Shear-induced molecular orientation and crystallization in isotactic polypropylene: Effects of the deformation rate and strain

Studies of dilute polymer solutions in shear flow suggest that the mean fractional extension of molecules increases gradually with the Weissenberg number (Wi = shear rate × longest relaxation time) and approaches an asymptotic value of 0.4-0.5, while in elongational flow it approaches full contour length above a certain critical strain rate. In an entangled polymer melt, this behavior is more complex due to inter- and intramolecular interactions. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) experiments were performed to investigate the effects of shear rate, shear duration, and Wi on the extent of molecular orientation/extension and crystal orientation in an isotactic polypropylene (iPP) melt. Two series of experiments were designed: (1) variation of shear rate (30, 45, and 60 s ^-1) at a constant shear duration (5 s) and (2) variation of shear duration (1.3, 3, and 5 s) at a constant rate (60 s ^-1). The degree of crystal orientation (Herman's orientation function, f) observed at 165°C and fraction of oriented crystals (X _o) observed in a fully crystallized sample at room temperature increased with both shear rate and shear duration. Interestingly, at a constant strain (rate × duration), short-duration shear at a high rate was found to be more effective (i.e., higher f and X _o) than long-duration shear at a low rate. The longest relaxation time for the iPP sample and Wi were estimated from the dynamic moduli data. Both f and X _o were found to gradually increase with Wi and approached plateau values at high values of Wi. Results indicated that, even under a very intense shear field (or high Wi values), molecules do not extend to full contour length, and there is a limiting value for mean orientation/extension and subsequent crystal orientation in a polymer matrix. Characteristic dimensions of the shish-kebab entity formed in a sheared iPP melt at 165°C were determined from the rheo-SAXS data. It was found that the average shish length was 700-750 nm and the average spacing between adjacent kebabs was 60-70 nm.