Mechanics Underpinning Phase Separation of Hydrogels.

This paper reveals the underpinning role of mechanical constraints and dynamic loading in triggering volume phase transitions and phase separation of hydrogels. Using the Flory-Rehner free energy that does not predict phase separation of hydrogels under equilibrium free swelling, we show that mechanical constraints can lead to coexistence of multiple phases. We systematically obtain the states of equilibrium for hydrogels under various mechanical constraints and unravel how mechanical constraints change the convexity of the free energy and monotonicity of the stress-stretch curves, leading to phase coexistence. Using a phase-field model, we predict the pattern evolution of phase coexistence and show that many features cannot be captured by the homogeneous states of equilibrium due to large mismatch stretch between the coexisting phases. We further reveal that the system size, quenching rate, and loading rate can significantly influence the phase behavior, which provides insights for experimental studies related to morphological patterns of hydrogels.