John H. Dillon Medal Talk: Skipping Polymer Physics

Associative polymer networks are one of the most pervasive categories of materials in polymer physics; however, self-diffusion in these systems has not been widely studied. Recent experimental measurements using forced Rayleigh scattering (FRS) from our lab have shown an apparent super-diffusive regime that is unexpected based on current theories but is well modeled by a simple empirical model for exchange of the polymers between fast and slow diffusing states. Brownian dynamics simulations suggest that these fast and slow states may be free molecules and physically attached molecules, suggesting that associative polymers may diffuse in a combination of hops and walking steps analogous to locomotion by skipping. Both the empirical two-state model and the simulations show mean square displacements that are linear in time; the superdiffusive regime observed by FRS results from the fact that there are fast and slow components to the diffusion. Beyond the obvious implications for systems such as self-healing elastomers and injectable biomedical gels where relevant properties are strongly influenced by the rate of self-diffusion, these results provide a potential pathway to a completely new mechanism of filtration in polymeric media. Inspired by a family of proteins called nucleoporins that regulate transport into the nucleus, we have developed a theoretical framework and demonstration materials that suggest how physical associations with a stationary medium may accelerate overall flux. The key hypothesis is that even though skipping is slower than free diffusion, an enhanced concentration of skipping molecules can lead to a large increase in flux. This insight suggests molecular designs for replicating the properties of nucleoporins in synthetic polymer gels.