Recent experiments on self-diffusion in associative networks have shown superdiffusive scaling hypothesized to originate from molecular diffusive mechanisms, which include walking and hopping of the polymer chains. Since hopping requires the release of all of the stickers on the chain, it is expected that as the sticker density is increased, the walking mode will become dominant such that eventually only Fickian scaling will be observed for polymers with sticker densities above a critical value. In this work, a set of copolymers of N,N-dimethyl-acrylamide and pendant histidine groups with sticker densities ranging from 4 to 15 stickers per chain was synthesized using reversible addition–fragmentation chain-transfer (RAFT) polymerization. The self-diffusion of the polymer chains in the gels in the unentangled regime was then studied using forced Rayleigh scattering (FRS). For the range of length scales measured, superdiffusive scaling was observed across the entire range of sticker densities. This suggests that molecular hopping is an important mechanism for diffusion, even for the polymer with the highest sticker density. Further analysis shows that hopping of the high-sticker-density polymer is promoted by the presence of a significant fraction of intrachain bonds, the entropic penalty associated with binding to the network, and the distribution of sticker densities inherent to copolymers synthesized through RAFT polymerization.

Exploration of effect of sticker clustering on dynamics of associative polymer networks showed trends in rheological relaxation and diffusion with clustering are different than for uniformly distributed stickers.

Understanding the physics of associative polymers is often limited by our inability to directly measure bond dissociation under deformation. In this work, we developed a rheo-fluorescence technique and applied it to characterize the nonlinear shear response of linear side-functionalized polymer chains cross-linked via nickel-terpyridine complexation. As the network was sheared, the fraction of dissociated bonds was quantitatively measured based upon a change in fluorescence with metal dissociation. Shear thinning of the gel was accompanied by only a small increase in the fraction of dissociated bonds. Comparison with several transient network models shows that the shear thinning within the constraint of the measured fraction of dissociated bonds cannot be explained by classical theories that include retraction of dangling chains alone; the rheological response likely involves alternative modes of stress relaxation.

The presence of entanglements in associative polymer gels has been shown to impart enhanced mechanical strength, toughness, and extensibility; however, the interplay between topological and binding interactions in these systems remains poorly understood. Here, the effect of entanglements on chain dynamics in a model associative network is investigated in the weakly entangled regime, corresponding to 1.1−3.1ϕ e , where ϕ e is the characteristic concentration for the onset of entanglement. The associative network is formed by a linear random copolymer of N,N-dimethylacrylamide and a histidinefunctionalized monomer crosslinked with Ni 2+ ions. Rheological characterization indicates that the concentrations investigated span the transition from unentangled to the weakly entangled regime, resulting in a subtle broadening of their relaxation spectrum. Selfdiffusion measurements using forced Rayleigh scattering demonstrate a pronounced suppression of apparent superdiffusive behavior with increasing concentration, revealing a stronger impact of topological entanglement on self-diffusion compared to overall network relaxation. This suppression in superdiffusive behavior is attributed to a reduction in the contribution of "hopping" diffusion due to the presence of entanglements, resulting in an approach to purely Fickian diffusion governed by a single "walking" diffusive mode at the highest concentration probed. These results demonstrate the marked effects of entanglements on self-diffusion and relaxation in associative networks, providing insight into the network response beyond that accessible by rheology alone.