Supramolecular Salts for Additive Manufacturing of Polyimides

Recent advances in vat photopolymerization (VP) additive manufacturing of fully aromatic polyimides employed photoreactive high-molecular-weight precursors dissolved at modest loadings (<20 wt %) in organic solvent. These earlier efforts revealed high isotropic shrinkage, approaching 52% on a linear basis while converting to the desired polyimide. To increase the polyimide precursor concentration and decrease shrinkage during VP processing of high-performance polyimides, photoreactive fully aromatic polyimide and thermoplastic polyetherimide (PEI) supramolecular salt precursors now serve as versatile alternatives. Both pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA) and 4,4′-(4,4′-isopropylidene-diphenoxy)diphthalic anhydride-meta phenylene diamine (BPADA-mPD) supramolecular dicarboxylate–diammonium salts, termed polysalts, provided prerequisite rheological performance and photoreactivity for VP. Solutions (50 wt %) of both photoactive polysalts exhibited viscosities more than two orders of magnitude lower than previously reported polyimide precursor solutions for VP. In addition, VP of 50 wt % polysalt solutions yielded high resolution, self-supporting organogel structures. During thermal postprocessing to the desired fully aromatic polyimide and PEI, photocrosslinked polysalt organogels exhibited retention of part shape in concert with linear isotropic shrinkage of only 26%, the lowest reported value using organogel strategies for VP of fully aromatic polyimides. Furthermore, the imidized structures exhibited comparable thermal and mechanical properties to analogous polyimides synthesized using classical methodologies for 2D films. The combination of facile synthesis and increased precursor concentrations designates polysalt polyimide precursors as a versatile platform for additive manufacturing of well-defined 3D polyimide structures.