Theses and Dissertations from UMD
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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.
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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 Long Term Stability and Implications for Performance of High Strength Fibers Used in Body Armor(2012) Forster, Amanda Lattam; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The objective of this work is to examine the relationship between structure (both molecular and morphological structure) and properties of high strength fibers. The superior performance of the high strength fibers is predicated on the development of a highly aligned molecular structure that allows the polymer to exhibit a superior strength in the axial direction of the fiber. Armor manufacturers have exploited the inherent strength of these materials to develop body armor that continues to defeat ever-increasing threats. However, even an ideal molecular structure will be subjected to a potentially hydrolytic or oxidative environment during use, which can reduce the high strength of these fibers, and impact their ability to protect the wearer. The effect of the wear environment on the molecular structure, which is responsible for the high strength of these fibers, has not been well understood by the scientific community. In this work, the chemical mechanisms of degradation were investigated at the molecular level to understand the effect of the environmental conditions on crystallinity, orientation, and molecular weight. The chemical mechanism and kinetics elucidated from these measurements are used to understand the reduction in strength of these materials after degradation. Hydrolysis was found to be the predominant mechanism of degradation for polybenzobisoxazole and goes to irreversible chain scission. Hydrolysis is also the primary mechanism of degradation for aramid fibers. Ultra-high molecular weight polyethylene (UHMWPE) fibers undergo an oxidative mechanism of degradation, and the activation energy for this mechanism was calculated. Additionally, the release of acids from aramid copolymer fibers, and the performance of these fibers in hydrolytic and thermooxidative environments were studied to determine that hydrolytic degradation is the predominant degradation mechanism for these fibers. Exploratory research was also performed in an effort to improve the stability of UHMWPE fibers by using radiation to crosslink the UHMWPE fibers and increase the temperature of their alpha relaxation. However, this radiation treatment was still found to reduce the overall tensile strength of these fibers. In summary, the wear environment and vulnerabilities of a material to degradation are essential when selecting materials or developing new materials for use in body armor.