Use of molecular dynamics to investigate polymer melt-metal wall interactions

The classical boundary condition of fluid dynamics, i.e. the no-slip condition is violated during the flow of various complex fluids including polymer melts and polymeric suspensions. It is recognized that the dynamics of the behavior of the macromolecules at the wall, their adsorption, and disentanglement from each other and from the wall all play significant roles during shearing and flow. During wall slip it is not clear whether the macromolecules detach from the wall (adhesive failure of the slip condition) or whether the macromolecules remain tethered to the wall but disentangle from the neighboring macromolecules (cohesive failure). In this study, we seek to shed light to the basic mechanisms of the wall slip of polymers by focusing on the dynamics of the polymer behavior at the wall for three polymers, two of which exhibit significant strong wall slip, high density polyethylene (HDPE) and poly(dimethylsiloxane) (PDMS), and one which does not exhibit wall slip under typical extrusion conditions, i.e. a block copolymer BAMO/AMMO, (crystalline blocks of poly(3,3-bis(azidomethyl)-oxetane), BAMO, and amorphous blocks of poly(3-azidomethyl)-3-methyl-oxetane, AMMO). The cohesive energy densities of the three polymers were found to be in the same range, with the cohesive energy density of BAMO/AMMO being slightly higher than those of the other two. The molecular dynamics based cohesive energy density values compared well with calculations based on the determination of the group molar attraction constants. On the other hand, the energy of adhesion value exhibited by the copolymer BAMO/AMMO/iron oxide is significantly higher than the energy of adhesion values for the iron oxide/PDMS and iron oxide/HDPE systems. Considering that over the same broad range of shear stresses the block copolymer BAMO/AMMO does not exhibit wall slip and the other two polymers HDPE and PDMS do, these findings suggest that at least for these three polymers wall slip is more likely to occur on the basis of an adhesive failure mechanism.