Wall Slip Behaviors\nof Crystallized Polypropylene\nunder Oscillatory Shear

Compared to the wall slip of polymer melts, less attention\nhas\nbeen paid to the wall slip of viscoelastic solids, such as crystallized\npolymer. The existence of the wall slip of crystallized polymers not\nonly prevents the precise measurement of their viscoelastic properties\nbut also leads to flow instabilities during polymer processing like\ninjection molding. In this work, a continuous cyclic strain sweep\ntest is designed to resolve the wall slip behaviors of crystallized\npolypropylene under large-amplitude oscillatory shear at different\ntemperatures. A stick–slip transition is identified, and its\ncritical conditions are discussed in relation to the crystallinity.\nA rheo-electric measurement is performed to reveal the interfacial\nprocess during the development of wall slip. At relatively low temperatures\n(below 145 °C), wall slip is found to take place in advance of\nbulk structural changes. In this case, a quantitative method is proposed\nto extract the transient slip rate from the apparent stress. A weak\nslip regime, a transition regime, and a strong slip regime where the\nslip rate exhibits different strain dependence are identified. Finally,\nthe effects of surface topography and surface free energy on the wall\nslip behaviors are discussed.