In situ observation of polyurea formation by rapid-scan time-resolved infrared spectroscopy

Polyurea and its derivatives exhibit excellent physical and chemical properties originating from intermolecular hydrogen bonding. Consequently, they have been used as coating materials that are resistant to heat, water, and chemicals, and in recent years, as materials for 3D printers. Their properties depend on their formation and hydrogen bonding mechanisms; however, these processes are very fast, making them challenging to study experimentally. In this study, polyurea formation via the polyaddition reaction of a diamine and diisocyanate was investigated in situ by fast time-resolved Fourier-transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. This method enabled the analysis of the polyaddition reaction within 1 s and observation of intermolecular hydrogen bond formation. We observed the polyurea formation of different aliphatic and alicyclic monomers by recording the FTIR-ATR spectra every ∼0.07 s and analyzed the reaction kinetics. In the case of aliphatic monomers, strong hydrogen bonds were formed in the initial stage of polyurea formation, whereas in the case of alicyclic monomers, weak hydrogen bonds were initially formed, which strengthened as the reaction proceeded. These findings elucidate the role of monomer structure in the formation and hydrogen bonding state of polyurea and demonstrate the effectiveness of time-resolved FTIR-ATR spectroscopy as a technique for observing very fast reactions and intermolecular interactions.