Solving the dilemma between reprocessability and mechanical properties faced by recyclable bio-based polyurethane foam through confining imine bonds in the hard domains

Abstract

Introducing dynamic covalent bonds (DCBs) into polyurethane (PU) foams can solve the problem of their poor recyclability. However, a dilemma arises between reprocessability and mechanical robustness because of the low bond energies of DCBs. In this work, an aromatic diol (VAN-AM) containing imine bonds is synthesized to serve as the polyol together with castor oil. A series of bio-based polyurethane foams (BPUFs) with tunable bio-based carbon content, apparent density, cell properties and mechanical properties are prepared by adjusting the proportions of VAN-AM and castor oil. Benefiting from the molecular rigidity of VAN-AM, the dynamic imine bonds are enriched in the hard domains of PU. This allows for the kinetic control of the dynamicity of the PU networks. When the foams work at ambient temperature, which is much lower than the glass transition temperature of PU (113 ºC), the frozen segments of the hard domains inhibit the dynamic behavior of the imine bonds even if the latter may be somewhat activated (their triggering temperature ≈ 50 ºC). Consequently, the good mechanical properties of BPUFs can be maintained during service. At higher temperatures (> 113 ºC) or swollen state, the segmental mobility in the hard domains and dynamicity of imine bonds are greatly enhanced. Recycling of BPUFs is enabled as evidenced by hot-pressing the crushed foams into the compact sheets under mild conditions. The findings would facilitate the rational design of crosslinked polymeric foams with both high reprocessability and mechanical performance.