Reexamination of self-assembly of nucleator and its influence on the crystallization of polypropylene

Abstract

In this study, the self-assembly phenomenon of nucleator N, N′-dicyclohexylterephthalamide (DCHT) in isotactic polypropylene (iPP) has been investigated and reexamined from the viewpoint of sublimation and condensation. A correlation between the crystallization temperature (Tc) of iPP and the final heating temperature (Tf) was constructed, in which a “V” shape phenomenon and its dependence on DCHT concentration are presented. With the increase of DCHT concentration, both the onset and the bottom of the “V” shift to a higher Tf, which is ascribed to faster saturation of DCHT vapor or slower sublimation rate of DCHT at same Tf for the hybrids with high DCHT concentration. A phase diagram distinguishing various assembly structures of DCHT with different nucleation ability is proposed based on the DSC and optical results, in which four regions are included. Considering the strong hydrogen bonding between DCHT molecules, a schematic condensation mechanism for the particular needle-like, agglomerates and flower-like structures were presented. Rheology results also provide indirect evidence to the phase transformation or condensation process, where two step condensation is observed and the lower temperature step could be attributed to the formation of agglomerates structure due to the “cage effect” of iPP supercooling melt to DCHT vapor.