Engineering of Chain Extender Flexibility and Hierarchical Hydrogen Bonding Toward Ultra‐Low‐Temperature Poly(Urethane‐Urea) Elastomers with Exceptional Toughness

Abstract The demand for elastomeric materials with exceptional mechanical properties at low temperatures is increasingly growing. However, meeting these requirements remains a significant challenge to date. The high strength and toughness of many PTMEG‐based elastomers are compromised at low temperatures. The ordered nature of their molecular chains leads to (semi‐)crystallization of the soft segments, preventing substantial recovery. In this study, a series of hydroxyl terminated polybutadiene‐based polyurethane (HTPB‐PU) elastomers (HPUs) with balanced rigidity and flexibility are synthesized by modulating the interplay between rigid (urethane and urea bonds combined with aryl groups) and flexible molecular segments (HTPB). Hierarchical hydrogen bonding and aryl π–π stacking form rigid nanostructured domains, together with the large polarity difference and absence of hydrogen bonds between the soft and hard segments of HPUs, resulting in pronounced microphase separation. The rigid nanostructured domains disrupt the regular alignment of HTPB chains, thereby enabling the material to maintain exceptional ductility at −70 °C. Consequently, the HPUs exhibit remarkable elongation at break (842.6 ± 7.4%), exceptional fracture toughness (254.1 ± 16.0 MJ m −3 ), as well as excellent tear resistance, oil resistance, solvent resistance, and fatigue resistance at −70 °C. These findings provide a promising pathway for designing next‐generation elastomers for extreme environments.