Evolution of Cellular Morphologies and Crystalline Structures in High-expansion Isotactic Polypropylene/cellulose Nanofiber Nanocomposite Foams

Herein, the development of cell morphology and crystalline microstructure of injection-molded isotactic polypropylene/cellulose nanofiber (PP/CNF) composite foams with 2-10-fold expansion ratios was investigated through scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). Compared with isotactic polypropylene (iPP) foams, the added CNF improved the cell morphology and resulted in a great reduction in cell size. Additionally, the PP lamella orientation and crystal type were notably altered during the core-back FIM process. As the expansion ratio increased, the original isotropic lamellae in the iPP foams were transformed into an oriented lamellar structure and then further transformed into a typical shish-kebab structure, while hybrid shish-kebab structures were simultaneously generated in the high-expansion PP/CNF nanocomposite foams. Accordingly, the highest content of β-crystals was observed in the low-expansion iPP foams. In contrast, the β-crystal content in PP/CNF composites decreased monotonously as the expansion ratio increased, which resulted from the combined effects of CNF's nucleating ability for α-crystals and the more dominant extensional flow effect assisted by the added CNF.