Kinetic-driven microphase separation and vitrification of crowded hydrogels with associative bonds

The thermal response of stimuli-responsive hydrogels is typically viewed through thermodynamic equilibrium. However, in "crowded" networks with dense associative bonds, elasticity and diffusion kinetics dictate distinct nonequilibrium fates. We investigate thermal transitions of a PAAc-CaAc₂ (polyacrylic acid complexed with calcium ions) hydrogel along two trajectories: a quasi-static isobaric pathway (path A) and a rapid quasi-isochoric pathway (path B). Along path A, the gel contracts homogeneously until reaching a hardening temperature via global vitrification (T_hard). In contrast, path B triggers confined microphase separation into polymer-dense and dilute phases at a critical temperature (T_dem), followed by a hardening transition via localized vitrification ([Formula: see text]). We demonstrate that T_dem is governed by a mechanical-thermodynamic competition: The internal pressure (P_c) to trigger demixing scales linearly with the permanent-network shear modulus (G_x), maintaining a ratio of P_c/G_x ≈ 100. We identify T_hard and [Formula: see text] as distinct hardening points, with elastically constrained local water content governing [Formula: see text]. This framework provides a strategy for programming nonequilibrium fates and "lockable" properties of responsive soft matter.