Molecular Scale Simulation of Homopolymer Wall Slip

Abstract

The first molecular scale simulation of highly entangled polydisperse homopolymers that is capable of capturing all three regions--no slip, weak slip, and strong slip--of the hydrodynamic boundary condition is presented. An on-lattice dynamic Monte Carlo technique capable of correctly capturing both unentangled and entangled polymer dynamics is used to study the molecular details of wall slip phenomena for homopolymers and energetically neutral walls. For unentangled chains (those exhibiting Rouse dynamics) weak slip is not present but evidence of strong slip is manifest at very high shear rates. For entangled chains (of sufficient length to exhibit reptation dynamics), both weak and strong slip are observed. Consistent with numerous experimental studies, disentanglement and cohesive failure occur at high shear rates. Disentanglement is clearly evidenced in a nonlinear velocity profile that exhibits shear banding, in an excess of chain ends at the slip plane, and perhaps most importantly in a nonmonotonic stress versus shear rate response. The chain end density exhibits a pretransitional periodicity prior to disentanglement. Unentangled Rouse chains do not show this pretransitional response or a bifurcation in their stress versus shear rate response. Finally, it is shown that when polydispersity is introduced, slip phenomena are severely reduced and the inherent constitutive bifurcation is limited to a small region. Predictions are in post facto agreement with many experiments, are distinct from existing results obtained using molecular dynamics simulation techniques, and shed light on fundamental mechanisms of polymer wall slip.

Molecular-scale simulation of cross-flow migration in polymer melts

“…The implementation of polydispersity [33], shear flow [30], and parabolic flow [31] have been successfully demonstrated. In addition, the algorithm accurately captures polymer dynamics [30] and wall-slip phenomena [34].…”

“…These simulations are at high shear rates and yet almost no additional migration other than that caused by the walls is evident. At even higher shear rates when slip and cohesive failure are present [34], additional migration can occur but this is accompanied by velocity banding, that is, by an inhomogeneous shear rate. The data of Figures 9 and 10 compared to that of Figs.…”

Effect of Chain Length Dispersity on the Mobility of Entangled Polymers

“…However, due to computational limitations, only the longer of the two chain lengths was well above the entanglement molecular weight M e . There have been few studies of polymer melts with a distribution of chains lengths, though mostly for short, unentangled polymers [19,23,[30][31][32][33][34][35][36][37]. Rorrer et al [19,[34][35][36] mapped a distribution of chain lengths on a small number of chain lengths and showed that for the same weight-averaged molecular weight, increasing the dispersity in chain lengths gives a lower Rouse time and introduces a broadening of the transition to reptation of the chains.…”

Local response in nanopores

“…As previously discussed, this is particularly useful for complex fluids which might exhibit layering or orientational ordering close to the boundaries. For example is known that polydisperse polymers, if confined, will distribute so as to have low molecular weight polymers collecting close to the boundaries [43]. Our study can therefore help the understanding and prediction of phenomena in small geometries and aid in the development of new nanodevices and lubricants.…”