Importance of chain tumbling and finite extension on the start­up and relaxation behavior of transient networks

Michelle K Sing
Department of Materials Science and Engineering, MIT
PhD Candidate

Abstract: 

Associative polymer networks are ubiquitous in tissue and biomedical engineering. However, the particular molecular attributes that contribute to the macroscopic behavior like shear thinning, self­healing, and yield stress are less well known. Here we incorporate chemical kinetics in the the Smoluchowski Equation capable of modeling the full network chain end­to­end distance distribution while tracking the fraction of looped, bridged, and dangling chains in the gel. In steady shear, we see the development of non­monotonic flow instabilities when the rate of chain association and dissociation are slower than the rate of chain relaxation. These instabilities arise due to a combination of chain finite extensibility and tumbling. During start­up of steady shear, the combination of these two phenomena also results in stress overshoots followed by multiple damped oscillations toward steady­state. The timescale of chain relaxation after the cessation of shear is dominated by the chain kinetics of association and dissociation as a function of the fraction of dangling chains present at any time post­shear.

Date of Talk: 

March 11, 2015