Hydrogels: From Simple Covalently Cross‐Linked Systems toward Hydrophobically Associating Adaptive Semicrystalline Materials

ABSTRACT

Hydrogels have evolved from simple water‐swollen polymer networks into sophisticated adaptive materials governed by hierarchical molecular interactions and structural organization. As discussed in this review, their macroscopic behavior cannot be explained by ideal network theories alone, but instead arises from the interplay of cross‐linking efficiency, network heterogeneity, finite chain extensibility, and dynamic intermolecular interactions across multiple length scales. The intrinsic non‐ideality of hydrogel networks, including structural defects and ineffective cross‐links, limits the applicability of classical elasticity models. These limitations are particularly evident in highly swollen and ionic hydrogels, where non‐Gaussian elasticity and ion‐induced osmotic effects dominate the mechanical response and produce strong environmental sensitivity. Consequently, hydrogel research has shifted from static covalently cross‐linked systems toward hydrophobically associating adaptive semicrystalline materials. Hydrophobically modified hydrogels exemplify this transition, as reversible hydrophobic associations act as physical cross‐links that enhance toughness, dissipate energy, and enable self‐healing. Micellar polymerization further allows the formation of transient hydrophobic domains, yielding materials with tunable mechanics and adaptive behavior. The incorporation of semicrystalline domains broadens the accessible property range by combining stiffness, toughness, extensibility, and shape‐memory functionality, reflecting a broader move toward bioinspired hydrogel design.