Compatibilisation of immiscible soft segments in self-healing thermoplastic polyurethane elastomers via Diels-Alder cycloaddition

Abstract

In this work, Diels-Alder (DA) chemistry has been used for the first time to make dual-soft segment thermoplastic polyurethane (TPU) elastomers (M_n = 34,000–42,000 g.mol⁻¹) with superior properties from incompatible precursors. Immiscible poly(1,4-butadiene) and poly(ɛ-caprolactone) polyols were used to prepare maleimide and furan-terminated poly(butadiene urethane) (PBU) and poly(ɛ-caprolactone urethane) (PEU) prepolymers (M_n = 7000–8000 g.mol⁻¹) from isocyanate terminated intermediates. These prepolymers were subsequently copolymerised via DA-cycloaddition reactions to prepare DA-(PBU-co-PEU) multiblock copolymers comprising 25, 50 and 75 wt.% PBU and PEU segments, which were compared to DA-100PBU or DA-100PEU copolymers containing 100 wt.% PBU or PEU segments. The morphology and thermomechanical properties of the DA-TPUs can be systematically controlled via altering PBU : PEU composition. Copolymers containing ≥75 wt.% PEU segments display predominantly semi-crystalline behaviour, whilst those containing ≤50 wt.% PEU segments exhibit amorphous behaviour. Copolymerisation via DA-cycloaddition enables soft segment compatibilisation providing the composition DA-(50PBU-co-50PEU) with excellent elastomeric properties (increased toughness by a factor of 7 over DA-100PBU and enhanced elastic recovery over DA-100PEU). The elastomers also display excellent thermal reprocessing and healing abilities under mild conditions (≤130 °C), retaining high stress recovery ratios over 85 % and rapidly healing 50 µm defects. The facile methodology established in this study could be applied to compatibilise other immiscible soft segments to obtain recyclable, self-healing materials with unique properties.