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SLIPPING TRANSITION AT A POLYMER/SOLID INTERFACE MIGLER K. *, HERVET H.** AND LEGER L.** *Exxon Research and Engineering Company, Clinton Township, Route 22 East, Annandale, N.J. 08801, U.S.A.. "**Collige de France, Laboratoire de Physique de la Matitre Condensde, 11 place MarcelinBerthelot, 75231 Paris Cedex 05, France. ABSTRACT

Direct measurements of slippage of a polymer melt within 100 nm of a solid surface, under simple shear, are reported. In the case of a high molecular weight polydimethylsiloxane (PDMS) interacting weakly with the solid surface, a sharp transition between weak and strong slippage is observed as a function of shear stress. For strong polymer-surface interactions, slippage is highly reduced, and no transition is observed. The results are compared to a theoretical model recently proposed by Brochard and de Gennes. INTRODUCTION For Newtonian flowing liquids, the normal boundary condition at the fluid-solid interface is non slippage. For viscoelastic fluids, like polymer melts however, the velocity at the solid surface can be finite, leading to a slippage effect. This phenomenon is of the outmost importance, especially for polymer processing, since the slippage at the extruder surfaces can be one cause of "extrudate roughness" or melt fracture. There is experimental evidence [1-5] that slippage occurs at the polymer melt-solid interface. However, previous techniques were either indirect or lacked the spatial resolution needed to examine the velocity very close to the surface, and were unable to give precise quantitative results on the slippage velocity and the associated extrapolation length, b (b is the distance from the wall where the fluid velocity extrapolate to zero). Furthermore it seems that slippage is very sensitive to the polymer surface interactions, and does not always show up. Theoretical models for slippage have also been developed, either purely hydrodynamic [3] or more phenomenological [6, 7]. De Gennes [6] has shown that for a polymer melt flowing on a smooth non adsorbing surface, slippage should always occur. But if there are some polymer molecules adsorbed or grafted on the surface, the slippage is strongly reduced [7]. In such conditions, it has also been suggested [3, 8] that slippage should occur above a critical shear stress related to the structure of the surface and the molecular weight of the melt.

Mat. Res. Soc. E

p. Proc. Vol. 290. 01993 Materials Research Society

14

EXPERIMENTAL SET-UP AND MATERIALS Determination of the slippage velocity of a polymer melt at a solid interface requires a technique able to measure the velocity of a fluid in a layer with a thickness comparable to that of a polymer chain, i.e. of the order of a few tens nanometers. Classical Laser Dopler Velocimetry does not match these requirements, so we have developed a new optical technique which combines

Evanescent Wave Induced Fluorescence (E.W.I.F.) [9] and Fringe Pattern

Fluorescence Recovery After Photobleaching (F.P.F.R.A.P.) [10]. The former technique gives the spatial resolution norm