The alpha beta-iPP growth transformation of commercial-grade iPP during non-isothermal crystallization

In the present study, the crystallization of commercial-grade isotactic polypropylene (iPP) was studied by optical microscopy, scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXD). It was found that isothermal crystallization of used iPP at 135 degrees C leads to the formation of pure alpha-iPP crystals. During non-isothermal crystallization at a cooling rate of 10 degrees C min(-1), growth transformation of alpha into beta-iPP occurs at the growth front of alpha-iPP spherulites. This is not common and has only been found during crystallization in a temperature gradient. The observed alpha beta-iPP growth transformation is confirmed to be a result of the existence of some impurities, which exhibited their ability to nucleate beta-iPP. The ability of the impurities to nucleate beta-iPP is dependent on concentration and temperature. Therefore, interlamellar rejection of the impurities caused by fast cooling (40 degrees C min(-1)) cannot induce the alpha beta-iPP growth transformation. On the other hand, even though isothermal crystallization of iPP at 135 degrees C can result in the accumulation of impurities at the spherulite growth front, it still cannot induce the alpha beta-iPP growth transformation at a low concentration.