Additive manufacturing of inherently porous polymers: Polymerized high internal phase emulsion (polyHIPE) structures via vat photopolymerization

High internal phase emulsions (HIPEs) show promise as vat photopolymerization additive manufacturing (AM) resins for creating innovative lightweight porous materials. Despite their potential, HIPEs often scatter light during the AM process, resulting in poorly defined and low-resolution structures. However, the incorporation of light absorbers can significantly enhance printing resolution. This study investigated the inclusion of light absorbers into the HIPE-based resin and assessed the compatibility of these resins with a commercial vat photopolymerization additive manufacturing setup. A water-in-oil emulsion, stabilized by the surfactant (hypermer) and formulated with 2-ethylhexyl-acrylate and isobornyl-acrylate was used in this study. Light absorbers, including hydrophobic beta-carotene and hydrophilic tartrazine molecules dissolving in the organic and aqueous phases respectively, were incorporated. The use of beta-carotene and tartrazine together was found effective in achieving the best 3D printing resolution. Moreover, the emulsion remained stable throughout the printing process, resulting in a porous polyMIPE structure with open surface porosity.
Expanding the application of 3D-printed polyHIPE structures, their utility as templates for electroless nickel plating to create highly porous metallic lattice structures with consistent wall thickness was examined. The electroless nickel plating process on polyHIPEs was optimized and the effects of different atmospheres during the heating process to remove the polyHIPE template from metallized 3D-printed polyHIPE lattice structures were explored. Distinct compounds were formed in various atmospheres, with fully oxidized nickel-coated polyHIPEs in an air atmosphere and the formation of nickel phosphide (Ni3P) structures in argon and reducing atmospheres alongside Ni metal. Notably, the porous structure of the polyHIPEs was retained in argon and reducing atmospheres, where the internal structure undergoes complete carbonization. This study demonstrated the versatility of intricate polyHIPE structures as templates for creating ultra-porous metal-based lattices.
Furthering the exploration, inherently porous polyHIPE lattices with three distinct porosities (80%, 85%, and 87.5%) at various temperatures (500ºC, 600ºC, 700ºC, and 800ºC) were pyrolyzed to fabricate porous carbon structures. The successful carbonization was confirmed through Raman spectra and XRD analysis. Mechanical testing results indicated a higher Young's modulus in carboHIPEs compared to polyHIPEs, with carboHIPE lattices demonstrating a lower Young's modulus compared to monolithic carboHIPE discs. This comprehensive exploration contributed valuable insights into the design and performance of hierarchically porous carbon materials fabricated using 3D-printed polyHIPE lattices.
In conclusion, this study established the potential of HIPEs as printing resins for creating inherently porous intricate structures and utilizing them as templates for producing highly porous complex metal-based and carbon structures.