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- All Subjects: Polymer Chemistry
- Creators: Williams, Christopher B
Description
High-performance polymers (HPPs) have dominated the synthetic polymer market for critical applications, including aerospace, energy, microelectronic, and transportation industries since their development in the mid-1900s. Although their structures share general similarities, such as high aromatic content, HPPs offer wide structural variance providing amorphous and semi-crystalline systems. As a result, conventional processing methods employed for HPPs are energy intensive and accessible part geometry is limited; often requiring subsequent subtractive techniques, i.e.,; milling, to obtain high quality and performant parts. Traditional processes were challenged by the emergence of advanced manufacturing techniques, such as 3D printing, which spurred significant academic and industrial interest. In the first project, poly(arylene ether sulfone)s (PSU) were chemically modified post-polymerization to enable ensuing photopolymerization of high molecular weight (Mn) PSU solutions into complex shapes with vat photopolymerization (VP). The resulting materials exhibited fast crosslinking, but low and unstable plateau storage moduli (G’). To overcome this, addition of low molecular weight crosslinker and precise control of UV irradiation increased crosslink density and inhibited photodegradation events, respectively. Ultimately, these modifications facilitated the first report of PSU structures fabricated with a UV-assisted AM modality. Next, 3D printable polyimides (PIs) were synthesized and extensively characterized to further expand the HPP AM toolbox. However, fully aromatic PIs pose a significant challenge as most are insoluble, intractable, and lack any discernable viscous flow. AM PIs were produced using two distinct approaches previously reported in the Long research group; the pendant salt approach imparts photoreactivity through the neutralization of the poly(amic acid) intermediate with small molecule amino-acrylates while the polysalt approach employs dicarboxylate-diammonium ionic organization to template the PI amongst an acrylic scaffold. Through the pendant salt approach, water soluble PI precursors enabled facile AM of complex structures, which served as efficient carbon precursors. The polysalt approach offers superior solid content and solution viscosities; however, these highly polar solutions initially exhibited deleterious side reactions. Application of acid-base fundamentals provided novel printable polysalt solutions with extended shelf-life, reproducible printing, and simplified processing. The relationships established from these projects expanded the applications of the most performant synthetic polymers and will inform future polymer design for additive manufacturing.
ContributorsWeyhrich, Cody (Author) / Long, Timothy E (Thesis advisor) / Williams, Christopher B (Committee member) / Biegasiewicz, Kyle F (Committee member) / Jin, Kailong (Committee member) / Arizona State University (Publisher)
Created2023
Description
Creating 3D objects out of high performance polymers, such as polyimides, is notoriously difficult since the highly stable polymer backbone limits processibility without extreme conditions. However, designing the polyimide precursor to crosslink upon photoirradiation enables the additive manufacturing of polyimides into complex, 3D objects. Crosslinking the photoactive polyimide precursor forms a solid 3D organogel, then subsequent thermal treatment removes the sacrificial scaffold and simultaneously imidizes the precursor into a 3D polyimide. The collaborative efforts of the Long and Williams group at Virginia Tech created three chemically distinct photoactive polyimide precursors to additively manufacture 3D polyimide objects for aerospace applications and to maintain the nuclear stockpile. The first chapter of this dissertation introduces fully aromatic polyimides and the additive manufacturing techniques used to print photoactive polyimide precursors. The second chapter reviews the common pore forming methods typically utilized to develop porous polyimides for low dielectric applications. The following chapters investigate the impact of the sacrificial scaffold on the thermo-oxidative aging behavior of the polyimide precursors after imidization, then focuses on lowering the imidization temperature of the polyimide precursor using base catalysis. These investigations lead to the creation of photoactive polysalts with polyethylene glycol (PEG) side chains to develop 3D, porous polyimides with tunable morphologies. Varying the molecular weight and concentration of the PEG side chains along the backbone tuned the pore size, and the photoactive nature of the polyimide precursor enabled 3D, porous polyimides printed using digital light processing.
ContributorsVandenbrande, Johanna (Author) / Long, Timothy E (Thesis advisor) / Williams, Christopher B (Committee member) / Jin, Kailong (Committee member) / Seo, Eileen (Committee member) / Arizona State University (Publisher)
Created2023