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Material Extrusion additive manufacturing (MEX) represents the most widely implemented form of additive manufacturing due to its high performance-cost ratio and robustness. Being an extrusion process in its essence, this process enables the free form fabrication of a wide range of thermoplastic materials. However, in most typical MEX processes, only amorphous polymers are being used as feedstock material owing to their smaller dimensional shrinkage during cooling and well-stablished process/structure/property (P/S/P) relationship. Semi-crystalline polymers, with their crystalline nature, possess unique properties such as enhanced mechanical properties and improved chemical resistance. However, due to the inherent processing challenges in MEX of semi-crystalline polymers, the P/S/P relationships are much less established, thus limits the application of semi-crystalline polymers in MEX. The overall aim of this thesis is to advance the understanding of P/S/P relationship of semi-crystalline polymers in MEX. This is accomplished through both experimental and simulation-based research. With a typical commodity semi-crystalline polymer, Poly (ethylene terephthalate) (PET), selected as the benchmark material. First, we experimentally explored the MEX printing of both neat and glass fiber (GF) reinforced recycled PET (rPET). Excellent MEX printability were shown for both neat and composite materials, with GF reinforced parts showing a significant improved mechanical property. Notably, a gradient of crystallinity induced by a different toolpathing time was highlighted. In the second project, to further investigate the impact of MEX parameter on crystallinity and mechanical properties, a series of benchmark parts were printed with neat PET and analyzed. The effect of part design and MEX parameter on thermal history during printing was revealed though a comparative analysis of IR thermography. Subsequent Raman spectroscopy and mechanical test indicated that crystallinity developed during the MEX process can adversely affects the interlayer adhesion. In the third project, a 3D heat transfer model was developed to simulate and understand the thermal history of MEX feedstock material during printing, this model is then thoroughly validated against the experimental IR thermography data. While good prediction accuracy was shown for some scenarios, the research identified and discussed several unreported challenges that significantly affect the model's prediction performance in certain conditions. In the fourth project, we employed a non-isothermal crystallization model to directly predict the development of crystallinity based on given temperature profiles, whether monitored experimentally or predicted by the heat transfer model. The research documented notable discrepancies between the model's predictions and actual crystallinity measurements, and the potential source of the error was addressed. In summary, this thesis explored the MEX printing of semi-crystalline polymer and its fiber reinforced composite. The influence of MEX parameters and part designs on the printed part's thermal history, crystallinity and mechanical performance was then thoroughly investigated. A heat transfer model and a non-isothermal crystallization model were constructed and employed. With rigorous validation against experimental data, previously unreported challenges in MEX thermal and crystallization modeling was highlighted. Overall, this thesis deepens the understanding of current semi-crystalline polymer's P/S/P relationship in MEX, and offers insights for the optimization and future research in the field of both experiment and simulation of MEX. / Doctor of Philosophy / Material extrusion additive manufacturing (MEX), also known as fused filament fabrication (FFF), is a popular form of 3D printing known for its cost-effectiveness and versatility in creating objects from plastic materials. Traditionally, MEX utilizes amorphous polymers because they are less prone to shrinkage and thus easier to print. However, semi-crystalline polymers, offer enhanced strength and chemicals resistance, yet they pose significant challenges in printing due to a limited understanding of their process/structure/property (P/S/P) relationships in MEX. This research aims to improve our understanding of P/S/P relationships of semi-crystalline polymers in MEX. The study utilizes a typical semi-crystalline polymer, Poly (ethylene terephthalate) (PET), as the benchmark material. The study begins with the exploration of the MEX printing of recycled PET (rPET) and its glass fiber composite, finding that with appropriate MEX parameters, both feedstocks are highly printable, and the incorporation of glass fibers substantially increased the strength of the printed parts. Subsequently, a comprehensive investigation regarding the intricate relationship between crystallinity development, mechanical properties, and the MEX printing process is conducted. Our research revealed that the MEX process and the design of the part both considerably affect the crystallinity of the final part, thereby influencing its mechanical properties. In the third chapter, a 3D heat transfer model is constructed to better understand and predict the temperature evolution of materials during MEX printing. Most importantly, the modeling results are rigorously validated against experimental data, showing promising results. However, it also reveals challenges in precisely predicting the temperature of parts under certain conditions. The research then evaluates the applicability of Nakamura non-isothermal crystallization model for MEX printing scenarios. It is found that this model underestimates crystallinity in MEX, primarily because it does not account for shear-induced crystallization, a critical factor in the process. This finding underscores the necessity for more advanced models that can effectively capture the complex dynamics of MEX. In summary, this dissertation significantly enhances our understanding of the behavior of semi-crystalline polymers in MEX printing. It sheds light on the complex relationship between the printing process, the structure of the material, and the final properties of the printed object. This work not only advances our knowledge in 3D printing but also paves the way for more sophisticated modeling approaches, optimizing the MEX process and expanding its potential applications.

One of the most critical issues for Polymer Matrix Composites (PMCs) in naval applications is the structural performance of composites at high temperature such as that experienced in a fire. A three-dimensional model including the effect of orthotropic viscoelasticity and decomposition is developed to predict the thermo-mechanical behavior and compressive failure of polymer matrix composites (PMCs) subjected to heat and compressive load. An overlaid element technique is proposed for incorporating the model into commercial finite element software ABAQUS. The technique is employed with the user subroutines to provide practicing engineers a convenient tool to perform analysis and design studies on composite materials subjected to combined fire exposure and mechanical loading. The resulting code is verified and validated by comparing its results with other numerical results and experimentally measured data from the one-sided heating of composites at small (coupon) scale and intermediate scale. The good agreement obtained indicates the capability of the model to predict material behavior for different composite material systems with different fiber stacking sequences, different sample sizes, and different combined thermo-mechanical loadings. In addition, an experimental technique utilizing Vacuum Assisted Resin Transfer Molding (VARTM) is developed to manufacture PMCs with a hypodermic needle inserted for internal pressure measurement. One-sided heating tests are conducted on the glass/vinyl ester composites to measure the pressure at different locations through thickness during the decomposition process. The model is employed to simulate the heating process and predict the internal pressure due to the matrix decomposition. Both predicted and measured results indicate that the range of the internal pressure peak in the designed test is around 1.1-1.3 atmosphere pressure. / Ph. D.

Inorganic nano- and micromaterials (NMMs) exhibit unique properties including high surface areas, tunable optical and electronic properties, low densities, thermal and chemical robustness, and catalytic capabilities, among others. One of the more novel subclasses of NMMs, metal-organic frameworks (MOFs), are crystalline porous coordination polymers consisting of metal nodes connected by organic linkers to form one-, two-, or three-dimensional frameworks. While the mechanism of MOF formation is complex, tuning the metal:ligand ratios, reaction temperature and vessel pressure, ligand concentration, modulator concentration, and H+ activity impacts particle size, morphology, dispersity, and isotropy of these materials. MOFs also exhibit post-synthetic modification capabilities, which, along with their tunable synthetic nature, make them promising candidates for composite materials such as functionalized nanofillers for reverse osmosis (RO) desalination. The work described herein investigates synthetic parameters of a zirconium-based porphyrinic MOF, PCN-222, to selectively control its crystal size, aspect ratio, and dispersity. Size-constrained PCN-222 was post-synthetically modified with fatty acids and zwitterions to be used as RO thin-film composite (TFC) membranes with improved membrane flux, salt rejection, and anti-fouling properties. The synthetic parameters of MOFs were also considered for the commercial scale-up of CO2 direct air capture (DAC) solid sorbents, including UiO-66, MIL-101-Cr, and Mg-MOF-74, to preserve CO2 uptake capacities between lab and industrial scales. / Doctor of Philosophy / Metal-organic frameworks (MOFs) are unique, highly porous materials that have garnered attention for their potential in many applications, including catalysis, drug delivery, energy, and gas storage. In this work, MOFs were produced for environmental applications, particularly for the conversion of salt water to drinkable water in a process known as reverse osmosis (RO) desalination. RO uses a thin membrane to separate dissolved salt, as well as organic materials such as decomposed organisms, from water. Though RO membranes are widely used commercially, they suffer from high costs and short lifetimes; however, their performance is improved through the incorporation of extremely small materials known as nanoparticles. MOF nanoparticles were grown small enough to be dispersed in the polymer matrix of the thin membrane, then functionalized to improve salt rejection and flux, or the speed at which clean water is produced from RO processes. They were also modified to improve lifetimes by preventing the build-up of organic materials on the surface. Besides clean water, MOFs were also prepared for capturing the greenhouse gas, CO2, directly from the air. Because MOFs can be made with many different functionalities, they are promising materials for many different research fields.

<p dir="ltr">Increasing power density and power consumption in electronic devices require heat dissipating components with high thermal conductivity to prevent overheating and improve performance and reliability. Polymers offer the advantages of low cost and weight over conventional metallic components, but their intrinsic thermal conductivity is low. Previous studies have shown that the thermal conductivity of polymers can be enhanced by aligning the polymer chains or by adding high thermal conductivity fillers to create percolation paths within the polymeric matrix. To further enhance the in-plane thermal conductivity, the conductive fillers can be aligned preferentially, but this leads to a lower increase in performance in the cross-plane direction. Yet, the cross-plane thermal conductivity plays a vital role in dissipating heat from active devices and transmitting it to the surrounding environment. Alternatively, when the fillers are aligned to enhance cross-plane thermal transport, the enhancement in the in-plane direction is limited. There is a need to develop polymer composites with an approximately isotropic increase in thermal performance compared to their neat counterparts.</p><p dir="ltr">To achieve this goal, in this study, I combine conductive fibers and fillers to enhance thermal conductivity of polymers without significantly inducing thermal anisotropy while preserving the mechanical performance of the matrix. I employ three approaches to enhance the thermal conductivity () of thermoset polymeric matrices. In the first approach, I fabricate thermally conductive polymer composites by creating an emulsion consisting of eutectic gallium indium alloy (EGaIn) liquid metal in the uncured polydimethylsiloxane (PDMS) matrix. In the second approach, I infiltrate mats formed from chopped fibers of Ultra High Molecular Weight Polyethylene (UHMWPE) with an uncured epoxy resin. Finally, the third approach combines the two previous methods by infiltrating the UHMWPE fiber mat with an emulsion of the liquid metal and uncured epoxy matrix.</p><p dir="ltr">To evaluate the thermal performance of the composites, I use infrared thermal microscopy with two different experimental setups, enabling independent measurement of in-plane and cross-plane thermal conductivity. The results demonstrate that incorporating thermally conductive fillers enhances the overall conductivity of the polymer composite. Moreover, I demonstrate that the network structure achieved by the fiber mat, in combination with the presence of liquid metal, promotes a more uniform increase in the thermal conductivity of the composite in all directions. Additionally, I assess the impact of filler incorporation and filler concentration on matrix performance through tension, indentation, and bending tests for mechanical characterization of my materials.</p><p dir="ltr">This work demonstrates the potential of strategic composite design to achieve polymeric materials with isotropically high thermal conductivity. These new materials offer a solution to the challenges posed by higher power density and consumption in electronics and providing improved heat dissipation capabilities for more reliable devices.</p>