(164n) Effect of Sticker Clustering on Self-Diffusion in Associative Polymer Gels Revealed By Brownian Dynamics Simulation

Associative polymer gels have attracted wide attention due to their useful viscoelastic properties and exquisite tunability for applications ranging from biomedicine to soft robotics. The macroscopic behavior of these materials is largely dictated by the dynamics of the transient cross-links, providing a handle to design their properties for specific applications. Recent efforts to engineer the properties of associative gels have explored chain architectures where the binding moieties (also referred to as stickers) are clustered in groups along the chain contour, resulting in an enhancement in the mechanical stiffness and toughness due to the cooperative effect of clustered stickers. However, the effect of sticker clustering on the microscopic chain diffusive behavior is still unknown, limiting the ability to design the transport properties of such materials. Recent experiments have shown, surprisingly, that clustering the stickers in groups can lead to faster chain diffusion than spacing them regularly along the chain, which has been hypothesized to occur from changes in network topology due to the proximity of the stickers. Further study is required to explain the molecular origin of the differences in network structure and chain dynamics caused by sticker clustering.

In this work, Brownian dynamic simulations are used to study the effect of sticker clustering on chain diffusion and relaxation in associative polymer gels, comparing systems in which the single-sticker bond energy is held constant and where the total cluster bond energy is held constant. A coarse-grained bead spring model is developed where chains undergo Rouse dynamics with stickers participating in pairwise binding interactions with a mean-field background. The number of clusters along the chain and the number of stickers per cluster are independently varied to probe the effect of chain design on gel dynamics. When the single-sticker bond energy is held constant, analogous to the recent experiments, the simulations reveal a strong enhancement in intra-cluster loop formation due to the proximity of the stickers. This reduces the number of stickers that are intermolecularly bound to the network and increases the chain diffusivity, consistent with experimental results. The simulations also reveal strong odd-even effects in the network topology due to the pairwise nature of sticker association, where having an odd number of stickers per cluster enhances intermolecular bond formation and slows chain diffusion. Increasing the number of clusters per chain is also found to slow chain diffusion due to the enhancement in the total friction of the chain. Finally, when the total cluster bond energy and kinetics are held constant but the number of stickers per cluster are varied, chains paradoxically exhibit slower long-time diffusion but faster end-to-end relaxation with increasing number of stickers per cluster. This surprising phenomenon arises due to the faster single-sticker bond kinetics when the total cluster kinetics are held constant, which allows local stress relaxation through a “walking” mechanism but slows diffusion on long length scales by increasing the rate of attachment to the network. Overall, the simulations reveal new and unexpected effects of clustered sticker domains on chain dynamics at different length scales and provide insight for the engineering of associative polymer systems with tailored transport behavior for various applications.