A Reversibly Cross‐Linked Network Enables Spider‐Silk‐Like Strain‐Adaptive Mechanical Behavior and Low‐Temperature Tolerance in Artificial Fibers

ABSTRACT Replicating spider‐silk‐like strain‐adaptive mechanical behavior remains challenging because elasticity, rapid stiffening, and energy dissipation must be sequentially activated across distinct deformation regimes. Here, we reproduce this behavior in reversibly cross‐linked fibers fabricated by wet‐spinning and post‐drawing of a poly(tetramethylene ether glycol) (PTMG)‐based polyurethane. The fibers exhibit high compliance and elasticity below 15% strain, rapid stiffening within 15%–35% strain, with the modulus increasing from 45 to 574 MPa, and strong energy dissipation above 35% strain, with a damping efficiency of up to 94%. The combination of stiffening and energy dissipation enhances damage tolerance, yielding a fracture energy of 2060 kJ m −2 . This behavior arises from strain‐induced sequential evolution in the reversibly cross‐linked network, in which amorphous PTMG strands are cross‐linked by co‐aligned PTMG crystallites and phase‐separated supramolecular nanodomains. Initial strand uncoiling accommodates low‐strain elasticity, subsequent load transfer to cross‐links induces intermediate‐strain stiffening, and cross‐link disruption driven by the transmitted stress enables high‐strain energy dissipation. The complete three‐regime mechanical response is retained down to −60°C. The synergy among these regimes enhances the impact‐buffering capability of fiber‐based nets. This work establishes a general strategy for designing high‐performance polymers in which mechanically antagonistic properties are not only integrated but also sequentially activated.