Delayed slip after step strains in highly entangled polystyrene solutions

Abstract

Apparent slip within a few micrometers of a glass surface is visualized with a microscope using small glass spheres as tracers suspended in a highly entangled 20% solution of polystyrene of molecular weight 8.42 million, confined between glass plates in a plane Couette device. After step shears of 0.5–5 strain units, particles within a few micrometers of each surface move by 70–500 μm, enough to unload more than half of the imposed shear strain at intermediate strains. The slip displacement, when plotted logarithmically against time, show a peculiar ‘‘kink’’ in the vicinity of the ‘‘Rouse’’ time of 100 s, similar to the kink reported for the shear stress. These results show that the peculiar rheological properties reported for highly entangled solutions are produced by an effective slip phenomenon that occurs either at the wall or in a thin disentanglement layer near it.