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Virginia Tech's open access policy enables researchers to deposit the accepted version of scholarly articles with no embargo.
Theses and Dissertations
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More than 50 freely available and openly licensed textbooks are among our most downloaded items.
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Recent Submissions
All-atom and Coarse-grained Molecular Dynamics Modeling of Various Polymeric and Composite Materials
Hao, Xi (Virginia Tech, 2025-11-14)
This dissertation leverages molecular dynamics (MD) simulations to reveal the structure-property relationship of various polymeric and composite materials, including polyacrylonitrile-poly(methyl methacrylate) (PAN-b-PMMA) copolymers, polyetherimide (PEI)-graphene composites, and epoxy network polymers, by bridging molecular-level behaviors and macroscopic thermomechanical properties. First, all-atom MD simulations are used to improve the design of porous carbon fibers derived from PAN-b-PMMA copolymers for energy storage applications. A new method is developed to characterize the interfacial area between different domains. Simulation results reveal a molecular mechanism underlying the experimental findings, demonstrating that the interfacial area -- a key predictor of the electrochemical performance of the resulting fibers after oxidation and carbonization -- reaches a maximal value when the two blocks are at a 50% volume fraction. This understanding paves the way for designing PCFs with optimal energy storage capabilities. Next, all-atom MD simulations are used to explore a strategy to enhance polymer nanocomposites by mitigating nanofiller aggregation. Experiments show that coating the surface of reduced graphene oxide (rGO) nanoparticles with PEI chains can improve their dispersion in a PEI matrix and thus lead to stronger composites. Simulations reveal that the PEI chains grafted to the edge surface a rGO particle form a protective layer of the particle, preventing particle aggregation and creating a more compatible interface with the host polymer. This enhanced compatibility makes the composites perform more strongly under mechanical loading, as seen experimentally. Polymer grafting is therefore confirmed as a powerful strategy for creating stronger, more reliable composite materials. Finally, a computationally efficient coarse-grained (CG) model is developed for epoxy resins based on EPON 862 (Diglycidyl Ether of Bisphenol F) monomers and diethyltoluenediamine (DETDA) curing agent, a critical component of high-performance composite materials. The CG model is transferable across a wide range of temperatures and is used to predict the mechanical properties of epoxy resins with reasonable accuracy. It provides a facile approach to creating large epoxy networks. Then via a backmapping procedure, the CG network is mapped to an all-atom network with the same topology. The all-atom and CG networks are used for understanding the fracture behavior of epoxy resins at experimentally relevant spatiotemporal scales. Collectively, this dissertation provides a suite of validated computational tools and fundamental molecular insights to advance the bottom-up design and optimization of next-generation polymeric and composite materials.
Cross-Laminated Timber made of Unmodified and Thermally Modified Yellow-Poplar Lumber
Masoumi, Abasali (Virginia Tech, 2025-11-14)
Cross-laminated timber (CLT), a sustainable construction material, is transforming the construction industry. To meet diverse demands, especially for exterior applications, its durability must be improved to withstand conditions ranging from dry indoor environments to moisture-prone settings. Thermally modified wood (TMW) is a potential solution to enhance CLT's moisture durability and reduce moisture-related issues. Yellow-poplar (YP), abundant in the Appalachian region, is a promising raw material due to its availability and favorable mechanical properties and incorporating its TMW into CLT outer layers offers a sustainable strategy to improve moisture resistance, reduce moisture-induced strain, and enhance durability for exterior applications.
The goal of this work was to enhance the moisture durability of CLT by integrating TMW in its outer layers. This goal was addressed through four objectives: (1) determining the water vapor permeability and resistance factor of CLT made from unmodified wood and hybrid CLT with TMW outer layers, including the influence of the one-component polyurethane (PUR) adhesive layer; (2) predicting long-term moisture diffusion performance using hygrothermal simulations; (3) establishing correlations between moisture-induced strain and water vapor diffusion to provide input for physics-informed modeling; and (4) developing a predictive model for water vapor permeability and resistance factor (µ-value) that accounts for swelling strain and the adhesive layer.
The first objective was achieved by investigating steady-state moisture diffusion (ASTM-E96) through CLT panels of various configurations, including 3-layer, 2-layer (to evaluate bondline effects), and 1-layer boards for both unmodified and hybrid CLT. Results showed that hybrid CLT exhibited significantly improved moisture resistance compared to unmodified CLT, with µ-values of 51.3 versus 32.7 for unmodified 3-layer CLT, and contributions from TMW (µ = 32.9) and PUR adhesive (µ = 1345), compared to 22.3 for unmodified wood.
The second objective involved simulating long-term moisture diffusion using WUFI software. While simulations indicated no significant improvement in moisture resistance for hybrid CLT compared to control, likely due to WUFI's lower sensitivity at low µ-values, this confirmed the absence of moisture accumulation in the middle layer of hybrid CLT, consistent with experimental results. Simulations also evaluated various insulation options, identifying Polyisocyanurate board (R=72) as an effective single-layer interior insulation.
The third objective focused on the correlation between moisture diffusion and moisture-induced strain. Dimensional changes were monitored in 1-layer boards, 3-layer and 5-layer CLT samples under controlled conditions, revealing strong relationships, such as R² = 0.99 between diffusion and volumetric strain in unmodified wood. Swelling strain, which reduces pore sizes in the microstructure, reduced diffusion by an average of 77%. Outdoor weathering over one year showed no delamination in the 5-layer hybrid CLT, while control YP CLT exhibited significant delamination on the surface and thickness; minor surface checks on TMW were attributed to lower elasticity. These results confirm that hybrid CLT with TMW outer layers offers superior long-term durability for outdoor applications.
Finally, the fourth objective was accomplished by developing a physics-informed data-driven machine learning (PIDDML) model to capture the interplay between diffusion, adhesive layers, and moisture-induced strain. Traditional Fick's law models fail to account for these effects, resulting in large prediction errors. The PIDDML model, trained on 11 features including humidity, temperature, and moisture, integrated diffusion and strain data, explicitly accounting for swelling and adhesive effects. It outperformed Fick's law, achieving a maximum absolute error (MAE) of 1.4–13.1% for spruce-pine-fir (SPF) CLT and 11.2% for hybrid CLT compared to 46.7–66.0% under Fick's law, with an overall R² of 0.96. Using a logistic regression moisture content-permeability submodel, the framework was extended to other wood species, generating data for untested species and achieving cross-validation R² = 0.94. By embedding physical constraints such as Fick's law, mass conservation, and absorption–diffusion kinetics, the PIDDML model ensures physically plausible predictions, reduces reliance on extensive experimental data, and generalizes across diverse conditions and species.
By integrating experimental measurements, hygrothermal simulations, and advanced PIDDML modeling, this study provides a robust framework for predicting CLT's moisture behavior. The hybrid CLT approach enhances dimensional stability and durability, advancing its use in moisture-exposed construction applications, contributing to sustainable building practices, and supporting wider adoption of CLT in North America.
Understanding Cooperative Extension Directors' Conceptualizations of, and Perceived Roles in, Internationalization
Grove, Benjamin Bryant (Virginia Tech, 2025-11-14)
The Cooperative Extension System has been engaged in internationalization for decades. Ludwig and Barrick (1996) outlined five indicators of an internationalized Extension system. In the broader higher education literature Knight (2003) has conceptualized internationalization as a process. Extension directors have been identified as key leaders of internationalization within the Extension system. This dissertation investigated directors' conceptualizations of internationalization and their roles in such pursuits. I conducted semi-structured interviews with directors from across the U.S. to ascertain their understanding of internationalization as a phenomenon. I used Knight's (1994) internationalization cycle as a primary analytic framework and also drew on additional arguments from the higher education and Cooperative Extension System literatures. The study participants revealed a lack of a shared definition of internationalization. They articulated a perceived imperative that domestic audiences receive the primary benefits of internationalization and financial considerations as prevailing lenses for considering engagement in such initiatives. They suggested process-based, position-based, rationale-based, and system leadership-based rationales for such efforts. They also acknowledged the mediating impact of university priorities and resources on whether their systems would become involved in internationalization.
Identifying solutions for youth participation in Agriculture in Rural Honduras
Skhisazana, Nompumelelo; Aljawharah, Aljohani; Humay, Sadig; Mitchell, Garber; Alsudairy, Dima; Oyedare, Israel (2025-05-19)
A Case Study of Irrigation Pondwater and Soilless Substrate Quality across Nine Large Nurseries in Eastern Virginia
Criscione, Kristopher S. (2025-11-11)
Nursery producers depend on clean irrigation and efficient soilless substrates to successfully produce salable plants. Little information is available regarding the quality of irrigation and pine bark used in large eastern Virginia nurseries, despite the impact eastern Virginia nurseries have on Virginia horticulture sales. The objective of this case study was to investigate whether large nurseries located in eastern Virginia maintained irrigation and substrate quality within recommended standards. Irrigation pondwater (n = 8) samples were collected and tested for chemical properties, including pH, electric conductivity, alkalinity, total suspended solids, hardness, calcium, iron, and sodium. Additionally, pine bark-based substrates (n = 9) were collected and tested for substrate physical properties, including water storage, air-filled porosity, total porosity, and bulk density. The results of this case study showed that nursery irrigation pondwater had lower alkalinity values than recommended standards, while pine bark-based substrates used by large nurseries slightly exceeded air-filled porosity recommendations. However, all other parameters measured were generally within acceptable recommended standards.