides: one side selectively absorbs water while the other side absorbs oil/solvents. Our oleo-sheets and omni-absorbent sheets could both be used in homes, hospitals, and various industries for cleaning up different spilled liquids.Item Effect of electrostatic interactions on biomolecular self-assembly processes(2018) Xu, Hongcheng; Matysiak, Silvina; Biophysics (BIPH); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Molecular level self-assembly processes are not only ubiquitous in living cells, but also widely applied in industry to synthesize and fabricate a variety of nanoscale biomaterials. The emergence of ordered aggregates from disordered components typically requires driving forces from electrostatic interactions to hydrophobic-hydrophilic effects. This thesis aims to elucidate the effect of electrostatic interactions, and the intricate balance between electrostatic and hydrophobic interactions in dictating spontaneous self-assembly processes with three case studies covering various types of biomolecules. For the first case study, we have examined the pH-induced polysaccharide hydrogel network formation. The polysaccharide molecule chitosan forms hydrogels composed of water-filled cross-linking polymer chains. The pH-responsive selfassembly behavior of chitosan hydrogel has been utilized in fabricating nanomaterials with a wide range of applications. To investigate the role of electrostatic interactions in the chitosan hydrogel network formation, we have developed a novel coarse-grained (CG) chitosan polymer model that captures the pH-dependent self-assembly behavior. The structural, mechanical, and thermodynamical properties of chitosan polymer hydrogel have been characterized well in the simulations and agree very well with experimental observations. For the second case study, the anticancer peptide folding induced by phospholipid membrane was investigated. Peptide folding in an aqueous environment is a self-assembly process that has been well studied over the years. However, the folding in a membranous environment is complicated by the heterogeneity in phospholipid distributions and membrane-peptide interactions. To provide information about the driving forces behind membrane peptide folding and the effect of lipid composition on folding behavior, my work has combined our recently developed Water-Explicit Polarizable Protein (WEPPRO) and Membrane (WEPMEM) model to explore the driving forces behind model anticancer peptide SVS-1 folding and how they can be affected by changing the membrane composition. For the third case study, we have studied the formation of nanodomains in mixed lipid bilayers. Phospholipid membranes are essential components in animal cells. The heterogeneous distribution of phospholipids on the membrane bilayer plays an important role in cellular structure and function such as signal transduction and membrane fusion. Interactions between a mixture of lipids and different ligands give rise to interesting patterns that are yet to be understood. Model lipid bilayers with a content of anionic lipids have been shown experimentally to be sensitive to the presence of certain ions. Monovalent cation Li+ induces membrane phase transition similarly as Ca2+ and Mg2+, while distinctive from other monovalent cations like Na+ and K+. We have evaluated the role of electrostatics interactions in the sizedependent cation-induced lipid nanodomain formation with binary mixed bilayers composed of zwitterionic and anionic lipids.Item Synthesis and Characterization of Hemostatic Foams(2018) Rudy, Michael; Raghavan, Srinivasa; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrophobically modified (hm) derivatives of biopolymers like chitosan have the ability to coagulate blood cells and thus stop bleeding from severe injuries (i.e., achieve hemostasis). Our lab has been particularly interested in developing foams based on these polymers for use as hemostastic agents. Foams are attractive because an expanding foam at a wound site can counteract blood loss without the need for mechanical compression. The amphiphilic nature of hm-polymers also enables them to stabilize such foams. Previous work centered around a foam based on hm-chitosan (hmC) that was delivered out of a canister. To effectively combat internal hemorrhaging, we recognized the need to develop foams that could be more precisely placed at the wound site and also had greater mechanical integrity. Towards this end, this thesis describes a new class of polymeric foams that can be delivered out of a double-barreled syringe (DBS) by combining precursors in the two barrels that produce bubbles of CO2 gas in situ. Moreover, we show that by combining hmC in one barrel with a second biopolymer – hm-alginate (hmA) – in the other, we can generate foams with enhanced rheological properties compared to foams of hmC alone. This rheological enhancement is quantified in our work, and is due to electrostatic interactions between the cationic hmC and the anionic hmA chains. Preliminary studies in animal wound models also confirm that hmC-hmA foams can be precisely directed to a wound using the DBS and that these foams form effective barriers to blood loss due to their greater mechanical integrity.Item ENHANCED PRODUCTION OF BIOSOLIDS BY IMPROVED ACTIVATED SLUDGE CLARIFICATION AND STRUCTURED WATER ANALYSIS(2017) Liu, Xiaocen; Kjellerup, Birthe Veno; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Biosolids contain high contents of soil-required nutrients, so that have been widely applied in land application. The production of the biosolids depends on the clarification performance and dewaterability of the sludge, which are influenced by bioflocculation and structured water content, respectively. Therefore, research on sludge bioflocculation improvement and structured water content determination were proposed in this study. The result indicated that different activated sludge exhibited various bioflocculation limitations. Influence of the sludge characteristics such as the extracellular polymeric substances (EPS) composition, viscosity and floc size on the structured water content were also investigated. The results indicated that no significant correlation was observed between the EPS composition and the structured water content, however, the sludge floc size was positively correlated with it. The bioflocculation limitations were pinpointed, and how floc size influenced the structured water content needed further studies to improve sludge dewaterability, therefore, enhance the biosolids production quantitatively and qualitatively.Item Experimental Modeling of Twin-Screw Extrusion Processes to Predict Properties of Extruded Composites(2016) Dryer, Benjamin; Bigio, David I; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Twin-screw extrusion is used to compound fillers into a polymer matrix in order to improve the properties of the final product. The resultant properties of the composite are determined by the operating conditions used during extrusion processing. Changes in the operating conditions affect the physics of the melt flow, inducing unique composite properties. In the following work, the Residence Stress Distribution methodology has been applied to model both the stress behavior and the property response of a twin-screw compounding process as a function of the operating conditions. The compounding of a pigment into a polymer melt has been investigated to determine the effect of stress on the degree of mixing, which will affect the properties of the composite. In addition, the pharmaceutical properties resulting from the compounding of an active pharmaceutical ingredient are modeled as a function of the operating conditions, indicating the physical behavior inducing the property responses.Item SYNTHESIS AND CONTROL OF MORPHOLOGY OF POLY METHYL METHACRYLATE AND POLY ACRYLIC ACID MICRO-PARTICLES BY THE MODIFIED SUSPENSION POLYMERIZATION TECHNIQUES(2015) Kim, Yunju Jung; Choi, Kyu Yong; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis studied the synthesis and control of morphology of two kinds of polymers by modified suspension polymerization techniques. The first polymer, poly (methyl methacrylate), is a transparent thermoplastic polymer, which is typically used in diffusing film in the backlight unit of an LCD. Also, the synthesis of micron-sized polymer particles with complex internal morphologies such as hollows, multihollows, and multiporous structures is of growing interest in many technological applications such as microelectronic displays and microencapsulation. The direct synthesis of such materials is carried out in heterogeneous processes with controlled phase separation mechanisms. In such systems, detailed knowledge of heterogeneous polymerization kinetics and phase separation phenomena is essential for investigating the process characteristics. An in situ polymerization and phase separation technique has been used to construct a ternary phase diagram for the free radical precipitation polymerization of methyl methacrylate (MMA), n-hexane, and poly(methyl methacrylate) (PMMA) system. The onset of the phase separation point during polymerization is directly monitored in real time by laser light scattering (LLS) technique for a broad range of polymer concentrations. The presented method overcomes the difficulty of determining the cloud points by titrating unreactive blends of polymer and solvent at high initial monomer concentrations that lead to high polymer concentration and high viscosity of the mixture fluid at the system phase separation point. We present the micro dispersive suspension polymerization (MDSP) process to produce complex particle morphologies in a single-stage process. MDSP is a hybrid of suspension and dispersion polymerizations. The micron-sized polymer particles are polymerized by suspension polymerization, and the internal morphology of particle is polymerized by dispersion polymerization inside the polymer particles. Varying the initial conditions for the phase separation in precipitation and dispersion polymerizations, final particles’ morphology may change from solid polymer particles to complex porous polymeric structures. In this heterogeneous process, the system evolution depends on the composition and molecular characteristics of the coexisting phases and on the characteristics of the interface. Using MDSP, we were able to develop a phase diagram to show the regions of multi-hollow/porous and core-shell/pomegranate-like poly (methyl methacrylate) (PMMA) particles. We also show that controlling morphology of polymer particles by thermodynamic and kinetic variables is technically feasible. The second polymer, poly (acrylic acid), is an absorbing polymer. Superabsorbent polymers (SAP) can absorb and retain extremely large amounts of water or aqueous solutions relative to their own mass. Partially neutralized sodium polyacrylate is industrially a very important polymer for many applications. However, in industry sodium polyacrylate is mostly manufactured by bulk polymerization, and the resultant bulk polymer is pulverized using a kneader to obtain small discrete polymer particles. It is environment-unfriendly process and the produced granules from bulk have irregular shapes, rather than a spherical shape. This study is aimed at investigating the inverse suspension polymerization of acrylic acid to make spherical polymer particles. In particular, the study is focused on how the resulting polymer morphology and characteristics are affected by the polymerization conditions. A feasible and simple technique to obtain Na-polyacrylate microparticles with sizes below 10 µm was investigated using a high shear mixing device. To maintain the stability of submicron size of aqueous droplets in the oil medium, a co-surfactant system containing Span 80 and Tween 80 was used. Neutralization of acrylic acid was proved from EDX analysis. Na-polyacrylate submicron particles were characterized by size, surface morphology, swelling capacity, and conversion. When the speed of homogenization was lowered from 3000 rpm to 1000 rpm, particles over 10 µm were obtained, but more nano-sized particles were present outside. We also developed the technique above to increase polymer particle size to tens of microns. In this process, a wrinkled and cracked surface of Na-polyacrylate particles was observed in the special environment of post treatment. Surface area, swelling capacity, and swelling speed of different morphologies and sizes were characterized and analyzed. In order to synthesize spherical Na-polyacrylate particles with smooth surface regardless of post treatment, polymerization time was progressed longer than 20 hr. Na-polyacrylate particles had a solid structure at high conversion over 0.996 after longer than 15 hrs of polymerization, which made particles maintain their shapes regardless of post treatment. When a high monomer concentration was used in this polymerization, perfectly smooth and spherical polymer particles were obtained after 9 hr, which was faster than when a lower monomer concentration was used.Item Nanomechanical Properties and Buckling Instability of Plasma Induced Damaged Layer on Polystyrene(2012) Lin, Tsung-Cheng; Phaneuf, Raymond J; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis we report on an investigation of an elastic buckling instability as a driving force for the roughening of polystyrene, a model resist, during Ar+ plasma etching. Polystyrene films etched by pure Ar+ plasma with different ion energies were characterized using both atomic force microscopy topography and force curve measurements. By using height-height correlation function in analyzing the AFM measured topography images, we find that surface corrugation of etched polystyrene film surfaces all display a dominant wrinkle wavelength (ë), which is a function of ion energy. Next, we characterized the mechanical properties of these samples using AFM force curve measurements in an controlled ambient environment. We analyzed the measured force curves using a systematic algorithm based on statistical fitting procedures, and taking into account the adhesive interaction, in order to determine the effective elastic modulus of the films. We find that the effective elastic modulus (EBL) of the etched samples increases monotonically with increasing ion energy, but the changes are rather subtle as compared to the elastic modulus (EPS) of the unetched one. In order to test the validity of a buckling instability as the mechanism for surface roughening in our polystyrene-Ar plasma system, the elastic modulus of individual layer (i.e. ion-damaged layer plus unmodified foundation) needs to be determined. We present a determination of the damaged layer elastic modulus (EDL) from the effective elastic modulus of the damaged layer/polystyrene bilayer structure (EBL), based upon a finite element method simulation taking into account the thickness and elastic modulus of the damaged layers. We extract the damaged layer elastic modulus versus etching ion energy initially within the approximation of a spherical tip in contact with a flat sample surface. We next extend our model, by considering a periodic corrugated film surface, with its amplitude and wavelength determined by AFM, to take into account the effect of roughness induced by plasma exposure. The damaged layer elastic modulus extracted from these two approximations gives of quantitative agreement, and thus evidence for the correlation between buckling instability and plasma-induced roughening.Item Thermomechanical Behavior of Polymer Composite Heat Exchangers(2011) Robinson, Franklin Lee; Bar-Cohen, Avram; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Industrial cooling with seawater, particularly at elevated temperature and salinity, shortens the useful lives of conventional metallic heat exchangers. Cost effective, corrosion-resistant heat exchangers are required to fully utilize available saline water resources. Polymer composites, which use carbon fibers for thermal and mechanical reinforcement, are a promising material for such heat exchangers. The present thesis provides a characterization and thermomechanical analysis of heat exchangers fabricated using thermally conductive polymers. The change in mechanical properties resulting from exposure to saltwater at elevated temperature is characterized for raw and reinforced polymers. Then, thermal performance of such heat exchangers is compared to that of heat exchangers fabricated from conventional corrosion-resistant materials. Finally, the mechanical and combined thermomechanical response of such heat exchangers to conditions typical of LNG operations is studied and compared to that of heat exchangers fabricated from conventional corrosion-resistant materials.