Mechanism Study on Mechanical Properties of Physical-Chemical Hybrid Hydrogels by Coarse-Grained Molecular Dynamics Simulations

The toughening mechanism has been a focal point for hydrogel studies. To solve the problems of the pure physical gel’s poor strength and the low toughness of pure chemical gels, hybrid physico-chemical hydrogels are devised. In this paper, we first studied the relationship between the mechanical properties and internal network structure of a physico-chemical hydrogel with hydrophilic-hydrophobic-hydrophilic triblock copolymers by coarse-grained molecular dynamics simulations. The stress and strain curves of the hydrogel system were obtained by tensile simulations, and it was found that the stress and strain curves clearly exhibit two stages with different slopes, which indicate that the aggregation of hydrophobic monomers produces a "hidden length" effect in the physico-chemical hybrid hydrogel. Then, by carrying out the cyclic tensile test and obtaining the loading-unloading curves of the hydrogel system, we found that the hydrophilic chains dominate the energy dissipation. Subsequently, the strain-stress curves at different strain rates revealed an optimal strain rate for the hydrogel’s mechanical property. Finally, we identify an optimal chain-length ratio of 7:1:7 to acquire the best mechanical properties of this hybrid hydrogel. Through this study, we have revealed that the physico-chemical hybrid hydrogel is an excellent candidate to combine the merits of a physical hydrogel and chemical hydrogel, and the length ratio between the hydrophobic and hydrophilic chains is a key for optimizing its mechanical properties.