Wall slip and absence of interfacial flow instabilities in capillary flow of various polymer melts

We conduct a comprehensive experimental study of melt/wall interfacial slip behavior of various polymer melts in capillary flow. Polymers under study include two polystyrenes of different molecular weights (M-W = 280 000 and M-W = 2x10(6), respectively), two low-density polyethylenes (LDPE), poly(ethylene vinyl acetate) (EVA), and polypropylene (PP). The experimental results reveal an important finding that has greatly extended our previous knowledge of the roles of melt/wall interfacial interactions and molecular entanglements in dictating capillary melt flow behavior. None of the six polymers exhibits an interfacial stick-slip transition or shows any sign of wall slip in bare aluminum dies. Yet, all are found to display a sizable wall slip when the strength of polymer/surface adsorption is reduced by lowering the die wall surface energy with a fluoropolymer coating. The degree of wall slip as characterized by the Navier-de Gennes extrapolation length is demonstrated to be explicitly proportional to the melt viscosity, such that the magnitude of wall slip in the coated die decreases with increasing shear thinning. Diminishing of a measurable wall slip at high stresses explains the well-known absence of spurt, flow oscillation, and sharkskin phenomena in capillary flow of commonly encountered polymers such as PS, LDPE, PP. (C) 1998 The Society of Rheology.