Comparative Investigation on the Crystallization Behavior of Polypropylene Nucleated by Sorbitol-Based Nucleating Agents with and without Fibrous Network Formation

Sorbitol derivatives are effective nucleating agents (NAs) for polypropylene (PP), and their nucleation capacity is generally accepted to associate with fibrous network formation before PP crystallization. However, whether the network must be preformed to promote PP crystallization is overlooked. To address this issue, this work adjusted the network to not form before PP crystallization and compared its effect on PP crystallization to NAs with the networks. Specifically, for PP containing 0.4 wt % Millad NX8000, a sorbitol-based NA, the network is controlled to form by cooling at 2 °C/min and not form by ice-water quenching. Both samples were heated to a temperature sufficient to erase the thermal history of PP while retaining the network of NAs present and absent (NA-p and NA-a), and subsequent PP crystallization was evaluated. Compared to PP/0.4NA-p, the slightly higher nucleation efficiency and density for PP/0.4NA-a indicate that the NA without networks exhibits comparable or even more effective nucleating effects. Additionally, NA-p restricts PP growth in the later crystallization stage and promotes the oriented growth after shear, whereas these phenomena weaken in PP/0.4NA-a. These findings provide new insights into the role of fibrous network formation in the effectiveness of sorbitol-based NAs.