New understanding in the influence of melt structure and β-nucleating agents on the polymorphic behavior of isotactic polypropylene

Abstract

The b-nucleation behavior of isotactic polypropylene (iPP) is a fascinating and important issue in polymer physics; however, little about this phenomenon or its physical nature has been clearly understood. In the present study, by tuning the heating temperature (fusion temperature, T f ), the amount of ordered structures in iPP melt was controlled. In this way, the influence of five types of representative bnucleating agents (b-NA) on the crystallization behavior of iPP with different melt structures (i.e. the amount of ordered structures) was comparatively studied by differential scanning calorimetry (DSC), polarized optical microscopy (PLOM), scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and rheological measurement. A surprising synergetic effect was observed between b-NAs with a-/b-dual polymorphic selectivity (dual-selective b-NA) and the ordered structure of iPP, resulting in significant increases of the b-nucleation efficiency and the b-phase proportion of the sample. PLOM observation of the crystallization process confirmed that with the presence of ordered structures in iPP melt, a mass of dark, small crystal embryos derived from self-nuclei uniformly distributed in the melt and exhibited b-nucleation efficiency. This work provides the first evidence that for iPP nucleated with dualselective b-NA, the ordered structures play a determining role in the b-nucleation of iPP. Under the influence of the dual-selective b-NA, the ordered structures exhibited b-nucleation efficiency and therefore encouraged b-nucleation. A possible mechanism was proposed.

Influence of melt structure on the crystallization behavior and polymorphic composition of polypropylene random copolymer

“…It should be noted that according to the results in the previous work [48] and related references [13,15], as well as the subsequent melting experiments at 153 C (at this temperature only a-crystals can survive, revealing the assignment of a-and b-crystals) the less birefringent crystals are a-crystals while the more birefringent ones are b-crystals, as indicated by arrows in Fig. 6.…”

“…Compared with the OSE behavior of b-iPP in the previous study [48], the temperature range of Region II of b-PPR (173 C ! T f !…”

“…From this point, our group [48][49][50] prepared iPP homo-polymer nucleated by 0.03 wt% b-NA (the b-NA has dual-selectivity, such as the rare earth based b-NA with tradename WBG-II, and the widely used N,N 0 -dicyclohexyl-2, 6-naphthalenedicarboxamide, DCNDCA). By tuning the fusion temperature (T f ), the melt structure (i.e., the content of ordered structures in the melt) of the sample was controlled, and the crystallization behavior of the sample with different melt structures was studied.…”

Effects of β‐nucleating agent and crystallization conditions on the crystallization behavior and polymorphic composition of isotactic polypropylene/multi‐walled carbon nanotubes composites

“…[45,46] During the subsequent melting, the lower the amount of β-phase that takes part in β-α recrystallization (in the case of T end < 100°C), the more the amount of β-phase measured from the DSC heating curves. Therefore, the final β c of the the sample is determined by the two aspects as discussed earlier.…”

“…Our previous studies [24,[46][47][48] proved that β-nucleated iPP with strong inherent α-nucleation capability can also form an amount of β-phase if the cooling rate is properly selected. However, the role of cooling rates and end temperature of cooling on the polymorphic composition of β-nucleated iPP/MWCNTs composites had not been studied before.…”