Dynamic crystallization and melting behavior of β-nucleated isotactic polypropylene with different melt structures

In the previous work, we reported that the β-selectivity, morphology and tensile behavior of isotactic polypropylene (iPP) can be efficiently tuned by the combination of controlling the melt structure status (namely, creation ordered structure by tuning the fusion temperature Tf) and the addition of β-nucleation agent (β-NA, tradename WBG-II, concentration 0.01 wt%), which was called “Ordered Structure Effect” (OSE). This study further investigates the dynamic crystallization and melting behavior of β-iPP with a different melt structure status by differential scanning calorimentry (DSC) and non-isothermal crystallization kinetics. The results revealed that under all the cooling rates studied (2, 5, 10, 20 and 40 °C/min), the crystallization temperature on the cooling curves increased gradually with the decrease of Tf; meanwhile, when the Tf was in the temperature range of 168–186 °C where the OSE occurs (defined as Region II), the crystallization activation energy ΔE was found to be evidently lower, compared with that when the Tf was higher than 186 °C or lower than 168 °C. The results of the subsequent heating showed that the occurrence of the OSE can be observed at all the cooling rates studied; the location of the Region II was constant when the cooling rate varied. Low cooling rate encouraged the formation of more β-phase triggered by OSE. Moreover, the role of OSE on the β-α recrystallization was comparatively studied by tuning the end temperature of recooling (Tend) after held at Tf, and it was found that the OSE encouraged the formation of β-phase with high thermal stability at the low temperature part of Region II, while enhancing the β-crystal with relatively low thermal stability at the high temperature part of Region II.