Cross-linked polyurethane with dynamic phenol-carbamate bonds: properties affected by the chemical structure of isocyanate

Abstract

Polyurethanes with covalent adaptive network (CAN) have received widespread attention due to their recyclability and self-healing properties. The strategy of regulating the dynamic network rearrangement kinetics through varying the monomer structure is particularly interesting. Herein, based on the phenol–carbamate dynamic bond, we design a strategy to regulate the rearrangement kinetics of the dynamic covalent network by adjusting the chemical structure of aliphatic isocyanates with the same initial cross-linking degree. Temperature-varying FTIR and stress relaxation experiments prove the feasibility of this strategy from the perspective of thermodynamics and kinetics. For the studied isocyanates, greater steric hindrance or attachment of benzyl groups can promote the rearrangement of CAN. The two factors can reduce the relaxation time by one order of magnitude, and the relaxation activation energy can be reduced by over 20 kJ mol⁻¹. Regardless of the network rearrangement kinetics, all polyurethane samples show good self-healing and reprocessing performance. In addition, the mechanical properties and stability in a hot air environment also vary with the chemical structure of the isocyanates, e.g., the isocyanates with alicyclic structure can improve the thermal stability, and the mechanical properties are almost unchanged after 24 h of treatment at 100 °C under air conditions, while the other polyurethanes show obvious softening. The method to regulate the rearrangement kinetics of phenol–carbamate CAN via changing the chemical structure of the monomers would be beneficial for developing self-healing and recyclable cross-linked polyurethanes with variable properties synthesized from industrial chemical feedstocks.