Emerging polyhydroxyurethanes as sustainable thermosets: a structure–property relationship

Polyhydroxyurethanes (PHU), obtained from CO2-based cyclic carbonates (CC) and polyamines, are known as greener and safer alternatives to conventional polyurethanes. Interestingly, the hydroxyurethane moieties present along the PHU’s backbone offer unexplored opportunities in terms of enhanced adhesion and mechanical properties that could be a major breakthrough in many structural applications. Furthermore, PHUs have shown thermomechanical recyclability arising from the ability of hydroxyurethane moieties to participate in reversible exchange reactions. However, the relationship between the macromolecular structure, the processability, and the final properties of these materials have not been evaluated to a sufficient extent to establish a comprehensive overview of these emerging thermosets. In this sense, this work aims to address this research gap by investigating the rheological and thermomechanical performances of PHU thermosets and opening an unexplored door for future sustainable engineered structural applications. A special emphasis was put on PHU thermosets formulated using potentially biobased monomers. The rheological behavior during cross-linking of the PHU formulations was studied and highlighted the importance of the number of CC functionalities in the viscosity and gel time, ranging from 10 min to nearly 3 h. Moduli superior to 2 GPa and glass transition over 50 °C were obtained for short multifunctional CC. Finally, the dynamic network behavior of these PHUs was also demonstrated through stress-relaxation and reprocessing. High temperatures (over 150 °C) and pressure lead to a good recovery of the thermomechanical properties. Such materials appear to be an interesting platform for structural applications, particularly fiber-reinforced polymers, that can overcome many sustainability challenges.