Mechanism study on structural and mechanical properties of triblock copolymer hydrogels by coarse-grained molecular dynamics simulations

In this paper, the fracture mechanism of a hydrophobic-hydrophilic-hydrophobic triblock copolymer physical hydrogel system is investigated through customized coarse-grained molecular dynamics simulations. The stress-strain behavior of the system subject to uniaxial tensile stretch is examined at various strain rates. The stretching of hydrogel network can be described as follows: (1) for hydrophilic chains: entangled-Tightened-Alignment-Coiled; (2) for hydrophobic chains: separated sphere-Coalescence-Ellipsoid-Pulled out-Coiled. The coalescence speed and rupture speed are found to be two competing factors that dominate the rupture process of the physical hydrogel, and both are correlated with the strain rate parameter. When the strain rate is quite high, the hydrophobic chains will be pulled out quickly without a chance to coalesce or get entangled, resulting in inferior mechanical properties. The optimum ratio of hydrophobic segment and hydrophilic segment for the best gel mechanics is then studied, resulting that a segment ratio of 1:6:1 for the hydrophobic-hydrophilic-hydrophobic hydrogel that may lead to the strongest mechanical properties. In summary, we revealed that the architecture of triblock copolymer can be optimized, and thus the mechanical properties of these advanced polymer networks can be tuned for specific applications.