Molecular weight and distribution dependency of spherulite growth rate of isotactic polypropylene with α phase

The relationship between the molecular weight of isotactic polypropylene (iPP) and the α-phase spherulite growth rate ( G ) was successfully established at different isothermal crystallization temperatures ( T c ). A significant weight-average molecular weight ( M w ) dependent G was identified that as M w increased, G first increased and then decreased at each T c . In contrast, the relationship between G and number-average molecular weight ( M n ) was weak. This behavior originated from the advantage of longer chains in secondary nucleation, the rate‑determining step of spherulites growth. M w captured this advantage because it emphasized the contribution of longer chains to the total mass of the system. The non-monotonic variation of G with M w resulted from the synergy of three factors: chain diffusion, nucleation probability, and the matched dimension of the radius of gyration of random coils in the melt and the final spacing of crystals. When iPP had a moderate M w , it showed a considerable nucleation probability, fast chains mobility, and ideal dimension matching between the random coils and the size of lamellar, leading to a maximum in G . Moreover, the molecular weight distribution ( MWD ) was a key variable in distinguishing the roles of M w and M n . As MWD narrowed, the correlation between G and M n increased significantly. Therefore, the difference between M w and M n in describing G was eliminated when MWD is smaller than 5.