Doctoral Dissertations (Winter 2014 to Present)
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New submissions to the University of Delaware Doctoral Dissertations collection are added as they are released by the Graduate College. The Graduate College deposits all dissertations from a given semester after the official graduation date.
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Browsing Doctoral Dissertations (Winter 2014 to Present) by Subject "3D printing"
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Item Interlaminar morphology and nonisothermal healing in fused filament fabrication(University of Delaware, 2021) Coasey, KeithFused filament fabrication (FFF), sometimes called material extrusion (ME) offers an alternative option to traditional polymer manufacturing techniques to allow the fabrication of objects without the need of a mold or template. However, these parts are limited in the degree to which the welding interface is eliminated (healed) post deposition, resulting in a decrease in the interlaminar fracture toughness relative to the bulk material (Z-strength). Here reptation theory under nonisothermal conditions is utilized to predict the development of healing over time, from the rheological and thermal properties of Acrylonitrile-Butadiene-Styrene (ABS). ABS is rheologically complex and acts as a gel and as such considerations had to be made for the relaxation time of the matrix which is important in predicting the degree of interfacial healing. The nonsiothermal healing model developed is then successfully compared to experimental interlaminar single edge notched bend (SENB) fracture experiments at variable printing temperatures, allowing future optimization of the process to make stronger parts. ☐ Additionally, we demonstrate a method to rapidly optimize print processing conditions to maximize both weld strength as well as the surface contact developed between layers using a modified ASTM D1938-19 trouser tear experiment. This creates a processing map, consisting of both interlayer morphology and fracture toughness, giving 3D printing operators a guide to picking processing conditions that maximize the part strength. Here we once again are using ABS, a complex structured copolymer that has been shown to exhibit gel-like behavior. ☐ In our previous studies, we focused on one grade of ABS, limiting us to a narrow understanding of potential variations in material behavior. ABS is a blend of grafted butadiene particles, and a copolymer matrix, both have respective variable molecular architectures and characteristics. This complex incompatible blend, both exhibits gel-like characteristics, and can further gelate at elevated temperatures (butadiene particle agglomeration). Here, we study six different grades of ABS. Each grade of ABS is rigorously characterized rheologically, including the strength and relaxation of the gel-like behavior. The exact mechanisms describing the gel-like behavior aren't fully understood, but characteristics about the gel-like content such as insoluble gel fraction, and the particle size distributions are provided to give context to the origin of the gel-like behavior. We provide an experimental framework for characterizing filled and unfilled polymeric materials, allowing conclusions to be made about the suitability of a material for printing or fiber drawing from the molecular characteristics. ☐ Again utilizing the processing map approach, the interlaminar morphology and welding trends for polylactic acid (PLA) are characterized and compared to the results for ABS. Despite large differences in the thermal and viscoelastic fluid properties of the two materials, the surface contact developed between single layers remains identical for printed parts. As hypothesized, we demonstrate that the relaxation time (of molecular diffusion) is a key metric in the assessment of a material, for the maximization of fracture toughness. ☐ Finally, composites consisting of talc blended in PLA were processed to exploit the role of talc as a nucleating agent for crystallization in PLA. The talc was unsuccessful at inducing crystallization at the high cooling rates experienced in FFF. However, we characterize the welding behavior of the as printed (amorphous) samples, thus isolating the role of talc on the fracture toughness. Then the welding behaviour of samples annealed to achieve full crystallization is compared to the amorphous samples, demonstrating the isolated effect of crystallization on fracture toughness of FFF specimens. We observe that higher crystallization may be undesirable as it lowers fracture toughness considerably, with possible implications for other semicrystalline polymers for use in FFF.Item Production of biocomposite materials from activated sludge: polyhydroxyalkanoates reinforced with filamentous bacteria(University of Delaware, 2023) Liang, XiangminPolyhydroxyalkanoates (PHAs) are intracellular microbial polyesters that serve as bacterial carbon and energy storage substances. They share similar properties with many petroleum-based polymers, thus attracted significant interest due to their potential biodegradability and environmental benefits. In order to improve the mechanical properties of PHAs, many types of natural plant-based fibers have been successfully used as reinforcement agents of PHA-based composites. The overall goal of this research is to obtain both the matrix (PHA copolymer) and the reinforcement agents (filamentous bacteria) for biorenewable composites from adapted wastewater treatment systems, thus, develop biocomposite materials that are derived entirely from microorganisms growing naturally in wastewater treatment processes. ☐ Biocomposite materials with commercially available poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as matrix and filamentous bacteria as reinforcement were fabricated by melt extrusion and 3D printing. Filamentous bacteria were cultivated and enriched in a laboratory membrane bioreactor. Low dissolved oxygen, and nitrogen and phosphorus limited conditions were provided in the MBR, resulting in proliferation of filamentous bacteria with the sludge volume index as high as 703 mL/g. Sphaerotilus spp. were identified as the dominant filamentous bacteria by 16S rRNA gene sequencing. A relative abundance of these filamentous bacteria was 19% in the mixed cultures. The morphology of filamentous bacteria was observed using optical microscopy and atomic force microscopy (AFM). The aspect ratio (length to width ratio) of the filamentous bacteria was 346. ☐ The thermal properties of PHBV and filamentous bacteria were evaluated by differential scanning calorimetry and thermogravimetric analysis. The results suggested that the processing temperature of these composite materials should be around 185 °C to minimize thermal degradation of both commercial PHBV and filamentous bacteria. Tensile properties (e.g., tensile strength, elongation at break, and tensile modulus) and notched impact strength of composites with different fiber contents were also evaluated. The results showed that tensile modulus and notched impact strength of the composite with 20% fiber content (mass basis) increased 12% and 95%, respectively. ☐ AFM-based nanoindentation method was used to examine the mechanical properties of filamentous bacteria. The measured Young’s modulus of Sphaerotilus spp. was 91.4% higher than that of non-filamentous bacteria. The Halpin-Tsai and Tsai-Pagano equations were used to determine the theoretical Young’s modulus of filamentous bacteria-reinforced poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The predicted values closely matched the experimental values, indicating that filamentous bacteria were randomly oriented and uniformly distributed in PHBV. ☐ A sequencing batch reactor was operated to enrich for PHA-accumulating bacteria, and the production of PHBV by the enriched cultures was maximized by a fed-batch reactor. The highest PHA content of 59% dry cell weight was obtained with 68% PHB and 32% PHV. PHBV extracted from the bacterial cells in the fed-batch reactor had a purity of 99.3%. A membrane bioreactor was used to cultivate filamentous bacteria-dominating cultures (19% relative abundance of Sphaerotilus spp.). Differential scanning calorimetry and thermogravimetric analysis of microbial PHBV suggested that the melt extrusion and 3D printing temperatures to be 170 °C and 180 °C, respectively, to minimize the thermal degradation of PHBV. Mechanical tests showed that the tensile modulus and notched impact strength of microbial PHBV improved by 57% and 45%, respectively, following the addition of 20% membrane bioreactor sludge. ☐ The research presented in this dissertation demonstrated that both biopolymer matrix and the natural fiber reinforcement for the composite materials can be recovered from microorganisms present in wastewater treatment systems. This development of biocomposite materials that are derived entirely from microorganisms growing naturally in wastewater treatment processes will help reduce the quantity of waste sludge from wastewater treatment plants and produce a green alternative to petroleum-based composite that are environmentally friendly, biodegradable, and sustainable.Item Tools and design considerations for improvements in fused filament fabrication 3D printing(University of Delaware, 2020) Phan, David D.Additive manufacturing, more commonly known as 3D printing, provides unlimited design freedom and access to geometries too complex for the more traditional means of manufacturing. Recent developments in this technique have contributed to a diverse palette of printable materials, some of which include metals, hydrogels, ceramics, and polymers. Complementary to this is a plethora of printing methods, such as the laser sintering of powders, the curing of photosensitive resins, and the controlled extrusion of thermoplastic materials. The last of these is often referred to as fused filament fabrication (FFF) and is the most commercially adopted printing technique. Despite the cost savings and design flexibility that FFF offers, products designed in this manner are weaker compared to parts made using other polymer processing operations. Addressing this shortcoming first requires an understanding of the strengthening mechanisms intrinsic to FFF, which is the primary objective of this dissertation, and is approached through the lens of rheological and heat transfer theory. ☐ First, a modified Cogswell rheological model is developed and is used to relate 3D printer extrudate temperatures to entry pressures developed within the nozzle flow field. Entry pressure measurements and calculations reveal unintuitive flow behavior, which are hypothesized to be attributable to heat transfer limitations within the melt zone. A dimensionless Nusselt-Graetz (Nu-Gz) number analysis is then presented and serves as strong evidence that heat transfer is a significant bottleneck in manufacturing the strongest printed parts. ☐ Next, computational fluid dynamics simulation tools are used to expand upon the understanding of the melting mechanism within the heated region. Extensive rheological characterization is used to guide computational efforts. Large pressure drops, which are observed in the simulations, are present through the system and are of similar magnitude to those obtained experimentally. These pressures are counteracted by the large viscosity within a recirculation vortex at the melting zone ingress. The large viscosity acts as a “seal,” preventing spillover and stabilizing the FFF operation. ☐ Then, polymer chain orientation as a means of imparting strength are explored. It was found that the deposition step in the FFF process is central to producing orientation in printed tracks. Thermal shrinkage and birefringence experiments are considered in tandem to quantify the degree of orientation for a given set of printing conditions. Low printing temperatures and high printing speeds are found to be critical in producing print tracks with the highest levels of orientation. ☐ Finally, in a slight departure from the previous topics, a study on six different commonly-used acrylonitrile butadiene styrene (ABS) 3D printing formulations was performed. ABS is a complex polymer which is composed of a styrene acrylonitrile (SAN) matrix that is then filled with butadiene spheres grafted with SAN (the grafted butadiene particles will referred to as the filler). The primary motivation for this work was to understand the rheological properties that would enable ABS to be drawn down for hybrid material applications. A secondary objective was to explore the effect of the rheological differences (if any) on end-use performance of objects fabricated with these different ABS formulations. Of the six ABS materials studied, four formulations had congruency of the storage and loss moduli at low frequencies, indicating gel-like behavior. One other formulation also had gel-like behavior, but due to parallel slopes in the moduli at low frequencies, and not because of congruency. The final formulation did not have any signs of gel-like behavior. The five gel-like ABS materials also never achieved steady-state elongational viscosity values. A modified Generalized Maxwell Model was used to separate the influence of the filler on the SAN matrix. The significant difference between the blend and matrix relaxation times is evidence of the filler’s enormous effect on the rheology. However,more work needs to be done to elucidate the structural reason for the filler’s influence. An analysis of the stress development down the draw line of a typical drawing process revealed that the formulation without gel-like behavior was most amenable to drawing. This was later confirmed experimentally by collaborators at the Army Research Laboratory (Aberdeen Proving Ground, Maryland, USA).Item Utilizing self-assembly and additive manufacturing to control mechanics and stimuli-response in structured polymer networks(University of Delaware, 2020) Thompson, Chase ByronThe function and characteristics of polymer materials are highly dependent on the network structure, where changes in polymer chemistry or architecture drive shifts in mechanics, thermal behavior, and stimuli-responsive properties of the final material. Supramolecular chemistry, or the design and organization of building blocks through non-covalent interactions, has gained traction as a convenient avenue for accessing mechanically resilient and stimuli-responsive polymeric materials by relying on the association and dissociation of non-covalent chemical motifs. In this work, we examine how changes in the polymer network architecture can be used as a platform for tuning the behavior of supramolecular polymer networks. ☐ First, we examine how the overlaying of a covalently crosslinked polymer network on to a metal-coordinating supramolecular polymer, forming a bio-inspired supramolecular semi-interpenetrating network (SIPN), can be used as a platform for tuning the self-assembly behavior of the network. These SIPN materials exhibit improved mechanics with a lower supramolecular content (30 wt%), allowing for energy dissipation through cavitation to increase material toughness. The shift in mechanical behavior is further attributed to the morphology, where the size of the phase-separated droplets and nature of the continuous phase in these SIPNs contributes to the material mechanics. Furthermore, chemical gradients are applied to these systems through exposure to a competitive ligand, offering control over the localization of supramolecular interactions. These materials offer a framework to mediate mechanics while maintaining the ability to program gradient supramolecular interactions. ☐ Control over self-assembly within a supramolecular polymer elastomer is also afforded by the addition of silica nanoparticles (NPs) to form nanocomposite films. Not only do the NPs increase the stiffness of the supramolecular polymer nanocomposite, their surface chemistry can also be changed as an avenue for tuning morphology of the polymer matrix. When complementary supramolecular motifs are present on the NP surface, long fibers are formed after film casting; however, neat silica NPs bearing silanol groups only facilitate nanofiber growth after a thermal annealing step. These morphological shifts offer functional handles to control nanocomposite mechanics and stimuli-responsive behavior. ☐ Finally, digital light processing (DLP) 3D printing is utilized to print thermoresponsive actuating bilayer hydrogels. A simple resin formulation is devised by dissolving 50 wt% N-isopropylacrylamide (NIPAM) into 2-hydroxylethyl acrylate (HEA) and a small amount of photoinitiator to yield a printable ‘active’ layer, where the lower critical solution temperature (LCST) behavior of the NIPAM is used to control the swelling of the printed hydrogel in water. An unresponsive passive layer is printed from a neat HEA formulation, and when the active and passive layers are printed to form bilayer structures, they show actuation behavior at elevated temperatures driven by the collapse of NIPAM out of the aqueous media.