Engineering Water‐Stiffening Polymers via PEG‐Sidechain‐Mediated Microphase Separation

Stiffness‐variable polymers have found their enormous potential in practical applications that require automatic adaptation and modulation. However, prevailing materials are often constrained by specific operational triggers, limited mechanical responsiveness, and inadequate universality. In this study, drawing inspiration from the mechanism of non‐solvent induced phase separation (NIPS), a novel series of polymers exhibiting reversible water‐triggered stiffening is designed and synthesized by covalently incorporating poly(ethylene glycol) (PEG) as a non‐solvent mediator onto the hydrophobic poly(meth)acrylates backbones. Owing to the hydrophilicity difference between PEG sidechains and backbones, these polymers display swift and substantial alterations in stiffness, with large‐scale and regulable changes (from several‐fold to exceeding 200‐fold) upon water penetration, resulting from water‐induced microphase separation. Meanwhile, the polymers also possess intrinsic dual‐responsive shape‐memory properties. Due to the excellent commercial availability, versatility, and designability of these polymers, the work provides novel perspectives for the advancement of water‐triggered stiffening materials and opens avenues for their prospective applications in various domains. A novel and universal approach for achieving stiffness modulation in polymers by covalently enchaining PEG onto rigid hydrophobic macromolecular backbones is proposed. The synthesized polymers exhibit significant alteration in stiffness, wide tunability, substantial versatility, and excellent processability, showcasing versatility and potential in load‐bearing and personal protective applications.