Unique crystallization behavior of isotactic polypropylene in the presence of L-isoleucine and its inhibition and promotion mechanism of nucleation

摘要

L-Isoleucine (L-Ile) was identified as an efficient anti-nucleating agent for isotactic polypropylene (iPP). At 0.08 wt %, L-Ile could significantly decrease the peak crystallization temperature (T-cp) of iPP by up to 8 degrees C at a cooling rate of 20 degrees C/min. Furthermore, L-Ile exhibited both anti-nucleation and pro-nucleation abilities; i.e., a low content of L-Ile inhibited iPP crystallization, whereas a high content promoted iPP crystallization. The unique crystallization behavior of iPP in the presence of L-Ile was investigated by differential scanning calorimetry, polarized optical microscopy (POM), and rheological measurement. According to POM, a low content of L-Ile completely dissolved in the iPP melt, whereas a high content of L-Ile did not. Therefore, a mechanism by which L-Ile inhibits and promotes the nucleation of iPP was proposed. Dissolving L-Ile molecules in the iPP melt hindered the homogeneous nucleation of iPP as a "dilution effect"; however, as the content increases, L-Ile could not be completely dissolved in molten iPP, and the residual crystals of L-Ile thus provided heterogeneous nucleation sites for iPP and further promoted its crystallization. Experimental evidence from rheology and POM supported this mechanism. (C) 2017 Wiley Periodicals, Inc.