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Author ORCID Identifier
Open Access Dissertation
Doctor of Philosophy (PhD)
Polymer Science and Engineering
Year Degree Awarded
Month Degree Awarded
Alan J. Lesser
Engineering | Materials Science and Engineering | Polymer Science
A unifying theme throughout this dissertation employs advanced experimental mechanics, including the design and development of new instruments, testing techniques, and analysis strategies. Chapter 1 covers the design and development of a new instrument for polymer and composite characterization. More specifically, a bi-fluidic, confining-fluid, pressurizable dilatometer (referred to herein as the BFCF-PVT). Whereas both classical and contemporary confining-fluid type pressurizable dilatometers utilize a bellows and / or piston system along with a Linear Variable Displacement Transducer to apply pressure and track volume changes, the BFCF-PVT that was designed and built utilizes a fluid-fluid interface (composed of two immiscible fluids that have different densities). This instrument is capable of transmitting pressure through the fluid-fluid interface from the pressurizing fluid to the confining fluid (within which the sample is located), and volume is directly measured through the integration of an optical encoder. In addition to instrument development and construction, a new calibration procedure was developed to characterize and account for instrument response. In order to verify that the BFCF-PVT can accurately measure and characterize polymeric systems (both solid and fluid polymer samples), results on model polymer systems are compared to published historical results for the same polymer systems that were generated using the pressurizable dilatometer developed by Zoller and it is demonstrated that there is excellent agreement.
Accelerated physical aging of aliphatic and aromatic thermosets is the focus of Chapter 2. Given the widespread use of glassy materials, especially polymeric glasses, there is considerable interest both in industry and academia to understand and be able to predict how the properties of the glass will change with time. To address this interest, two acceleration strategies are investigated and compared. The traditional route of thermal annealing and a newly developed strategy referred to as ultra-high-pressure conditioning. A set of three stoichiometric epoxy-based thermosetting glasses were prepared for the aging studies, and the formulations of these glasses were designed such that they possessed controlled molecular architecture (i.e. crosslink density and backbone stiffness). The BFCF-PVT is utilized to perform the ultra-high-pressure conditioning, and in doing so the utility of the instrument to be used not only as a characterization instrument but also as a conditioning device is demonstrated. Characterizations of the aged glasses and comparisons of the acceleration strategies are performed using thermal and mechanical techniques. The results indicate that ultra-high-pressure conditioning ages the glasses in a different but consistent way when compared with thermal annealing. Additionally, it is observed that molecular architecture of the polymeric glasses influences the kinetics of aging.
In Chapter 3, mechanical, dilatometric, and optical measurements are brought together to fully characterize three mechanical properties of model isotropic polymeric systems, more specifically Young’s modulus, bulk modulus, and Poisson’s ratio. The materials used in this work are epoxy-based thermosetting glasses, the same glasses that were used in the physical aging work of Chapter 2. To measure bulk modulus of the samples, bulk compressibility tests were performed using the BFCF-PVT described in Chapter 1. Compression tests were performed to measure Young’s modulus. In order to evaluate Poisson’s ratio, another new instrument was developed: a bi-directional optical extensometer (referred to herein as the BD-OE). This optical extensometer was synchronized with the mechanical tests on the Instron, and it permitted the tracking of displacements over a full two-dimensional field. Subsequent custom image analysis, based off of an edge detection and tracking strategy, enabled the characterization of Poisson’s ratio as a function of both position on the sample and time during the mechanical test. Additionally, an analysis routine was developed to reconstruct three-dimensional sample volumes from the detected edges, and this enabled the evaluation of volume changes (and subsequently density changes) that occur during compression testing. Upon comparing the calculated and measured Poisson’s ratios, the results indicate that the bi-directional optical extensometer and accompanying analysis work well.
Ondra, Brendan Robert, "CHARACTERIZING NON-LINEAR STRUCTURAL, MECHANICAL, AND VOLUMETRIC PROPERTIES OF ALIPHATIC AND AROMATIC THERMOSETS" (2020). Doctoral Dissertations. 1902.
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