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.