Crystal Morphology of Polypropylene/Graphene Nanoplatelets (PP/GnPs) Fibers in Isothermal and Non-isothermal Crystallization

crystallization behavior of polypropylene (PP) crystals including nucleation, growth, and morphology is significantly affected in the presence of graphene nanoplatelets (GnPs). In the present work, spherulites formation and their growth in the PP/GnPs nanocomposite fibers have been investigated by hot-stage polarized optical microscopy at isothermal and non-isothermal conditions. The results show that the crystallization rate of PP/GnPs nanocomposite fibers is higher than that of neat PP fiber whereas the spherulites with smaller size and more irregular shape are formed in the PP/GnPs samples. Moreover, nucleation time decreases with the addition of GnPs since they have a nucleating effect. The results obtained from non-isothermal crystallization suggest that the more nucleating sites are formed in the presence of GnPs, resulting in PP crystallization at higher temperatures. Furthermore, most spherulites are firstly formed around GnPs. Nucleation density of neat PP is significantly lower than that of PP/GnPs samples due to heterogeneous nucleation of GnPs. By increasing the GnPs content from 0.5 wt% to 1 wt%, no appreciable changes are observed in nucleation density, suggesting that the nucleating effect of GnPs is saturated at 0.5 wt%.