Robust Nanofiller with Rigid Armor and Stretchable Core for Polymer Strengthening and Toughening

The intrinsic trade‐off between mechanical strengthening and toughening restricts the advancement of high‐performance polymer nanocomposites. Conventional rigid fillers often induce brittleness via stress concentrations, while liquid inclusions compromise load‐bearing capacity. To resolve this fundamental material paradox, we presented a bioinspired “rigid armor‐stretchable core” nanostructure architecture: liquid metal nanoparticles encapsulated by MXene nanosheets (LMNPs@MXene), synthesized via a thermodynamically driven, hyperbranched polymer‐mediated interfacial assembly. This hybrid design functioned as a dynamic interfacial stress regulator to alter the stress transfer paradigm within polymer matrices: the rigid MXene shell ensured robust interfacial adhesion and efficient load transfer, while the liquid core underwent adaptive, reversible deformation to serve as a high‐capacity energy dissipation sink. Upon integration into a polyvinyl alcohol matrix, the composite films exhibited a simultaneous breakthrough in tensile strength (58.1 MPa) and elongation at break (297.2%), culminating in a toughness of 102.8 MJ m −3 . This strategy proved universal across diverse matrices, ranging from brittle cellulose nanofibers to engineering polyimide and elastomeric polyurethane. Furthermore, the photothermal conversion capability of the hybrid filler endowed the composites with rapid, light‐triggered self‐healing properties. This work establishes a new paradigm for designing adaptive, stress‐managing materials, transcending the limits of static reinforcement.