Date of Award

5-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

S. Michael Kilbey II

Committee Members

Bin Zhao, Chris Baker, Joshua Sangoro

Abstract

Additive Manufacturing (AM), or 3D printing, provides an alternative route to generate end-stage products by coupling advanced manufacturing techniques with computer modeling. However, parts fabricated by AM are known to have inferior mechanical properties compared to parts prepared by traditional methods, such as injection molding. This principal drawback is attributed to the presence of voids and inefficient adhesion between adjacent filaments, or beads, due to limited diffusion of polymer chains across interbead interfaces. Together these shortcomings also lead to anisotropic mechanical properties in printed parts. While optimizing print conditions or applying post-printing procedures decreases the anisotropy, these methods are incapable of obtaining significant enhancements in material properties. To address this, my dissertation work examines how incorporating nanoscopic additives, including bare (unfunctionalized) nanoparticles, poly(methyl methacrylate)-grafted-nanoparticles (PMMA-g-NPs), and macromolecules containing self-complementary, multiple hydrogen bonding motifs that trigger supramolecular assembly, into PMMA filaments affects structure formation at the nanoscale and impacts the resultant macroscopic properties of PMMA parts manufactured by Fused Filament Fabrication (FFF). Results indicate that incorporating bare nanoparticles, which arrange as well-dispersed mass fractals throughout the matrix, into PMMA filaments leads to a slight increase the thermomechanical properties. Adding PMMA-g-NPs significantly improves material properties relative to samples printed with bare nanoparticles. These enhancements are attributed to increased interactions across grafted nanoparticle/matrix interfaces because there is a direct correlation between loading level and changes in thermomechanical properties. In addition to using inorganic additives, my research efforts demonstrate that copolymeric additives capable of forming thermoreversible physical crosslinks are advantageous. They increase part performance at use temperatures, but the dissociation of physical crosslinks at high temperatures (used for polymer melt processing) alleviates any deleterious effect on the viscosity, rendering them highly processable. These results demonstrate that molecular engineering can be used to effectively manage interactions on the nanoscale, leading to substantial increases in the performance of FFF-printed parts. These studies, which highlight the importance and potential of non-bonded interactions, provide a compelling and useful pathway for addressing challenges associated with the inferior performance of 3D printed polymeric materials.

Comments

Portions of this document were previously published in journal Polymer and Macromolecules.

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