Summary

Understanding how interfacial interactions can be manipulated to drive assembly at the nanoscale and provide reinforcement at the macroscale is crucial for expanding the known boundaries of material-property space for 3D printed materials. Specifically, my dissertation research examines how controlling interfacial interactions across the matrix /nanoparticle interface in polymer nanocomposites and how using hydrogen bonding interactions in multicomponent blends impacts the resultant properties of parts printed by Fused Filament Fabrication (FFF). The structure-property relationships established from these research endeavors provide a foundation for future efforts that use nanoscopic additives to control the performance of parts printed by the non-isothermal, nonequilibrium FFF process. The first research effort describes how the thermomechanical properties of FFF-printed PMMA parts are impacted by the addition of unfunctionalized (bare) Si NPs.

This research work demonstrates that increases in the macroscopic performance of PMMA nanocomposites containing Si NPs are due to favorable, hydrogen bonding interactions between hydroxyl groups on Si NP surface and carbonyl groups within the backbone of PMMA chains. The number density of polymer-particle interactions increase as the loading level of Si NPs increases, which enhances stress transmission and results in an increase in the performance of PMMA printed nanocomposites. This effort is noteworthy because it highlights the importance of interfacial interactions as a way to enhance the performance of polymer nanocomposites in the context of 3D printing.

The second major theme of my dissertation research describes the first example of incorporating polymer-grafted nanoparticles in FFF-printed nanocomposites. In that research effort, I examined how attaching end-tethered chains to the surface of Si NPs

controls organization on the nanoscale and alters the macroscopic properties polymer-grafted nanocomposites. Specifically, PMMA-polymer-grafted nanoparticles (PMMA-g-NPs) having low molecular weight graft chains and an intermediate grafting density are demonstrated to arrange in surface fractals, or connected sheets, throughout PMMA nanocomposites generated by FFF. Macroscopic assessments coupled with rheology measurements suggest that these interconnected sheet-like nanostructures effectively dissipate stress throughout the nanocomposite through graft chain interactions with matrix chains and graft chains on neighboring particles. Additionally, the mechanical performance of FFF-printed parts described in this work surpass those obtained when bare Si NPs were used, which further highlights how manipulating interfacial interactions in polymer nanocomposites, conveyed here by grafted polymer chains, controls organization on the nanoscale and affects the performance of polymer nanocomposites. This work is significant because it emphasizes how the performance of polymer nanocomposites depend on both the spatial distribution of nanoparticles and interfacial interactions. From this research effort, additional studies examining how variation in the grafting density of chains or graft chain length impact nanoscale organization can be completed. These research efforts may provide a useful way to effectively control the stress-transmission from the matrix chains to Si NPs and also may provide a way to promote particle diffusion across bead-bead interfaces.

Lastly, I examined how the melt characteristics and thermomechanical properties of FFF-printed PMMA parts were affected by the addition of polymeric additives containing self-complementary, hydrogen bonding groups. These research studies provide the first example showing how thermoreversible, hydrogen bonding interactions offer

mechanical reinforcement at use temperatures, but exhibit no delirious impact on the processability at temperatures above the dissociation temperature of the hydrogen bonding interaction. Specifically, I synthesized random copolymer additives consisting of methyl methacrylate and 2-ureido-4[1H]-pyrimidone methacrylate (UPyMA). By varying the UPyMA content, the number density of self-complementary, hydrogen bonding interactions between UPy groups could be manipulated. Results from macroscopic assessments demonstrate that increasing the number of physical crosslinks, conveyed by the self-dimerization of UPy groups, increased the mechanical properties of PMMA parts printed by FFF, but no changes in the melt characteristics were observed. Results from this research show that the properties of parts printed from these multicomponent blends depend on the self-dimerization strength of the hydrogen bonding groups and the number density of non-bonded interactions. This work is impactful because it demonstrates how the strength and number density of non-bonded, physical interactions can tuned to effectively manage the properties of FFF-printed materials at use and production temperatures.

While these research efforts provide strong proof for the concept that the performance of FFF-printed parts are dictated by the interactions on the nanoscale, improvements in the properties of FFF-printed parts are limited by the amount of additive that can be successfully incorporated in PMMA filaments. For instance, all multicomponent or nanocomposite filaments generated herein (by using either Si NPs, PMMA-g-NPs, or SCMHB copolymer additives) exhibit degradation in the structural integrity of filaments and the appearance of macroscopic agglomerates as the loading level of the additive increases above 1.0 wt%. These problems are attributed to inefficient mixing

during either mechanical mixing or melt extrusion. Two methods are suggested for resolving this problem: First, it may be useful to use a different preparation procedure to incorporate additives in PMMA filament. For instance, instead of using a mechanical mixing procedure, one may use a solution mixing method which has been used previously to create nanocomposites. By allowing the matrix chains and additive to mix in solution, a more homogenous nanocomposite (or blend) may be obtained, and this strategy may help to eliminate macroscopic agglomerates. In addition to altering how the different components are mixed prior to filament extrusion, it may be useful to consider using an extruder with a longer length-to-diameter (L/D) ratio of the extrusion screw and multiple heating zones, as these may enhance mixing during melt processing. This method also may decrease the presence of macroscopic agglomerates and allow structurally stable filaments that can be used for printing to be generated.