Effect of deformation rate on the phase transition process of syndiotactic polypropylene

Xiaoli Sun, Tiantian Gao, Shuyang Wang, Shujing Zhang, Ce Mi, Huihui Li, Shouke Yan

Abstract : The effect of deformation rate and temperature on the crystal−crystal transition of sPP with [rrrr] = 78% was studied by wide-angle X-ray diffraction measurements, using synchrotron radiation. Three stretching temperatures (25, 40, 90 °C) and two different deformation rates (10 and 20 mm/min) were selected. At lower stretching temperature (e.g. 25 °C), the transformation of form I to mesomorphic phase occurs at both deformation rates. Lower deformation rate favors the occurrence of transition at lower strain. By contrast, higher deformation rate can speed up the transition process, although the start of transformation is observed at higher strain. At the stretching temperature of 40 °C, form I transits to mesomorphic phase at lower deformation rate. At higher deformation rate, the transformation of form I to form III through mesomorphic phase is evidenced. At the stretching temperature of 90 °C, the form I transits to form III and it does not depend on the deformation rate. From the results of stress-strain curves, it can be known that the phase transition from form I into mesomorphic phase is neither strain-induced nor stress-induced transition. These results indicate that the structural evolution of sPP with [rrrr] = 78% in the stretching process strongly depends on the deformation rate and temperature.