Wall Slip Behaviors of Crystallized Polypropylene under Oscillatory Shear

Compared to the wall slip of polymer melts, less attention has been paid to the wall slip of viscoelastic solids, such as crystallized polymer. The existence of the wall slip of crystallized polymers not only prevents the precise measurement of their viscoelastic properties but also leads to flow instabilities during polymer processing like injection molding. In this work, a continuous cyclic strain sweep test is designed to resolve the wall slip behaviors of crystallized polypropylene under large-amplitude oscillatory shear at different temperatures. A stick-slip transition is identified, and its critical conditions are discussed in relation to the crystallinity. A rheo-electric measurement is performed to reveal the interfacial process during the development of wall slip. At relatively low temperatures (below 145 degrees C), wall slip is found to take place in advance of bulk structural changes. In this case, a quantitative method is proposed to extract the transient slip rate from the apparent stress. A weak slip regime, a transition regime, and a strong slip regime where the slip rate exhibits different strain dependence are identified. Finally, the effects of surface topography and surface free energy on the wall slip behaviors are discussed.