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The authors use neutron spin-echo spectroscopy and forced Rayleigh scattering to study the interplay between conformational fluctuations and transient binding in associative protein hydrogels.

Transient network theory is the state of the art for understanding the mechanical response of associative networks. However, the predictions of the models are difficult to verify in the absence of direct observations of the bond states. This manuscript introduces a method to quantitatively measure force-induced bond dissociation in associative networks through the design of an opto-mechanically coupled model polymer system and a rheo-fluorescence set-up. The findings show that shear thinning in the model associative polymers cannot be explained by classical theories, and likely involves alternative modes as suggested by newer models.

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, suggesting that these enhanced mechanical properties result from molecular-scale topology. Previously, artificially engineered associative proteins were designed to provide an ideal model system to investigate the role of entanglement on gel mechanics via well-defined entangled or unentangled physical gels. Herein, uniaxial strain-induced structural changes in these model gels were observed using in situ small-angle x-ray scattering (SAXS) and in situ polarized optical microscopy (POM) up to 2000% engineering strain. Anisotropic optical responses to uniaxial strain at the nano-, micro-, and macroscales suggest that stress dissipation mechanisms enable high extensibility and toughness. Nano- and microscopic anisotropy observed by SAXS indicate stretching and alignment of flexible polymer strands along the straining axis, and nonmonotonic macroscopic anisotropy observed by POM suggests relaxation within the hydrogel due to rearrangement of associative network junctions. Unentangled hydrogels exhibit low toughness and a strain-rate-dependent transition from ductile to brittle tensile behavior, which is typical for associative polymer solutions. 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.

Polymer networks invariably possess topological defects: loops of different orders which have profound effects on network properties. Here, we demonstrate that all cyclic topologies are a universal function of a single dimensionless parameter characterizing the conditions for network formation. The theory is in excellent agreement with both experimental measurements of hydrogel loop fractions and Monte Carlo simulations without any fitting parameters. We demonstrate the superposition of the dilution effect and chain-length effect on loop formation. The one-to-one correspondence between the network topology and primary loop fraction demonstrates that the entire network topology is characterized by measurement of just primary loops, a single chain topological feature. Different cyclic defects cannot vary independently, in contrast to the intuition that the densities of all topological species are freely adjustable. Quantifying these defects facilitates studying the correlations between the topology and properties of polymer networks, providing a key step in overcoming an outstanding challenge in polymer physics.