UV-Assisted Direct Ink Writing of Dual-Cure Polyurethanes

Direct ink writing (DIW) offers a unique avenue for printing a variety of soft materials into complex objects. However, rapid DIW of elastomeric materials with control over the final printed shapes remains a challenge. In this work, we present a methodology for printing a commercial thermally curable polyurethane elastomer via UV-assisted DIW (UV-DIW) through a dual-cure approach. The hybrid dual-cure resin consists of photopolymerizable acrylate monomers for rapid shape fixation and thermally curable polyurethane monomers to provide tailorable elastomeric mechanical properties. By tuning the composition of acrylate and polyurethane networks, a wide range of mechanical properties were achieved, ranging from soft elastomers (E ∼ 2 MPa) to rigid plastics (E ∼ 1 GPa). Phase behavior and network interpenetration were investigated through atomic force microscopy, revealing that the dual-cured polymers had two-phase microstructures with submicron domain sizes and a matrix inversion as the composition varied. The polyurethane elastomers were printable via UV-DIW with minimal acrylate content (20 wt %) and have excellent mechanical properties, including high elongation (>600%) and toughness (>10 MJ m^-3). It was demonstrated that this approach can generate multimaterial parts with regions of disparate stiffness, useful in applications such as pneumatic soft actuators, with excellent adhesion between adjacent regions and layers.