Isothermal crystallization behavior of β-nucleated isotactic polypropylene with different melt structures

A previous work reported that the polymorphic crystallization behavior of isotactic polypropylene (iPP) can be efficiently tuned by the combination of controlling the melt structure (i.e., creation of an ordered structure by tuning the fusion temperature T_f by self-nucleation) and the addition of the α-/β-dual-selective nucleation agent (dual-selective β-NA, tradename WBG-II). In this study, we further investigated the impact of isothermal treatment on the polymorphic behavior and the isothermal crystallization kinetics of a β-iPP sample with different melt structures by differential scanning calorimetry (DSC). The results of isothermal crystallization kinetics study illustrated that as T_f decreased gradually from Region I to Region II and Region III, two sharp increases of the crystallization rate took place at the transition temperatures of Region II, showing that the enhancement of β-phase crystallization took place in a certain crystallization rate window. The calculation of the Avrami exponent n revealed that the two-dimensional growth of crystallites with instantaneous nucleation took place before and after the occurrence of the synergetic effect between the ordered structures and the dual-selective β-NA (when Tf ≥ 168 °C). Moreover, it was found that the occurrence of the synergetic effect in the fusion temperature range of Region II can evidently enhance the β_c of the sample: by tuning the fusion temperature T_f and the isothermal crystallization temperature T_c, the relative percentage of β-phase (β_c) of the single β-iPP sample with only 0.03 wt.% β-NA can be efficiently tuned in the wide β_c range of 0 %–95.0 %; meanwhile, the sensitivity of β_c to the isothermal crystallization temperature T_c was reduced.