Physically Cross-Linked Hydrogels with High Mechanical Properties and Temperature Sensitivity Achieved by High Initiator Content

Optimizing the mechanical properties (including stiffness,
toughness,
operational stability, and lifetime) of hydrogels is of great interest
toward broad applications of hydrogels such as soft actuators and
smart switches. Currently, common strategies to enhance the mechanical
properties of hydrogels generally change the components of the original
hydrogel, which inevitably produce side effects, especially the loss
of reversibility for physically cross-linked hydrogels. In this article,
we develop a strategy of adjusting the polymer chain length and internal
force by changing the initiator content to improve the mechanical
properties of physically cross-linked hydrogels without changing the
original components. The improved mechanical properties of physically
cross-linked hydrogels are derived from two factors: increased cross-linking
points induced by a higher monomer conversion ratio and shorter polymer
chains and strengthened interchain interactions by weakening the polymer
chain bonding to water, simultaneously achieved by increasing the
initiator content. Based on this mechanism, we report hydrogels with
stiffness up to ∼480.12 MPa and toughness up to ∼4.44
MJ/m3, superior to the reported high stiffness and toughness
hydrogels. Moreover, the hydrogels are temperature sensitive with
a wide range of stiffness changes between 20 and 70 °C (∼18-fold),
facilitating their application in shape memory polymers and smart
switches.