(38a) 3D printing all-aromatic polyimides with light: Photoreactive supramolecular polymeric salts as a versatile printing platform

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. Moreover, research must impose a lens of sustainability to ensure next-generation advanced materials for advanced manufacturing minimize negative consequences on the environment. 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 diene-based 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. This lecture will focus on printed three-dimensional all-aromatic polyimide objects that 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 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 (Figure 1A). Polymeric salts, commonly termed polysalts, that are derived from various photo-reactive dicarboxylic acids with various diamines are amenable to vat photopolymerization and with subsequent thermal processing to fully aromatic polyimides. This approach allows for low solution viscosities, which minimizes dipolar aprotic organic solvents and leads to reduced shrinkage in printed lattices. Recent efforts have expanded the library of suitable precursors with attention to the formation of more stable polymeric salts, thus allowing for longer shelf lives for polyimide precursors and more reproducible printing. This advance was based on an in-situ FTIR spectroscopic investigation, which demonstrated the importance of understanding aryl amine-containing monomer nucleophilicity in the presence of adjacent photo-reactive acrylate functionality. Avoiding the Michael addition reaction allowed for the formation of polymeric salts of amine-containing aromatic monomers with acrylate functionalities with a preservation of photo-reactivity. Current efforts involve an investigation of the role of crosslink density of the organogels on the printing process and structure of the intermediate high modulus green bodies.

Polymeric salts have also enabled pathways for the formation of porous printed objects with either the introduction of porogenic sequences as graft copolymers or facile solvent swapping technique followed with lyophilization to form aerogels (Figure 1B). Scanning electron microscopy (SEM) and small angle x-ray scattering (SAXS) revealed the formation of well-defined porous structures and demonstrated the opportunity to print porous polyimides with the preservation of geometric complexity. This lecture will also illustrate the opportunity to print monolithic carbon upon pyrolysis of the fully aromatic polyimide structures. The geometric complexity was preserved in the printed object upon pyrolysis, suggesting opportunities for printed catalysts, electrodes, and membranes. Raman spectroscopy confirmed the balance of disordered and graphitic carbon as a function of polyimide composition and processing conditions.

This lecture will introduce the concept of printing hydrogels and organogels with a focus on recent efforts to print all-aromatic polyimides. Success demanded facile and versatile synthetic strategies coupled with tailored post-process imidization strategies, representing the criticality of concurrent design of advanced materials and next-generation machines.