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
43 results
Search Results
Item DESIGN AND PERFORMANCE EXPLORATION OF A SCALED-UP MILLIGRAM-SCALE FLAME CALORIMETER(2024) Cromwell Reed, Kyra; Raffan-Montoya, Fernando; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fire causes thousands of lost lives and injuries, as well as billions of dollars of property damage, each year. It is critical to understand the fire hazard associated with materials used in the built environment. One method to evaluate the flammability properties of a material is through bench- scale and milligram-scale testing with apparatus such as the Milligram-Scale Flame Calorimeter (MFC). The MFC has previously been used to test samples ranging from 30 mg – 50 mg in mass. The small samples were useful for testing materials under development or materials cost prohibitive to test at larger sizes, but presented some difficulties in testing, including in sample preparation and as inconsistency in the results of testing on inhomogeneous materials. Furthermore, the small size of the MFC caused difficulty in heater manufacturing, requiring laborious by-hand construction. The size of the MFC crucible and apparatus was increased in this work to allow testing on larger sample masses, ranging in size from 90 mg – 150 mg, and for the exploration of five alternate heater manufacturing techniques. The MFC was rebuilt with a larger heater and optimized to create the best possible test conditions for this work. Tests were conducted on five polymers: polymethyl methacrylate (PMMA), polyethylene (PE), polyvinyl chloride (PVC), and polyether ether ketone (PEEK), and on a wood-based material: oriented strand board (OSB). The tests showed general consistency when materials were tested at different sample masses and sample presentations. The results for the heat release rate and heat of combustion of the materials also aligned well with testing conducted using the previous version of the MFC apparatus. The updates to the MFC conducted in this work constitute an improvement to the versatility of the apparatus, allowing for testing on larger sample masses, but future work is needed to resolve flow and exhaust issues that caused some inconsistency in the test results and to further explore and develop alternate heater manufacturing techniques.Item MOLD PROCESS INDUCED RESIDUAL STRESS PREDICTION USING CURE EXTENT DEPENDENT VISCOELASTIC BEHAVIOR(2024) Phansalkar, Sukrut Prashant; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Epoxy molding compounds (EMC) are widely used in encapsulation of semiconductor packages. Encapsulation protects the package from physical damage or corrosion due to harsh environments. Molding processes produce residual stresses in encapsulated components. They are combined with the stresses caused by the coefficient of thermal expansion (CTE) mismatch to dictate the final warpage at room and reflow temperatures, which must be controlled for fabrication of redistribution layer (RDL) as well as yield during assembly. During molding process, EMC is continuously curing and the mechanical properties continue to evolve; more specifically, the equilibrium modulus and the relaxation modulus. The former defines behavior after complete relaxation while the latter describes the transient behavior. It is thus necessary to measure cure-dependent viscoelastic properties of EMC to be able to determine mold induced residual stresses accurately. This is the motivation for this thesis. In this thesis, a set of novel methodologies are developed and implemented to quantify a complete set of cure-dependent viscoelastic properties, including the fully cured properties. Firstly, an advanced numerical scheme has been developed to quantify cure kinetics of thermosets with both single and dual-reaction systems. Secondly, a unique methodology is proposed to measure the viscoelastic bulk modulus -K(t,T) of EMC using hydrostatic testing. The methodology is implemented with a unique test setup based on inert gas. The results of viscoelastic testing along with the shear modulus (G) and bulk modulus (K) master curves and temperature-dependent shift factors (a(T)) of fully-cured EMC are presented. Thirdly, a novel test methodology utilizing monotonic testing has been developed to measure two sets of equilibrium moduli of EMC as a function of cure extent (p). The main challenge for the measurements is that the equilibrium moduli could only be measured accurately only when the EMC has fully relaxed. The temperatures for complete relaxation typically occur above the glass transition temperature, Tg (p), where the curing rate is also high. A special measurement procedure is developed, through which the EMC moduli above Tg can be determined quickly at a near isocure state. Viscoelastic testing of partially-cured EMC is followed to determine the cure-dependent shift factors of EMC. The test durations have to be very long (several hours) and it is conducted below Tg (p) of the EMC where the reaction is slow (under diffusion-control) Finally, a numerical scheme that can utilize the measured cure-dependent viscoelastic properties is developed. It is implemented to predict the residual stress evolution of molded packages during and after molding processes.Item UNVEILING THE SELF-ASSEMBLY OF POLYMER-GRAFTED NANOPARTICLES IN SELECTIVE SOLVENTS(2023) Lamar, Chelsey; Nie, Zhihong; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The self-assembly of inorganic nanoparticles (NPs) has garnered considerable attention due to the potential for fabricating functional structures with unique collective properties. In recent years, polymers have emerged as valuable candidates in assisting the organization of NPs into complex architectures with multiple capabilities. Researchers have shown that polymer-grafted nanoparticles (PGNPs) facilitate the use of advanced nanostructures with tailored properties in biomedical applications. Although, continued exploration of the rational design and tailoring of PGNP assemblies is needed to expand our understanding before we can fully realize the potential of these structures in desired applications. My dissertation aims to investigate the fundamental aspects and elucidate the underlying mechanisms in the self-assembly of PGNPs for modern biomedical applications. A facile and versatile solution-based strategy was utilized to explore the individual self-assembly of PGNPs with anisotropic NPs and the co-assembly of binary PGNPs with distinct sizes. We focused on designing, characterizing, and exploring the optical properties of hierarchical assembly structures produced from inorganic NPs tethered with amphiphilic block copolymers (BCPs). Individual PNGPs with anisotropic NPs and binary mixtures of small and large PGNPs produce vesicle structures with well-defined packing arrangements. My work shows how key parameters, including polymer chain length, nanoparticle size, and concentration, influence the self-assembly behavior and the formation of vesicles in each system. Through a combination of experimental observations and theoretical considerations, I highlight the significance of polymer shell shape in dictating the self-assembly behavior of individual anisotropic PGNPs. Moreover, I demonstrate that elevated temperatures impacted the stability and optical responses of the vesicle structures. In co-assembly studies, my work describes the macroscopic segregation of PGNPs with different sizes in the vesicular membrane, which is attributed to the conformation entropy gain of the grafted copolymer ligands. This research will provide valuable insights into the self-assembly behavior and fundamental design of PGNP structures relevant to biomedical applications.Item MECHANICS AND THERMAL TRANSPORT MODELING IN NANOCELLULOSE AND CELLULOSE-BASED MATERIALS(2023) RAY, UPAMANYU; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cellulose, the abundantly available natural biopolymer, has the potential to be a next generation wonder material. The motivation behind this thesis stems from the efforts to obtain mechanical properties of two novel cellulose-based materials, which were fabricated using top-down (densified engineered wood) and bottoms-up (graphite-cellulose composite) approaches. It was observed that the mechanical properties of both the engineered wood (strength~596 MPa; toughness ~3.9 MJ/m3) and cellulose-graphite composite (strength~715 MPa; toughness ~27.7 MJ/m3) surpassed the equivalent features of other conventional structural materials (e.g., stainless steel, Al alloys etc.). However, these appealing properties are still considerably inferior to individual cellulose fibrils whose diameters are in the order of nanometers. A significant research effort needs to be initiated to effectively transfer the mechanical properties of the hierarchical cellulose fibers from the atomistic level to the continuum. To achieve that, a detailed understanding of the interplay of cellulose molecular chains that affects the properties of the bulk cellulosic material, is needed. Modeling investigations can shed light on such underlying mechanisms that ultimately dictate multiple properties (e.g., mechanics, thermal transport) of these cellulosic materials. To that end, this thesis (1) applies molecular dynamics simulations to decipher why microfibers made of aligned nanocellulose and carbon nanotubes possess excellent mechanical strength, along with understanding the role of water in fully recovering elastic wood under compression; (2) delineates an atomistically informed multi-scale, scalable, coarse grained (CG) modeling scheme to study the effect of cellulose fibers under different representative loads (shearing and opening), and to demonstrate a qualitative guideline for cellulose nanopaper design by understanding its failure mechanism; (3) utilizes the developed multi-scale CG scheme to illustrate the reason why a hybrid biodegradable straw, experimentally fabricated using both nano- and micro-fibers, exhibits higher mechanical strength than individual straws that were built using only nano or microfibers; (4) investigates the individual role of nanocellulose and boron nitride nanotubes in increasing the mechanical properties (tensile strength, stiffness) of the derived nanocellulose/boron-nitride nanotube hybrid material; (5) employs reverse molecular dynamics approach to explore how the boron nitride nanotube based fillers can improve thermal conductivity (k) of a nanocellulose derived material. In addition, this thesis also intends to educate the readers on two perspectives. The common link connecting them is the method of engineering intermolecular bonds. The first discussion presents a few novel mechanical design strategies to fabricate high-performance, cellulose-based functional materials. All these strategies are categorized under a few broad themes (interface engineering, topology engineering, structural engineering etc.). Another discussion has been included by branching out to other materials that, like nanocellulose, can also be tuned by intermolecular bonds engineering to achieve unique applications. Avenues for future work have been suggested which, hopefully, can act as a knowledge base for future researchers and help them formulate their own research ideas. This thesis extends the fundamental knowledge of nanocellulose-based polymer sciences and aims to facilitate the design of sustainable and programmable nanomaterials.Item Versatile Strategies for Multifunctional Polyolefins(2023) Fischbach, Danyon Miles; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polyolefins have quickly become one of the world’s most utilized products since their discovery in the 1950s. With 350 million tons produced each year, it is clear that the use of polyolefins is not subsiding in the near future. Instead, it is imperative to develop novel materials that are more efficient than their current counterparts. As the function of a plastic is derived from its properties, creating polyolefins with designable and targetable attributes is a major priority. The Sita group has played a huge role in the development of ‘precision’ polyolefins. The techniques employed allow for the scalable synthesis of a plethora of polyolefins. To do this, input variables such as the monomer, tacticity, molar mass, and molar mass distribution are controlled in an organized manner to affect output properties such as crystallinity, elasticity, and tensile strength. The ability to create diverse plastics is necessary for the functions asked of them, however, the missing element in almost all polyolefin synthesis is chemical functionality. The inert nature of polyolefins leads to limited reactivity, therefore, reducing possible chemical reactions, such as recycling. The goal of this work is to increase the scope of functional polyolefins so that new materials with improved properties can be produced. The first step in adding functionality is choosing the proper functional group. A drawback to many polyolefin functionalities currently under study is that they have a very limited scope. Functional groups are designed and used individually, requiring different compounds for each target functionality. To overcome this obstacle, aryl functional groups were targeted in this report. Phenyl functionalities are known for undergoing a range of chemical transformations leading to a wide variety of possible materials. Described in this report, aryl-functionalized polyolefins were synthesized using three different techniques. Each method has been shown to later undergo post-synthetic transformations to yield new functional groups that can either be used as contact points for macromolecular building blocks or as chromophores for optical observation. The single use or combination of these techniques has led to polyolefin-based materials that may in fact lower the barrier for the next-generation of functional polyolefins.Item USE OF MILLIGRAM-SCALE FLAME CALORIMETRY FOR CHARACTERIZING FLAMMABILITY OF FABRIC SAMPLES WITH FLAME RETARDANT TREATMENTS(2023) Roche, Thomas William; Raffan-Montoya, Fernando; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The fire hazard associated with fabrics threatens everyone’s safety, and the current standards used to reduce those hazards are expensive and time-consuming. Fabrics are a key component in clothing, upholstery, and carpentry and are present in nearly every built environment. The inherent flammability of fabrics leads to the application of flame-retardant treatments on nearly all commercial fabric products. Recently, environmental, economic and performance concerns have driven research to develop new flame retardants across a variety of materials. The military industry in particular has focused recent research efforts on flame retardant treatments for fabrics, given the challenging environments that military uniforms must endure. Current methods for testing performance of novel flame retardants, such as the Cone Calorimeter and Microscale Combustion Calorimeter can be prohibitively expensive or only provide a limited understanding of flame-retardant action. Fabrics present additional testing challenges due to their low density and thickness, effectively reducing the amount of fuel available for testing. A novel apparatus, the Milligram-scale Flame Calorimeter (MFC), has been used to test flame retardants in polymeric materials, successfully capturing gas-phase activity and with favorable comparison to Cone Calorimeter results. This study aims to expand the use of the MFC to the testing of fabrics and flame-retardant treated fabrics. Optimization tests were run to find the optimal number of fabric layers and best method for preparing samples for use in MFC. Subsequently, cotton fabrics (untreated and treated with phosphoric acid), as well as Nylon fabrics (untreated and treated with tannic acid) were characterized with MFC, and results were compared to those from the Microscale Combustion Calorimeter and Cone Calorimeter. The MFC showed similar trends in the onset of ignition, peak heat release rate, average heat release rate, char yield, and heat of combustion for the untreated fabrics with the Cone Calorimeter and Microscale Combustion Calorimeter results. The results for the flame-retarded fabrics are inconclusive and require additional testing, though the observations of the condensed-phase and gas-phase activity for the MFC samples does provide important insights on how the mechanism for the flame retardants operate.Item Magnetic nanoparticle inks for syringe printable inductors(2023) Fedderwitz, Rebecca; Kofinas, Peter; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Direct Ink Writing (DIW) additive manufacturing (AM) has the transformative potential to construct complex shapes and devices with a single apparatus by exchanging the printable material at the print head. Iron cobalt (FeCo), permalloy (Ni80Fe20), and iron (II,III) oxide (Fe2O3·FeO) nanoparticles with varying magnetic properties were incorporated in resins to explore the influence of particle loading on printability and inductor device performance. It was generally found that increasing particle loading increased ink viscosity, with a loading maximum approaching 29 – 42 vol% depending on the particle type and resin mixtures due to differences in particle shape and size and resin viscosity. With more magnetic content, composites had higher magnetic permeability and inductance. Syringe printable, colloidal, aqueous magnetic inks were made using both stabilized iron oxide and MnZn doped ferrite nanoparticles with added free polymers. MnZn doped ferrite inks are printed into toroids, sintered to improve magnetic permeability and mechanical robustness, and constructed into an inductor device. Inductors with high magnetic permalloy nanoparticle content were also syringe printed into toroids and hand-wound to demonstrate their viability in fabricating three-dimensional inductors. The effect of particle size on stability and printability was observed. The research presented in this thesis investigates various methods for formulating magnetic nanoparticle inks and evaluates the contributions of particle stabilization, free polymer content, solvent composition, and particle loading on the rheological behavior required for syringe printing. Material properties and device performances were characterized using methods such as zeta potential and settling studies to observe particle functionalization and stability, rheology to study viscoelastic flow behavior, and vector network analysis to measure inductance and device efficiency to showcase the viability of this technique to manufacture passive electronic devices.Item Simulation of polymeric drop dynamics: Effect of photopolymerization, impact velocity, and multi-material coalescence(2023) Sivasankar, Vishal Sankar; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Over the past couple of decades, additive manufacturing has emerged as one of the most promising manufacturing tools and has rightfully garnered the attention of researchers across various fields ranging from biochemistry and medicine to energy and infrastructure. Especially, direct-ink-writing methods (e.g., inkjet printing, aerosol jet printing, or AJ printing, etc.) have been widely studied because of their ability to print highly complex geometries with finer resolution. In order to design a more efficient droplet-based direct ink writing system, it is essential to understand the deposition process and the post-deposition dynamics of the drop. The post-deposition drop dynamics dictate the spreading radius of the drop and hence the print resolution. Such an understanding is even more critical when there are multiple drops interacting with each other, given the fact that such interactions determine the presence/absence of surface defects in addition to determining the print resolution. Moreover, to have a holistic understanding of the post-deposition process, it is essential to further account for the droplet solidification mechanisms (for example, through effects such as in-situ curing) that might interplay with multiple drop dynamics events (such as drop spreading, drop coalescence, drop impact, etc.). In this dissertation, computation fluid dynamics (CFD) frameworks have been developed to investigate the facets dictating the post-deposition dynamics of one (or several) solidifying polymer drops, with these dynamics show-casing the different post-deposition events that are intrinsic to the droplet-based additive manufacturing processes. First, we considered a situation where the polymeric drop undergoes simultaneous spreading and photopolymerization, with the timescales of the spreading and photopolymerization events being τ? and τ? respectively. The findings from this work confirmed the significant impact of the ratio of timescales (τ? and τ?) on the thermo-fluidic-solutal dynamics of the polymeric drops. Moreover, the evolution of the curing front showed distinct behaviors as a function of the timescale ratio.Subsequently, the effect of the interaction of multiple polymeric drops during the post-deposition event, as seen in the typical printing process, was investigated. Specifically, we studied the effect of drop impact on the coalescence dynamics of two polymeric drops of identical and different sizes. The study revealed the presence of two distinct stages of coalescence. The early-stage coalescence was found to be enhanced with an increase in the impact velocity; however, the late-stage coalescence behavior remained unaffected by the impact velocity. Further, the coalescence dynamics of polymeric drops of different materials, as witnessed in multi-jet printing, was probed. This study shed light on the mechanisms that drive the mixing process at different stages of drop coalescence. Finally, we evaluated the effects of the in-situ photopolymerization on the coalescence dynamics of multiple polymeric drops deposited on a substrate. Here too the comparative values of the drop dynamics timescale and the photopolymerization became important. Our results show three-distinct regimes characterizing the bridge growth which was further validated through physics-based theoretical scaling. This study would provide key insights into the direct-ink writing process and would aid in designing parameters for polymer-based additive manufacturing and product repair.Item DESIGN AND SYNTHESIS OF POLYOLEFIN MATERIALS FOR NANOSTRUCTURED SELF-ASSEMBLY: BUILDING BLOCKS, COPOLYMERS, AND POLYMER CONJUGATES(2022) Wentz, Charlotte Maria; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polyolefin based materials are essential to today’s society in both simplistic commodity plastics to complex nanostructured materials and optoelectronic devices. In order to better understand these materials and make new impactful innovations, there is a barrier of fabrication, scalability, versatility, and programmability. The answer to the world’s plastic waste problem lies not in removing our use of polymers but relies in better understanding their properties, utilizing them as building blocks in advanced materials, and creating a long-lasting advanced material. Towards the goal of overcoming limitations in fabrication and scalability the work herein presents on utilizing a toolbox of living polymerization techniques such as living chain transfer polymerization (LCCTP) where new functionalities, stereochemical microstructures, optical properties, and physical properties of the polyolefin can be designed and systematically controlled. The polyolefins made through these techniques are scalable and versatile with end-group functionalization creating a seemingly endless choice of polymer building blocks and polymer materials. In line with creating new technologies that are programable the polyolefin building blocks made herein are utilized in multiple conjugates to create and understand methods and mechanisms of solid-state nanostructured self-assembly and access rare nonclassical phases that are highly desirable for their properties and uses in a plethora of applications. The conjugates investigated involve either a sugar-based head group covalently bond to a polymer tail to access rare and misunderstood Frank Kasper phase order-order transitions, or a perylene chromophore core covalently bond on both sides of the core in a linear fashion to polymer domains to create highly florescent or optically active materials that are useful in organic technologies such as solar cells, light emitting diodes, or nanotechnology. These perylene based conjugates can self-assemble into unique columnar phases and single gyroid phase. These results with conjugates provide methods for reliable and programmable access to rich phase behavior through the design of the polyolefin domains.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.