Recyclable Polyurethane Thermosets with High Rigidness and Toughness via Side Chain Induced Dual Phase Separation Strategy.

Polyurethane thermosets with high strength, rigidity, and cost efficiency are highly desirable for industrial applications. However, they always suffer from brittleness and poor recyclability owing to their crosslinked structure, causing an unacceptable level of sustainability concerns. Here, a side chain induced dual phase separation strategy is proposed to tackle these limitations. Specifically, a crosslinked rigid polyurethane is designed, synthesized from small-molecule diols with a side chain. The urethane groups form hard domains by hydrogen bonding and induce a primary phase separation. Moreover, the curing process immobilizes part of the polymer segments with side chains to promote a secondary phase separation and generate a heterogeneous crosslinked structure. The primary phase separation enables energy dissipation, while the secondary phase separation serves as stress concentrations or crack initiation points to enhance the toughness. Notably, the transesterification between urethane groups, coupled with hydrogen bonding, facilitates catalyst-free reprocessability, extending thermosets lifespan. This recyclable polyurethane thermoset exhibits a high Young's modulus (1.13 GPa) and exceptional toughness (77.76 MJ m-3). Furthermore, its application and recyclability in glass fiber-reinforced composites are demonstrated. This strategy provides a viable route toward sustainable thermosetting polymers, addressing key environmental concerns associated with their widespread use.