Surface mobility and slip of polybutadiene melts in shear flow

Surface mobility and wall slip of entangled polybutadiene melts were studie d with attenuated-total-reflectance infrared spectroscopy at stresses chara cteristic of the sharkskin, spurt, and melt-fracture regimes. Small-scale s lip, accompanied by an apparent decrease in transverse mobility, occurs in the sharkskin regime, but at a stress above the visual onset of sharkskin i n capillary viscometry. Simulations cannot distinguish between a cohesive m echanism and a lubrication mechanism that might follow from a stress-induce d phase transition, but an adhesive failure seems to be excluded. The near- surface length scale is of the order of four to six times the equilibrium r oot-mean-square end-to-end distance, and the estimated slip velocity is ins ensitive to molecular weight. Strong slip occurs in the spurt regime, eithe r at the wall or within one radius of gyration. Substantial apparent slip o ccurs with a fluorocarbon surface, but the mechanism does not appear to be an adhesive failure; there seems to be a substantial decrease in the fricti on coefficient of chains over a distance of order 300 nm or more from the f luorocarbon surface, and the transverse chain mobility in this region appea rs to be enhanced rather than retarded. Overall, the results of this study indicate that the influence of the wall extends farther into the sheared me lt than would be expected from the chain dimensions, except in the case of strong slip. (C) 2000 The Society of Rheology. [S0148-6055(00)00503-4].