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

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 (Tcp) of iPP by up to 8 °C at a cooling rate of 20 °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. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 135, 45956.