Synergistic reinforcement of Diels–Alder cycloadducts with hydrogen bonding interactions in recyclable dual-dynamic polyurethane networks

Abstract

Here we report a facile, efficient strategy to prepare dual-dynamic networks (DDNs) comprising both thermally reversible Diels–Alder (DA) covalent bonds and non-covalent hydrogen bonds which combine excellent mechanical properties and creep resistance with facile processability at mild temperatures. A series of DDNs was synthesised via the copolymerisation of maleimide-terminated poly(ε-caprolactone urethane) or poly(1,4-butadiene urethane) prepolymers with multifunctional furan crosslinkers containing ester, urethane or urea functional groups. The mechanical properties of the resulting DDNs are enhanced by increasing the strength of crosslinker hydrogen bonding or reducing the polarity of the bismaleimide backbone, achieving a broad range of tensile strength (11.7–26.5 MPa), elongation (210–690%) and toughness (14.4–75.7 MJ m−3) values. DDNs comprising crosslinkers with stronger hydrogen bonding groups produced higher gel transition temperatures (Tgel), creep-resistance and tensile strength, implying synergistic network reinforcement. Furthermore, DDNs comprising the non-polar poly(1,4-butadiene) also presented improved creep resistance. For these materials, rubbery plateaus extended over broader temperature ranges resulting in higher Tgel up to 150 °C. Poly(ε-caprolactone) conferred networks with superior Young's modulus, tensile strength, toughness and flexibility. We have shown that materials can be thermally reprocessed multiple times whilst maintaining high stress recovery efficiencies and display rapid healing abilities under mild temperatures. This work highlights the crucial role of crosslinked network reinforcement via hydrogen bonding interactions to design high-performance yet recyclable polymer networks with tailored properties.