Tensile Strain Softening of Associative Polymer Networks: Understanding Nonlinear Mechanics

Biological systems often exhibit strain stiffening to prevent deformation and softening to avoid catastrophic failure. Associative polymer networks are inspired by the dynamic nature of biomaterials and have broad applications as stimuli-responsive materials. Herein, we investigated the nonlinear extensional response of associative polymer networks with a focus on tensile strain softening. Metallo-supramolecular networks formed by pendant group cross-linking with different metal ions and a low degree of entanglements were used to access different network temporal parameters. While all networks exhibited shear-thinning, the ones with fast metal–ligand cross-links showed tensile strain softening with a yield point, while networks with slow cross-links did not. This indicates a fundamental difference in how associative networks respond to force under shear versus tensile deformations. The mechanism of yielding was explained by comparing the changing instantaneous extension rate at different stages of the tensile test with the network relaxation times probed by the sticky Rouse model and KWW fitting.