Paper Number
SM23 

Session
Polymer Solutions and Melts

Title
Molecular scale simulations of rheological behavior – Understanding slip of molten polymers

Presentation Date and Time
October 7, 2014 (Tuesday) 2:20

Track / Room
Track 3 / Commonwealth C

Authors (Click on name to view author profile)

1. Dorgan, John R. (Colorado School of Mines)
2. Rorrer, Nichoals A. (Colorado School of Mines)

Author and Affiliation Lines (in printed abstract book)
John R. Dorgan and Nichoals A. Rorrer
Colorado School of Mines, Golden, CO 80401

Speaker / Presenter 
Dorgan, John R.

Text of Abstract
New simulation results of slip phenomena at the molecular level shed new fundamental insight into this important phenomena. While most experimental studies of homopolymer slip treat drag and pressure drive flows as equivalent, simulations show this is not the case. In particular, molecular weight fractionation means that slip in pressure driven flows (Pousielle flow) are distinct from drag flows (Couette). For monodisperse melts in shear flow, slip is not observed at low shear rates, however, a cascade of wall slip followed by cohesive failure is observed with increasing shear rates. In comparison, parabolic flow exhibits slip at all apparent wall shear rates, in agreement with the theories originally proposed by Brochard and deGennes. The slip velocity at the wall is found to scale with molecular weight and shear rate. When polydisperse melts are simulated there is a small preference for lower molecular weight chains at the wall under quiescent conditions which is not enhanced in planar Coutte flow. However, at all flow rates there are gradients in shear rate for parabolic flow, and thus there are always migration effects present. The cascade of molecular events in rectilinear shear is thus distinct from parabolic flow. Interestingly, early experiments recognized this difference but more recent work has not. Polydispersity serves to alleviate slip in both cases; higher shear rates can be reached prior to the onset of slip. Reduction of slip is observed in the shear stress response and in the chain end density for shear flow, and is observed in a reduction of the slip velocity in parabolic flow. These simulations shed light onto the fundamental microscopic and molecular origins of slip, and are in post facto agreement with experimental and theoretical investigations.