(240b) Effects of Chain Length and Polydispersity on Shear Banding in Simple Shear Flow of Entangled Polymeric Melts

The characteristics of shear banding were investigated in entangled, polydisperse, linear polymer melts under steady-state and startup conditions of simple shear flow. The occurrence of shear banding has been traditionally associated with the negative slope of the steady-state shear stress flow profile. However, many advanced experimental and theoretical studies point out that even in absence of concentration or stress coupling and demixing, entangled polymeric fluids can exhibit shear banding under a variety of conditions, namely, monotonic and nonmonotonic flow curves. To this end, a molecular picture that paves the way for a full mechanistic understanding of the occurrence of shear banding in entangled polymeric fluids is needed. We have conducted virtual experimentation using course-grained nonequilibrium dissipative particle dynamics simulations. The equilibrium and nonequilibrium DPD simulations were performed to investigate the equilibrium properties and the flow characteristics in moderate and fast regimes in range 10<Wi <5000. We’ve examined three different melts with mean molecular bead numbers of N_n=250 having polydispersity indexes of 1.025, 1.05 and N_n=400/1.025. Our data indicate that the wide range of relaxation timescales in the polydisperse melts decreased the nonmonotonic character of the steady-state shear stress vs. shear rate profile compared to a monodisperse linear melt. Such that for melts N_n=250 , the flow curves are monotonically increasing function of strain rate, while for the more entangled melt some level of nonmonotonicity was observed.
Startup shear flow simulations revealed the development of strain localization in polydisperse fluids containing both monotonic and nonmonotonic steady shear stress profiles. The development of shear banding relies on the topological relaxation of macromolecular chain and chain segments within an entangled network. The molecular level simulations show the onset and underlying mechanism leading to the formation of shear bands were generally universal. Perturbations arose soon after the occurrence of a large stress overshoot under startup conditions, and that banded structures stemmed from local reorientation and subsequent deconstruction of the entanglement network. Furthermore, data indicated that the inception of strain localization occurred at shear rates near the reciprocal of the Rouse characteristic timescale, γ' _imposed>1/τ_R.
We have observed both transient and steady shear banded velocity profiles, for monotonic and nonmonotonic flow curves, respectively. Transient shear banding was observed in shorter chain melts undergoing startup of shear flow. These instabilities eventually decayed, but only long after the stresses had attained their steady-state values. The longer chain melt, however, exhibited a shear band structure that remained indefinitely, long after the stresses had attained steady state. The data indicates the decrease in the life span of the banded structure and formation of short-lived transient inhomogeneity especially for the flow rates γ'_imposed>1/τ_e.