(283g) Molecularly Based Criteria for Shear Banding in Transient Flow of Entangled Polymeric Fluids

Dissipative Particle Dynamics (DPD) simulations of polymeric melts in a start-up of shear flow as a function of ramp-time to its steady state value is studied. Here, the molecular findings showing the effect of ramp-time on formation of shear banded structure and chain relaxation behavior is presented. Specifically, it is shown that shear banding emerges at rapid start-up, however homogenous shear prevails when the deformation rate ramp time is sufficiently slow. This finding is in full consistency with prior continuum level linear stability analysis of shear banding in startup of shear flows [1] as well as experimental observations of entangled DNA and polymer solutions [2]. Moreover, it has been revealed for the first time that the ratio of the chain longest orientation relaxation time to that of the time for the imposed deformation rate to reach its steady state value, denoted as MK, plays a central role in determining whether local inhomogeneities are created in the fluid; a prerequisite for shear banding formation. Specifically, if MK is greater than one, local inhomogeneity in the entanglement density in the velocity gradient direction appears and in turn flow transitions to a shear banded structure. Otherwise, homogeneous shear flow prevails. Moreover, local process for the formation of shear banding is examined and is shown that the gradient of average number of entanglements per chain along the velocity gradient direction should reach a critical value for local inhomogeneity and in turn a fluid interface to be formed. Finally, a molecular picture for the interrelation between the chain longest orientation, stress relaxation time, entanglement network dynamics, local inhomogeneities, and shear banding is proposed.

References:

1. R.L. Moorcroft, S.M. Fielding, Physical Review Letters, 110 (2013) 086001.
2. P.E. Boukany, S.-Q. Wang, Macromolecules, 43 (2010) 6950-6952.