Mechanisms of Diffusion in Associative Polymer Networks: Evidence for Chain Hopping

Abstract

Networks assembled by reversible association of telechelic polymers constitute a common class of soft materials. Various mechanisms of chain migration in associative networks have been proposed, yet there remains little quantitative experimental data to discriminate among them. Proposed mechanisms for chain migration include multichain aggregate diffusion as well as singlechain mechanisms such as "walking" and "hopping", wherein diffusion is achieved by either partial ("walking") or complete ("hopping") disengagement of the associated chain segments. Here we provide evidence that hopping can dominate the effective diffusion of chains in associative networks due to a strong entropic penalty for bridge formation imposed by local network structure; chains become conformationally restricted upon association with two or more spatially separated binding sites. This restriction decreases the effective binding strength of chains with multiple associative domains, thereby increasing the probability that a chain will hop. For telechelic chains this manifests as binding asymmetry, wherein the first association is effectively stronger than the second. We derive a simple thermodynamic model that predicts the fraction of chains that are free to hop as a function of tunable molecular and network properties. A large set of self-diffusivity measurements on a series of model associative polymers finds good agreement with this model.

Effect of sticker clustering on the dynamics of associative networks

“…Superdiffusive scaling that transitions to Fickian diffusion at length scales larger than the R g has been previously reported in other unentangled associative networks as well. 3,5,6,38 A previously proposed two-state model 5 demonstrated that the presence of two diffusive modes, with distinct diffusivities can lead to the appearance of superdiffusive scaling at length All gels prepared at a concentration of 25% (w/v). The dashed lines are fits to the two-state model 5 .…”

Non-Covalently Associated Streptavidin Multi-Arm Nanohubs Exhibit Mechanical and Thermal Stability in Protein-Network Materials

“…50, 51 Biosynthetic coiled-coil protein structures, inspired by a variety of innate proteins such as cartilage oligomeric matrix protein and troponin, have been utilized as cross-linkers for the development of protein-network hydrogels with customizable network properties. 27, 28, 38, 50, 51 Coiled-coil cross-linkers, where multiple ɑ-helical structures physically inter-wound each other, undergo transient binding kinetics where cross-linkers can bind and unbind periodically. These temporary bonds promote protein-network homogeneity by allowing protein building blocks to diffuse throughout the hydrogel over time.…”

“…These temporary bonds promote protein-network homogeneity by allowing protein building blocks to diffuse throughout the hydrogel over time. [49][50][51] This ability to reorganize protein networks presented a viable option for improving network homogeneity in our tetra SAv-based protein-network design, engineering a monomer of pentameric coiled-coil cross-linker (P) at the N-terminus to form P-C24-SAv (Figure 4a; Table S1). SAv monomers in each protein led to form SAv tetramers with four protein arms (Figure 4a) due to its high affinity 23,35 as we confirmed on the SDS-PAGE 42 (Figure 4d).…”

“…In addition to the specificity of biomolecular cross-linkers, their binding kinetics, dependent upon their binding affinity of non-covalent interactions, 49 provide the ability to restructure the protein network and improve homogeneously distributed protein building blocks. 50, 51 Biosynthetic coiled-coil protein structures, inspired by a variety of innate proteins such as cartilage oligomeric matrix protein and troponin, have been utilized as cross-linkers for the development of protein-network hydrogels with customizable network properties. 27, 28, 38, 50, 51 Coiled-coil cross-linkers, where multiple ɑ-helical structures physically inter-wound each other, undergo transient binding kinetics where cross-linkers can bind and unbind periodically.…”

Hierarchy of relaxation times in supramolecular polymer model networks

“…14 In general, hopping describes the diffusive motion when a star-polymer detaches all four arms from the network and its center-of-mass travels an unexpected large distance. Rapp et al 30 showed that the equilibrium binding constant of a sticker is drastically reduced if other arms of the same molecule are already connected to the network. The chain conformation of an un-attached arm is limited by the surrounding local network structure and binding is therefore entropically unfavoured.…”