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-CaAc 2 (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 ( T hard PS ). 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 T hard PS as distinct hardening points, with elastically constrained local water content governing T hard PS . This framework provides a strategy for programming nonequilibrium fates and “lockable” properties of responsive soft matter.