Hydration-Mediated Multiscale Structural Evolution in Polyurethane Hydrogels: From Water States to Microphase Reconstruction and Dynamic Networks

Polyurethane (PU) hydrogels combine high water content with the intrinsic structural heterogeneity of segmented polyurethane, giving rise to strong coupling among hydration, intermolecular interactions, microphase organization, and network dynamics. This review treats hydration as an active structural variable rather than solely as a swelling medium. The effects of soft- and hard-segment chemistry and network topology on the pre-hydration state are first summarized, followed by analysis of the multiscale sequence from water uptake and water state to competitive hydrogen bonding, segmental mobility, microphase reconstruction, and dynamic-network evolution. Particular attention is paid to distinguishing hydration-induced reconstruction of pre-existing domains from water-induced microphase separation, in which hydration generates new compositional segregation or phase connectivity. Direct evidence from PU hydrogels is also distinguished from mechanistic evidence derived from related hydrated PU, polyurethane–urea, and thermoplastic PU systems. The consequences of these processes for strengthening, toughening, cyclic recovery, stimuli responsiveness, wet interfaces, and representative applications are discussed. A system-dependent hydration window is further introduced to define the conditions under which sufficient molecular mobility for reversible reorganization coexists with persistent load-bearing structures. Remaining challenges include synchronized wet-state characterization, long-term fatigue and aging, and the establishment of predictive composition–hydration–structure–property relationships.