(75h) Designing Advanced Materials for Advanced Manufacturing: Striving for Sustainability

Additive manufacturing (AM) offers the promise for addressing looming concerns for materials sustainability with chemical processes that require less energy and result in less material consumption. However, research must impose a lens of sustainability earlier in the innovation process, where processes and materials are designed to adhere to the principles of green chemistry and strive for more sustainable engineering. Novel materials for AM continue to emerge at a feverish rate, and it is imperative that we begin to consider end-of-life (EOL) earlier in the material and process design. This lecture will highlight our efforts to forge a more sustainable field of additive manufacturing, a field that continues to diversify material composition due to the unique functionality that is required for successful printing. Our research has focused on light-based AM modalities due to the unique combination of low energy consumption, exceptional resolution that enables less material consumption in latticed architectures, and the opportunity to print high molecular weight polymers with low viscosities. An important motivation is the opportunity to intensify chemical processes, wherein the synthetic chemistry occurs in a printed shape that serves as the reactor that ultimately results in a printed object.

Future advances in additive manufacturing (AM) will require the rethinking and redesign of traditional macromolecular structure. Advanced macromolecular materials for advanced manufacturing require a precisely tailored balance of reactivity and rheological performance that collectively ensure precise resolution from diverse additive manufacturing modalities. Our recent research has focused on the printing of high molecular weight polymers using vat photopolymerization (VP) and UV-assisted direct ink write (UV-DIW) where printed organogel and hydrogel precursors enabled the printing of high-performance fully aromatic polyimides and styrene-butadiene copolymer elastomers, respectively. Our recent results illustrate the solvent-free stereolithographic printing of high molecular weight polymers as low viscosity aqueous colloids to prepare elastomers with unprecedented three-dimensional geometries. Printed three-dimensional all-aromatic polyimide objects present identical thermomechanical properties compared to polyimide films that are derived from legacy manufacturing processes. Acrylate and epoxide functionalities continue to play an important role in the design of photoreactive polymers for AM, however, recent adaptations of thiol-ene chemistry, photo-acid and photo-base generators, and unsaturated co-polyesters exemplify the rapidly expanding toolbox of functional polymers for additive manufacturing. The necessity to maintain a lens of materials sustainability will fuel the discovery of functional polymers that are amenable to recyclability and circularity, thus catalyzing the emerging field of sustainable additive manufacturing (SAM).

Supramolecular chemistry also offers unique opportunities for the design of thermo-reversible polymers for additive manufacturing. Our earlier efforts demonstrated the importance of electrostatic interactions and multiple hydrogen bonding for fused filament fabrication of water- soluble polyesters and polyurethanes with improved isotropic properties. Most recently our research has focused on the formation of polymeric salts as precursors for vat photopolymerization. Polysalts that are derived from various photo-reactive dicarboxylic acids with various diamines are amenable to vat photopolymerization with subsequent thermal processing to fully aromatic polyimides. This approach has allowed for the use of alcohols and water with the removal of organic solvents for printing. Moreover, the polysalts exhibit low solution viscosities and hence high polymer concentrations are permitted, leading to less shrinkage in the final printed objects.

Vat photopolymerization of aqueous colloids continues with the introduction of inorganic nanoparticles for the formation of nanocomposites and a demonstration of a vast array of polymeric latexes. 3D printed nanocomposites were achieved with UV assisted direct ink write processes due to the desirable shear sensitivity of the hybrid colloid. Our research includes the design of photo-reactive systems for the printing of various high-performance polymers from elastomers and elastomeric composites to high performance engineering polymers.

This lecture will highlight recent advances in polymer design with a molecule to manufacturing perspective, and tailored crosslinking plays a major role in the design of elastomers for 3D stereolithographic printing, polyimides for vat photopolymerization, and novel polymer architectures for binder jetting of water-soluble adhesives. Star-shaped architectures of water- soluble poly(N-vinyl pyrrolidone) allow high concentrations for jetting solutions into powder beds for pharmaceutical manufacturing. The star-shaped polymers, which are prepared using RAFT polymerization, result in reduced entanglement concentrations, and higher c* concentrations that permit a higher maximum jetting concentration. Photo-reactive, fully aromatic polyimides that are decorated with acrylate substituents provide chemically crosslinked organogels that are amenable to thermal post-processing, and thermal decomposition of the network scaffold occurs without structural detriment to the polyimides. Our latest efforts involve the preparation of semi-interpenetrating networks of latexes and crosslinked functional poly(ether) scaffolds using vat photopolymerization. Recent efforts have demonstrated three-dimensional elastomeric objects that approach the performance of conventional elastomers in the presence and absence of the scaffold network. This lecture will illustrate various crosslinking strategies where the crosslinks are susceptible to reversibility using various external stimuli, ranging from heat and pH to photo-irradiation and ionic salts.

The following Figure illustrates recent vat photopolymerization of photo-functional polymeric salts for the 3D printing of all aromatic polyimides.