Anomalous diffusion and sticky Rouse dynamics in associating networks

Shengchang Tang
Department of Chemical Engineering, MIT
PhD Candidate

Abstract: 

Natural and synthetic materials based upon associating polymers have attracted significant interest. While much is known about their dynamics on the molecular and macroscopic level, knowledge of self-diffusive dynamics of the network-forming constituents remains limited. Here, self-diffusion of model associating proteins is probed by forced Rayleigh scattering and anomalous diffusion is observed for the first time. The diffusion can be quantitatively explained by a two-state model, which accounts for the diffusive motion of proteins at the molecular and associated state, as well as the interconversion kinetics between the two states. In addition, a sticky-Rouse-like relaxation is found in the low-frequency regime from bulk rheological measurements, suggesting collective relaxation of associative proteins. From time-temperature-concentration superposition, the rheological dissociation time is shown to be 2-3 orders of magnitude shorter than the molecular dissociation time obtained from diffusion studies, which provides a direct evidence of the hierarchy in relaxation dynamics from a submolecular to a molecular level. Finally, the diffusion and rheological behavior from systems with different associating strengths and kinetics are compared.

About the Speaker: 

Shengchang Tang is currently a Ph.D. candidate in Chemical Engineering Department at MIT, under the supervision of Prof. Bradley Olsen. He received his B.S. degree in Polymer Materials and Engineering from Tsinghua University (China) in 2010, and obtained M.S. degree in Chemical Engineering Practice in 2014. His thesis project focuses on mimicking the mechanical properties of biological tissues using hierarchically structured protein/polymer hydrogels.

Date of Talk: 

March 11, 2015