Mechanism of High Extensibility in Entangled Associative Protein Hydrogels*

When topological entanglements are incorporated into physically associating polymer gels, they can exhibit remarkably large ultimate extensions, exceeding 3,000% engineering strain. Here, we show that these large strains are related to molecular alignment within the gel under deformation that is not observed in unentangled gels. Uniaxial strain-induced structural changes are investigated in an associative protein hydrogel up to ~600% engineering strain using in situ laser light scattering and small-angle X-ray scattering. These methods reveal that the hydrogel develops an anisotropic optical response to uniaxial strain at the nano-, micro-, and macro-scales. Macroscale anisotropy suggests bulk chain alignment occurs along the straining axis, which is confirmed with depolarized light scattering. This behavior does not emerge in hydrogels with molecular weight below the entanglement cutoff. The findings suggest that both entanglements and freedom of individual chains to align at the nanoscale due to junction relaxation are critical to achieving high extension in physical gels.