Rheo-Dielectric Responses of Entangled cis-Polyisoprene under Uniform Steady Shear and LAOS

Extracting microscopic insight from transient dielectric measurements during large amplitude oscillatory shear

Probing the transient microstructure of soft matter far from equilibrium is an ongoing challenge to understanding material processing. In this work, we investigate inverse worm-like micelles undergoing large amplitude oscillatory shear using time-resolved dielectric spectroscopy. By controlling the Weissenburg number, we compare the non-linear microstructure response of branched and unbranched worm-like micelles and isolate distinct elastic effects that manifest near flow reversal. We validate our dielectric measurements with small angle neutron scattering and employ sequence of physical processes to disentangle the elastic and viscous contributions of the stress.

Primitive chain network simulations for the interrupted shear response of entangled polymeric liquids

The non-linear viscoelastic response under interrupted shear flows is one of the interesting characteristics of entangled polymers. In particular, the stress overshoot in the resumed shear has been discussed concerning the recovery of the entanglement network in some studies. In this study, we performed multichain slip-link simulations to observe the molecular structure of an entangled polymer melt. After confirming the reasonable reproducibility of our simulation with the literature data, we analyzed the molecular characteristics following the decoupling approximation. We reasonably found that the segment orientation dominates the stress overshoot even under the resumed shear with minor contributions from the segment stretch and entanglement density. We defined the mitigation function for the recovery of the stress overshoot as a function of the rest time and compared it with the relaxation of the molecular quantities after the initial shear. As a result, we have found that the mitigation of the stress overshoot coincides with the relaxation of entanglement density.

Linear and Nonlinear Universality in the Rheology of Polymer Melts and Solutions

Understanding the dynamics of polymeric liquids has great importance in the design and processing of soft materials. While slow flow dynamics is now resolved, fast flow dynamics is still unsolved, especially due to the lack of experimental evidence. We here manipulate a poly(methyl methacrylate) solution into exhibiting the same flow behavior as a polystyrene melt. Strikingly similar responses of the fluids are seen both in slow and very fast flow. With this discovery we show that dynamics in polymeric liquids can be generalized and captured in one single polymer physics model.

Origin of Stress Overshoot under Start-up Shear in Primitive Chain Network Simulation

Birefringence measurement demonstrates that the segment orientation of entangled polymers overshoots on start-up of fast shear [Pearson et al. J. Rheol. 1989 33, 517-535]. The stress-optical rule holds for those polymers, so that the overshoot of orientation results in the overshoot of shear stress. On the other hand, an opposite result was deduced from the recent molecular dynamics simulation for bead-spring chain [Lu et al. ACS Macro Lett. 2014 3, 569-573]: the evolution of segment orientation does not overshoot but the chain stretch induces the stress overshoot, even at the shear rate γ̇ smaller than the reciprocal Rouse time, 1/τ_R. In this study, we performed the primitive chain network simulation to find that our simulation reproduces the overshoot of both stress and orientation and the chain stretch exhibits a slight, monotonic increase but no overshoot. Our result is thus fully consistent with the experiment.