Thermally Activated Stress Relaxation and Creep in Ideal Hydrogel Elastomers: Rupture of Tensile Strands

The microscopic origins of time-dependent mechanical responses, including stress relaxation and creep, in a hydrogel film composed of a chemically cross-linked network are investigated using dissipative particle dynamics. After free relaxation following uniaxial stretching, the model hydrogel exhibits full recovery at moderate strains but develops a small residual strain at larger deformations, indicating an elastomeric behavior accompanied by irreversible plasticity. Microscopic analyses reveal the evolution of the bond length, strand length, internal energy, strand stretch ratio, and bond rupture during deformation and relaxation. Stress relaxation and creep are essentially absent at moderate strains, consistent with negligible frictional dissipation, but become pronounced at larger strains due to the occurrence of rare bond-rupture events. During stress relaxation, the decay of stress closely correlates with reductions in the bond length and internal energy with the response dominated by bond-length relaxation associated with network reorganization. During creep, the mean bond length remains nearly constant, whereas the mean strand length evolves through primary/secondary regimes and accelerates into tertiary creep-to-failure. Both time-dependent behaviors arise from structural reorganization induced by rare thermally activated bond-rupture events localized within highly stretched tensile strands.