(289d) Molecular Anisotropy and Rearrangement As Mechanisms of Toughness and Extensibility in Entangled Physical Gels

Dynamic networks formed by physically crosslinked, entangled polymers have emerged as self-healing, stretchable, and functional materials. Entangled associative gels with remarkable toughness and extensibility have been produced by several distinct chemical approaches, which suggests that these enhanced mechanical properties result from molecular-scale topology. Previously, artificially engineered associative proteins were designed to form well-defined entangled or unentangled physical gels and to provide an ideal model system to investigate the role of entanglement on gel mechanics.

Here, we observe uniaxial strain-induced structural changes in entangled protein hydrogels up to ~800% engineering strain using in situ small-angle X-ray scattering and in situ polarized optical microscopy. Entangled hydrogels develop anisotropic optical responses to uniaxial strain at the nano-, micro-, and macro- scales, and this alignment is not observed in hydrogels with molecular weight below the entanglement cutoff. Nano- and microscopic anisotropy suggest stretching and alignment of polymer chains along the straining axis, and non-monotonic macroscopic anisotropy suggests relaxation within the hydrogel due to rearrangement of associative domains. These findings indicate that topological entanglements and the freedom of individual chains to align at the nanoscale due to junction relaxation are both critical to achieving high toughness and elongation in entangled physical gels.