Polyimides provide an attractive complement of thermal and chemical physical properties for harsh environments, which is unattainable for most polymeric materials. The conventional manufacturing of polyimides limits the processability to 2D form factors due to the rigid and unreactive backbone. Previous reports entail an energy-intensive sintering technique to manufacture 3D polyimide objects, but the method offers limited obtainable geometries. The advancement of the Long and Williams group permitted the additive manufacturing of fully aromatic polyimides using three complimentary synthetic techniques: poly(amic acid) pendant salts, poly(amic diacrylate esters), and polysalts. The acrylate side groups along the precursor backbone promoted the organogel formation upon UV irradiation. Thermal treatment of the printed organogel yields a darkened, black polyimide. The discoloration, after imidization at 400 °C, is a result of leftover carbonaceous char from the degraded sacrificial scaffold. Disrupting the charge-transfer complexes (CTCs) in the polyimides prevents the electronic transitions, between an electron-rich dianhydride and an electron-poor diamine, responsible for the orange color. Typical colorless polyimides utilized fluorinated monomers or bulky side groups to disrupt the CTCs, producing an optically clear, colorless polyimide. Reducing the rigidity of the polyimide backbone decreases the T<sub>g</sub>, thereby improving the imidization degree at lower temperatures (&lt;200 °C). The reduced imidization temperature prevents the degradation of the sacrificial scaffold in the photoactive polyimide precursors, and hence prevented discoloration of the polyimide. Herein is the comparison of three complimentary synthetic techniques to develop reduced-color polyimide objects using additive manufacturing.

Additive manufacturing of polyimides with reduced color and improved aging

ACS Spring 2022

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