Wall slip and extrudate distortion of three polymer melts

A high-density polyethylene (HDPE), a poly(dimethylsiloxane) (PDMS), and an oxetane based alternating block thermoplastic elastomer (TPE) were subjected to steady torsional and capillary flows. HDPE and PDMS exhibited wall slip in steady torsional flow. The shear stress ranges at which wall slip became apparent during steady torsional flow for HDPE and PDMS coincided with the wall shear stress ranges in capillary flow at which distortions of the extrudates emerging from capillary die were first noted. In contrast, TPE did not exhibit wall slip in steady torsional flow (for shear rates up to 200 s(-1) and strains up to 25). None of the three polymers exhibited any overshoots of shear stress or first normal stress difference under steady flow conditions, indicating that the often-reported stress overshoots could be artifacts of wall slip. Furthermore, the extrudates of the TPE were relatively smooth and exhibited only minor and nonperiodic surface blemishes. The absence of extrudate distortions under the presumed no-slip condition for TPE emphasizes the intimate link between wall slip and the development of extrudate distortions and strengthens the premise that extrusion flow instabilities can be eliminated under conditions of stable wall slip or stick. (C) 2003 The Society of Rheology.