Understanding the effects of nucleating agent concentration on the polymorphic behavior of β-nucleated isotactic polypropylene with different melt structures.

To elucidate the synergetic effect in enhancing β-crystallization between β-nucleating agent (β-NA) and ordered structures of isotactic polypropylene (iPP), i.e., ordered structure effect (OSE), in this study, the role of β-NA concentration (C) was studied by differential scanning calorimetry (DSC) and in-situ small-angle X-ray scattering (in-situ SAXS). Results showed that C was a determining factor of OSE: C < 0.01 wt% was too low to induce β-crystallization while C ≥ 0.09 wt% was saturated β-NA concentration, and none of them can induce OSE; only when C = 0.01-0.08 wt% (i.e., unsaturated concentration) can OSE take place. There existed a competitive relationship in the enhancement of β-phase between OSE and conventional β-nucleation effect of β-NA. Moreover, in-situ SAXS revealed that at T = 190 °C (around upper limiting temperature for occurrence of OSE), when C was different, the geometric shapes of ordered structures in iPP melts were quite different from each other. The involved mechanism was proposed. [ABSTRACT FROM AUTHOR]