Breaking the Trade‐Off of Mechanical Robustness and Energy Storage Capacity in Phase Change Materials Through a Molecular Design Strategy of Hard‐Segment‐Anchored/Side‐Chain‐Storage

ABSTRACT Phase change materials (PCMs) have garnered significant attention for personal thermal management, yet conventional solid‐liquid PCMs suffer from potential leakage issues, and existing solid‐solid polyurethane‐based PCMs (PU‐PCMs) often face an inherent trade‐off between mechanical robustness and energy storage capacity. Here, we develop a molecular design strategy of hard‐segment‐anchored/side‐chain‐storage to decouple these competing functions by relocating the phase‐change units from the polymer main chain to side chains. Specifically, a series of flexible side‐chain PU‐PCMs were synthesized using 2,2'‐(octadecylimino)diethanol (SDEA) bearing crystallizable C18 alkyl side chains and 1,4‐butanediol (BDO) as chain extender. The hard segments formed by BDO and hexamethylene diisocyanate provide a robust physical network, while the hanging C18 side chains independently crystallize to enable solid‐solid phase transition and latent heat storage. By adjusting the BDO‐to‐SDEA molar ratio, the optimized material, SPU‐BDO _0.20 , exhibits a desirable combination of high toughness, excellent flexibility (elongation at break >600%), high phase change enthalpy (51.1 J g ^{− 1} ), and outstanding thermal cycling stability (500 cycles). Notably, electrospun fibrous membranes fabricated from SPU‐BDO _0.20 demonstrate effective temperature regulation, maintaining a surface temperature 3°C–4°C lower than cotton controls under heating. This work provides a new strategy for developing high‐performance, flexible, and leakage‐free PCMs for wearable thermal management applications.