Effect of Sticker Clustering on the Dynamics of Metal Coordinate Associative Polymer Networks

Clustering of stickers along an associative polymer chain often leads to enhanced toughness and creep resistance. Sticker clustering changes network topology, and it can also change sticker dynamics due to the proximity of the associating groups. Here, the effect of clustering is investigated by comparing associative polymer hydrogels with randomly-distributed metal coordinate groups to triblock copolymers with the metal-coordinate groups concentrated in the endblocks. Polymers were synthesized with histidine associative groups that coordinate Ni atoms in a 2:1 ligand:metal ratio. The combination of rheology, neutron scattering, and forced Rayleigh scattering (FRS) are used to study the structure, mechanics, and molecular-level dynamics of these systems. Although rheology shows slower stress relaxation for clustered polymers due to the need for cooperative dissociation of multiple bonds, faster diffusion is observed in clustered polymers by FRS due to the presence of topological defects. Brownian dynamics simulations of the clustered systems provide further insight into the molecular origin of these dynamic differences, demonstrating the key cooperative effect of proximate stickers in producing loop defects which alter the dynamic properties of these systems.