Doctoral Dissertations

Date of Award

12-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Mark D. Dadmun

Committee Members

Alexei P. Sokolov, Shawn R. Campagna, Rigoberto Advincula

Abstract

Two widely used polymeric additive manufacturing (AM) techniques are powder bed fusion (PBF) and fused filament fabrication. These techniques enable the creation of complex geometries from digital models in a layer-by-layer fashion. However, structures created using these methods often exhibit weak mechanical performance and anisotropy compared to those produced by traditional manufacturing techniques, such as injection molding. Factors such as poor interlayer and interparticle adhesion, residual stress, large voids, and insufficient chain entanglement between layers contribute to the inferior mechanical performance and anisotropic behavior of parts processed by AM. Additionally, the range of polymeric feedstocks available for AM techniques is limited. To address these issues, this dissertation applies polymer science principles to design polymeric materials by controlling their molecular characteristics to form robust structures using polymeric AM. In Chapter 2, the coalescence in polypropylene (PP) powders is enhanced by blending low molecular weight (LMW) PP with high molecular weight (HMW) PP. Using the Hopper coalescence model, the results indicate that the viscosity of the powder and extensional flow dominates the coalescence process. In Chapter 3, the impact of blending LMW with HMW PP on the properties of structures processed via PBF are examined. The results demonstrate that structures formed from the PP MW blend exhibit reduced void space, increased crystallinity, and improved thermomechanical behavior compared to those printed from neat HMW PP powders. Chapter 4 employs liquid-liquid phase separation to create powders that are blends of polycarbonate (PC) and PP matrix, with the goal of correlating the presence of PC in the powder on the crystallization of PP in the PBF process. The results reveal notable variation in the crystallization behavior and crystal structure of PP, including an v accelerated crystallization rate and reduction in lamellar thickness. Finally, Chapter 5 examines the effect of ultraviolet (UV) light on the material extrusion AM of PC. UV exposure causes fragmentation of PC chains, creating smaller chains that facilitate interlayer diffusion, reduce interlayer voids, and strengthen the PC-printed parts. Ultimately, the feedstock formulation protocols presented in this dissertation offer straightforward and cost-effective approaches to develop and produce mechanically robust polymeric structures using AM techniques.

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