Li, J.; Hu, W.-B.* Biased diffusion induces coil deformation via a ‘cracking-the-whip' effect of acceleration generated by dynamic heterogeneity along polymer chain. Polymer International 64(1), 49-53(2015). Journal Link
[ABSTRACT] We performed Brownian dynamics simulations of a single polymer with monomer diffusion biased in a field of constant activation forces. We compared coil shapes among the synchronous and non-synchronous (random and sequential) updating schemes of monomer positions. The synchronous scheme makes an ideal linear integration of monomer motions, while the random scheme mimics the real nonlinear situation holding dynamic heterogeneity along the polymer chain, and the sequential scheme represents its extremely inhomogeneous situation. We found that, in contrast to the synchronous scheme that raises no deformation, the sequential scheme accumulates the local acceleration generated by dynamic heterogeneity and reveals a ‘cracking-the-whip’ effect along the chain from one end to the other to stretch the polymer coil. Meanwhile, the random scheme accumulates the local acceleration towards the middle segment of the chain and thus raises an internal tension for coil deformation as well. Our results demonstrate the dynamic heterogeneity source of coil deformation on biased polymer diffusion, which implies a molecular-level nonlinear factor in the non-Newtonian-fluid behaviors of polymer flows. © 2014 Society of Chemical Industry.
[ABSTRACT] We performed dynamic Monte Carlo simulations of biased diffusion of 3D phantom single lattice polymer. We observed spontaneous deformation of polymer coil when the external driving forces exceed a critical strength. In addition, longer chains require lower critical strengths, at which their activated velocities deviate from Newtonian-fluid behaviours and merge into a master curve exhibiting shear-thinning followed with shear thickening. We attributed the cause of deformation to the random updating of monomers. The latter represents the dynamic heterogeneity along the real polymer chain, and raises a nonlinear asymmetric accumulation of local acceleration and then an internal tension between chain middle and chain end, as evidenced by our previous Brownian Dynamics simulations. Our results unravel a single-molecular-level source of nonlinear dynamics, which has been overlooked in current theoretical considerations on the basis of Rouse ideal-chain model.
[ABSTRACT] By means of dynamic Monte Carlo simulation of bulk lattice polymers in Couette shear flow, it was demonstrated that in addition to velocity gradient the constant driving forces acting as the activation aspect of shear stresses can also raise polymer deformation. Moreover, enhancing driving forces in a flow without any velocity gradient can reproduce non-Newtonian fluid behaviors of long-chain polymers. The simulations of Poiseuille shear flow with a gradient of shear stresses show that, the velocity gradient dominates small deformation in the flow layers of low shear stresses, while the shear stress dominates large deformation in the flow layers of high shear stresses. This result implies that the stress-induced deformation could be mainly responsible for the occurrence of non-Newtonian fluid behaviors of real polymers at high shear rates.
[ABSTRACT] Dynamic Monte Carlo simulations of bulk lattice polymers driven through planar geometries with sequentially converging, parallel and diverging spaces between two neutrally repulsive solid plates are reported. The spatial profiles of polymer velocity and deformation along the course of such a laminar extensional flow have been carefully analyzed. The results appear consistent with experimental observations in literature. In the entrance and exit regions, a linear dependence of chain extension upon the excess velocity has been observed. Moreover, an annexed shear flow and a molecular-dispersion effect are found. The results demonstrate a useful strategy of this approach to study polymer flows and bring new insights into the non-Newtonian-fluid behaviors of bulk polymers in capillary rheometers and micro-fluidic devices.