Polymerizable inverse HIPEs-based inks towards 3D printing of highly tailorable polymeric materials

Due to its processing requirements, extrusion 3D printing of plastics is limited to a rather narrow range of thermoplastic polymers. Here we present a strategy to formulate 3D printing polymerizable inks with rheological features compatible with extrusion-based approaches and applicable to hardly processable polymers, surpassing their thermal limitations. To do so, we formulate inverse oil-in-water high internal phase emulsions (i-HIPEs) with dispersed phase consisting of polymeric precursors and reaching 90 % volume fraction. As opposed to their water-in-oil counterparts, inverse-HIPEs polymerization, after 3D deposition, yields dense plastic rather than macroporous structures, mimicking the deposition of a fused thermoplastic filament. To demonstrate the validity of the approach, we formulate highly tailorable inks for room-temperature 3D printing of polystyrene (PS) and polymethyl methacrylate (PMMA), which are notoriously difficult to process via conventional thermal strategies. Despite their potential, inverse-HIPEs are far less characterized than classical ones. Here we provide a comprehensive rheological characterization, showing that these emulsions behave as Herschel-Bulkley fluids, exhibiting controllable yield stress, elasticity, and shear-thinning behavior, which are modulated by droplet packing, phase hydrophobicity, and continuous phase viscosity. The resulting inks display shear recovery and good printing potential. Preliminary curing tests demonstrate that methacrylate-based HIPEs undergo rapid UV polymerization, whereas styrene-based systems are thermally polymerized but require alternative strategies for efficient photopolymerization. This work provides a robust framework for the design of broadly applicable, monomer-in-water inks that unlock new possibilities in additive manufacturing with chemically diverse polymer targets.