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.