Infrared Spectroscopic and Mechanical Analysis of Supramolecular Self-Healing in 3D Printable Urea Photoresins

Supramolecular interactions provide 3D printable materials with noncovalent cross-linking and stimuli-responsive properties to improve their processability and functionality. Here, we developed photoprintable acrylate polymer networks that contain pendant aliphatic or aromatic urea groups at different molar ratios to improve their toughness and enable self-healing ability via hydrogen bonding. We synthesized two methacrylate monomers containing either an aliphatic or an aromatic urea motif, and we formulated them in liquid photoreactive resins. We 3D printed these formulations and evaluated their tensile mechanical strength, elongation, toughness, and self-healing properties. By increasing the supramolecular cross-linking density provided by the urea groups, we improved the mechanical strength up to 119%, and the toughness up to 205%, compared to the control, in formulations with 10% molar ratio of the aromatic urea monomer, without sacrificing the elongation of the printed parts. Physical evidence for the presence of hydrogen bonding was provided with variable temperature Fourier transform infrared (VT-ATR-FTIR) spectroscopy and van’t Hoff analysis. The self-healing efficiency of these formulations was characterized by measuring the recovery of their tensile mechanical properties. Resins that contained 10 mol % of the aliphatic or aromatic urea monomers recovered more than 100% of their original mechanical strength. These results show the ability of supramolecular cross-linking via urea hydrogen bonding in improving the toughness, tuning the mechanical properties, and imparting stimuli-responsiveness in materials that are compatible with photoprinting methods.