(6hl) Mechanisms of Diffusion in Associative Polymer Networks: Evidence for Chain Hopping

Research Interests: design and evolution of artificial proteins, physics of soft materials, biocatalysis with artificial enzymes, antigen-specific immunotolerance

Teaching Interests: chemical engineering thermodynamics, reactor design, mass and heat transport, polymer physics and polymer chemistry, protein design and evolution, modern methods in chemical biology, advanced organic synthesis and organocatalysis for engineers, chemical engineering research as teaching, creating active learning environments for specifically upper level engineering coursework

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 single chain 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.