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

In the previous work, we reported that the beta-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 T-f) and the addition of beta-nucleation agent (beta-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 beta-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 A degrees C/min), the crystallization temperature on the cooling curves increased gradually with the decrease of T-f; meanwhile, when the T-f was in the temperature range of 168-186 A degrees C where the OSE occurs (defined as Region II), the crystallization activation energy Delta E was found to be evidently lower, compared with that when the T-f was higher than 186 A degrees C or lower than 168 A degrees 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 beta-phase triggered by OSE. Moreover, the role of OSE on the beta-alpha recrystallization was comparatively studied by tuning the end temperature of recooling (T-end) after held at T-f, and it was found that the OSE encouraged the formation of beta-phase with high thermal stability at the low temperature part of Region II, while enhancing the beta-crystal with relatively low thermal stability at the high temperature part of Region II.