Solving the difficult recyclability of conventional thermosetting polyurea elastomers based on commercial raw materials in a facile way

Abstract

Great efforts have been devoted to solving the issue that covalent thermosetting polymers and their composites, unlike thermoplastic polymers, are unable to undergo reprocessing via secondary melting or recycling through injection or extrusion after curing, which prevents them from forming three-dimensional network structures. However, tremendous attention has been focused on introducing specific dynamic covalent bonds or noncovalent bonds into the molecular structures of thermosetting polymers, which requires choosing or synthesizing particular monomers, greatly limiting the industrial application potential. Herein, we have devised and prepared a series of conventional thermosetting polyurea elastomers (TPUEs) by utilizing the commercial raw materials of poly(propylene oxide)diamine and different diisocyanate monomers. These TPUEs have remarkable solvent resistance, great thermal stability, and outstanding transparency in the visible light range. In particular, the toughness, the breaking strength, and the elongation at break of TPUEs can reach maximum values of 300 MJ m⁻³, 31.4 MPa, and 1752%, respectively, making them superior to most dynamic materials and commercial elastomers that exhibit exceptional mechanical properties. Surprisingly, the TPUEs devised here have a superior dynamic nature owing to the classic urea bond, as confirmed via analysis of small molecules and dynamic properties, such as stress-relaxation and creep–recovery. These TPUEs can maintain remarkable mechanical properties, chemical stability, and solvent resistance after being recycled multiple times. Additionally, we propose a universal mechanism by which these TPUEs can simultaneously have mechanically robust properties in the range from the glass transition temperature of the soft segment (T_gs) to their topology freezing-transition temperature (T_v) (about −40 °C to 150 °C) and excellent recyclability at temperatures ranging from T_v to the thermal decomposition temperature (T_d) (about 150 °C to 220 °C). Integration of mechanical robustness and a dynamic nature into one network can provide a facile way to solve the recyclability issues relating to conventional TPUEs.