SCHEDULED MAINTENANCE
The αβ-iPP growth transformation of commercial-grade iPP during non-isothermal crystallization
Abstract
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 °C leads to the formation of pure α-iPP crystals. During non-isothermal crystallization at a cooling rate of 10 °C min−1, growth transformation of α into β-iPP occurs at the growth front of α-iPP spherulites. This is not common and has only been found during crystallization in a temperature gradient. The observed αβ-iPP growth transformation is confirmed to be a result of the existence of some impurities, which exhibited their ability to nucleate β-iPP. The ability of the impurities to nucleate β-iPP is dependent on concentration and temperature. Therefore, interlamellar rejection of the impurities caused by fast cooling (40 °C min−1) cannot induce the αβ-iPP growth transformation. On the other hand, even though isothermal crystallization of iPP at 135 °C can result in the accumulation of impurities at the spherulite growth front, it still cannot induce the αβ-iPP growth transformation at a low concentration.
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