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 or- dered structures of isotactic polypropylene (iPP), i.e., ordered structure effect (OSE), in this study, the role of β-NA concen- tration (Cβ-NA) was studied by differential scanning calorim- etry (DSC) and in-situ small-angle X-ray scattering (in-situ SAXS). Results showed that Cβ-NA was a determining factor of OSE: Cβ-NA<0.01 wt% was too low to induce β- crystallization while Cβ-NA≥0.09 wt% was saturated β-NA concentration, and none of them can induce OSE; only when Cβ-NA=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 re- vealed that at Tf=190 °C (around upper limiting temperature for occurrence of OSE), when Cβ-NA was different, the geo- metric shapes of ordered structures in iPP melts were quite different from each other. The involved mechanism was proposed.