(289j) Understanding the Hierarchical Relaxation Dynamics in Associating Polymer Networks

Authors: 
Tang, S., Massachusetts Institute of Technology
Wang, M., Massachusetts Institute of Technology
Kulik, H. J., Massachusetts Institute of Technology
Olsen, B. D., Massachusetts Institute of Technology

Natural and synthetic materials made from associating polymeric networks have been of longstanding interest due to their applications in actuators and sensors, drug delivery carriers, separation technologies, and next generation energy devices.  Because many of the material properties (such as adaptive mechanics and responsiveness to external stimuli) originate from the dynamic and reversible nature of physical bonds, it is critical to develop a mechanistic understanding of the relaxation mechanisms governing associating networks over many time scales. 

We first investigate the dynamics of model associating proteins on the molecular level, i.e., self-diffusion, in hydrogels by using forced Rayleigh scattering.  Super-diffusion is observed for the first time in a regime where the association time scale becomes comparable to the diffusion time scale.  We explain the experimental observation by a two-state model, which accounts for the diffusive motion of proteins at the molecular and association states, as well as the interconversion kinetics between the two states.  The associative diffusion manifests in the shear rheology data, displaying a sticky-Rouse-like relaxation in the low-frequency regime.  A time-temperature-concentration superposition, which is obtained with the use of the molecular dissociation time from diffusion measurement, provides direct experimental evidence that the sticker dissociation is two to three orders of magnitude faster than molecular dissociation. 

We then study the dynamics of associating polymers based on metal-ligand coordination bonds.  Unexpectedly, self-diffusion experiments reveal two relaxation processes, but consistent with the prediction from the two-state model.  Furthermore, an additional relaxation mode is observed in the high-frequency regime due to sticker exchange, as confirmed by EXSY nuclear magnetic resonance experiments and density functional theory calculations.  Taken together, these studies illustrate how differences in bond kinetics and thermodynamics affect the bulk mechanical properties and relaxation mechanism of associating polymeric networks.