Microstructural evolution and reverse flow in shear-banding of entangled polymer melts.

The temporal and spatial evolution of shear banding under startup of shear flow was simulated for highly entangled, linear, monodisperse polyethylene melts of differing molecular weight, C750H1502, C1200H2402, and C3000H6002, using a high-fidelity coarse-grained dissipative particle dynamics method. It was determined that shear stress was dominated by segmental orientation of entangled strands at low shear rates, but at a critical shear rate below the reciprocal of the Rouse time, flow-induced disentanglement resulted in the onset of chain tumbling that reduced the average degree of orientation, leading to a regime of decreasing shear stress, with a commensurate onset of increasing average chain extension imposed by the strong flow kinematics that ultimately drove the steady-state shear stress higher. During startup of shear flow, shear band development began immediately after the maximum in the first normal stress difference, where distinct fast and slow bands formed. The slow bands consisted of relatively entangled and coiled molecules, whereas the fast bands consisted of more disentangled and extended chains that experienced quasiperiodic rotation/retraction cycles. The simulation results often exhibited a generation of temporary reverse flow, in which the local fluid velocity was temporarily opposite to that of the bulk flow direction, at the onset of the shear-banding phenomena; this effect was consistent with earlier experiments and theoretical results. The physical mechanism for the generation of reverse flow during shear-band formation was investigated and found to be related to the recoil of the molecules comprising the slow band. Overall, the phenomenon of shear banding appeared to arise due to flow-induced disentanglement from orientational ordering and segmental stretching that affected individual chains to different degrees, ultimately resulting in regions of relatively coiled and entangled chains that evolved into a slow band, whereas the locally disentangled chains, experiencing quasiperiodic stretch-rotation cycles, formed a fast band. The transitional period resulted in a kinematic instability that generated the temporary reverse-flow phenomenon.

Molecular Processes Leading to Shear Banding in Entangled Polymeric Solutions.

The temporal and spatial evolution of shear banding during startup and steady-state shear flow was studied for solutions of entangled, linear, monodisperse polyethylene C3000H6002 dissolved in hexadecane and benzene solvents. A high-fidelity coarse-grained dissipative particle dynamics method was developed and evaluated based on previous NEMD simulations of similar solutions. The polymeric contribution to shear stress exhibited a monotonically increasing flow curve with a broad stress plateau at intermediate shear rates. For startup shear flow, transient shear banding was observed at applied shear rates within the steady-state shear stress plateau. Shear bands were generated at strain values where the first normal stress difference exhibited a maximum, with lifetimes persisting for up to several hundred strain units. During the lifetime of the shear bands, an inhomogeneous concentration distribution was evident within the system, with higher polymer concentration in the slow bands at low effective shear rate; i.e., γ˙<τR-1, and vice versa at high shear rate. At low values of applied shear rate, a reverse flow phenomenon was observed in the hexadecane solution, which resulted from elastic recoil of the molecules within the slow band. In all cases, the shear bands dissipated at high strains and the system attained steady-state behavior, with a uniform, linear velocity profile across the simulation cell and a homogeneous concentration.