THE EFFECT OF MOLECULAR-MASS AND TEMPERATURE ON THE SLIP OF POLYSTYRENE MELTS AT LOW-STRESS LEVELS

The slip of monodisperse polystyrene melts next to a solid, inhomogene ous, metal substrate (stainless steel) is measured at small stress lev els for a variety of temperatures. A critical stress, below which no s lip occurs, is not seen and the polystyrene melts used here slip at al l stress levels. The slip velocity is quantified by the slip length (b , equal to the slip velocity divided by the shear rate) and friction c oefficient (k, equal to the slip velocity divided by the shear stress) . The slip length shows complicated dependence with both the molecular mass and temperature, however, when converted to the friction coeffic ient a master curve with molecular mass results for temperatures above 170 degrees C. The data are compared to contemporary theories for sli p. It is concluded that none of the present theories accurately repres ent the data and that the number of adsorbed molecules are in a dynami c equilibrium which affects the slip behavior. Analysis of the force o n the adsorbed molecules during shear demonstrates this may cause adhe sive failure which contributes to the slip. (C) 1998 The Society of Rh eology. [S0148-6055(98)00406-3].