Influence of Stereoregularity on the Wall Slip Phenomenon in Polypropylene Prashant S. Tapadia, † Yogesh M. Joshi, † Ashish K. Lele,* ,† and R. A. Mashelkar ‡ Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India, and Council of Scientific and Industrial Research, Anusandhan Bhavan, 2 Rafi Marg, New Delhi 110 001, India Received August 30, 1999 Introduction. The phenomenon of wall slip occurring during extrusion of certain polymer melts has been extensively investigated for several decades. The history of research on wall slip and the physical mechanisms of slip have been recently reviewed by Wang. 1 It is generally accepted that for walls of high adhesive energy slip occurs by sudden disentanglement of bulk chains from the adsorbed chains at a critical stress, 2,3 while for walls of low adhesive energy slip occurs by sudden debonding of the adsorbed chains. 4 We have recently proposed a unified model for wall slip that combines the two mechanisms of slip into one mathematical frame- work. 5 Thus, although a lot is now known about the mechanisms of the interfacial slip, it is still not clear as to how the subtleties of molecular architecture influence the slip behavior. Thus, the effects of molec- ular structural parameters such as molecular weight, molecular weight distribution, tacticity, short- and long- chain branching, and the branching distribution are still not completely understood. In this work we show for the first time that the tacticity of polymer chains can significantly influence the slip behavior. We show that syndiotactic polypropylene shows sharkskin distortions on the extrudate surface, which is an indication of wall slip, while an isotactic polypropylene of similar molec- ular weight and molecular weight distribution does not show any wall slip. Experimental Section. Four grades of metallocene catalyzed syndiotactic polypropylene (m-sPP) and two grades of metallocene catalyzed isotactic polypropylene (m-iPP) were supplied by the FINA Oil and Chemical Co., USA. One grade of conventional Zeiglar-Natta catalyzed isotactic polypropylene was obtained from Indian Petrochemicals Company Ltd., India. The rel- evant characteristics of the polypropylene grades used in this work are reported in Table 1. All extrusion experiments were done on a CEAST Rheovis 2100 rate controlled capillary rheometer. Iso- tactic polypropylene grades were extruded at 190°, while syndiotactic polypropylene grades were extruded at temperatures of 190 and 140 °C. The PP samples were extruded through case hardened steel dies of 1.0 mm diameter and L/D of 20 and 30. The dies had an entry angle of 60°. Some experiments were also done using flat entry dies. In some other experiments the internal surface of the dies were modified by coating them with a fluoroelastomer (SUMMIT, Sumitomo Corp., Japan). Coating was done following the procedure recommended by Wang and Drda. 6 A 5% solution of the fluoroelas- tomer in acetone was made to flow through the capillary for several times followed by drying the capillary at 220 °C for 12 h. Results and Discussion. Figure 1 shows the photo- micrograph of the extrudate surfaces of m-sPP-1 and m-iPP-2 extruded at shear rates of 50, 600, and 2000 s -1 . All samples were extruded at 190 °C using a die of 60° conical entry angle unless otherwise mentioned. The extrudate of m-sPP-1 at 50 s -1 shows a smooth surface, while that at 600 s -1 shows a typical sharkskin surface similar to that seen in LLDPE. Gross melt fracture was seen at the shear rate of 2000 s -1 . All the four grades of m-sPP showed sharkskin above a critical shear stress. Interestingly, none of the grades showed a distinct stick-slip phenomenon or flow oscillations. Importantly, Figure 1 shows that isotactic polypropylene (m-iPP-2) of the same molecular weight as m-sPP-1 and of similar molecular weight distribution does not show sharkskin distortions on the extrudate surfaces. In fact, none of the iPP grades of high or low molecular weights and of broad or narrow molecular weight distributions showed sharkskin extrudates over the entire range of shear rates studied. Figure 2 shows the flow curves for all the four grades of m-sPP. It can be seen that the shear rate at which the sharkskin was first seen increases as the molecular weight decreases from m-sPP-1 to m-sPP-4. However, sharkskin was seen at the same shear stress of about 0.2 MPa for the three high molecular weight resins m-sPP-1 to m-sPP-3. For the m-sPP-4, weak sharkskin distortion was seen at much higher shear stresses and was not as predominant as for the higher molecular weight grades. Extrusion of high molecular weight m-sPP at a lower temperature of 140 °C showed that the sharkskin appeared again at a shear stress of around 0.2 MPa (and consequently at lower shear rates). None of the shear rate-shear stress plots showed any nonmonotonic behavior such as that seen typically in the case of a strong stick-slip transition. At higher shear stresses the extrudate surface of the syndiotactic as well as the isotactic polypropylene samples showed gross spiral distortions as seen in Figure 1. We believe that this distortion is due to entry flow instabilities at high flow rates. When the syndiotactic polypropylene samples were extruded through a fluoroelastomer coated die under identical conditions, it was found that while the sharkskin disappeared and the extruded surface became smooth, the spiral gross distortion continued to occur at higher pressure drop. This confirms that the gross distortions are entry instabilities, while the sharkskin defects are indeed interfacial phenomena at the die wall. Also, when a flat entry die was used during extrusion, the gross distortions occurred at a shear rate lower than that for a conical entry die. † National Chemical Laboratory. ‡ Council of Scientific and Industrial Research. * Corresponding author: E-mail lele@che.ncl.res.in. Table 1. Molecular Parameters of Syndiotactic and Isotactic Polypropylenes material Mw Mw/Mn power law index 13 m-sPP-1 160 000 4.5 0.32 m-sPP-2 120 000 4.1 0.34 m-sPP-3 115 000 3.6 0.35 m-sPP-4 87 000 3.4 0.44 m-iPP-1 203 000 2.5 0.36 m-iPP-2 160 000 2.8 0.41 ZN-iPP-1 >10 0.36 250 Macromolecules 2000, 33, 250-252 10.1021/ma991480l CCC: $19.00 © 2000 American Chemical Society Published on Web 01/05/2000