The melt–recrystallization behavior of highly oriented α-iPP fibers embedded in a HIPS matrix

The melt–recrystallization behavior of α-iPP fibers embedded in an amorphous HIPS matrix has been studied by means of optical microscopy. The amorphous HIPS serving as a supporter of iPP fibers does not become involved in the nucleation and crystallization process of the molten highly oriented iPP fibers. It also does not provide any birefringence under the optical microscope with crossed polarizers. This enables the study of orientation-induced β-iPP crystallization through a control of the melting status of the fibers. Through melting the fibers at different temperatures above 175 °C and subsequent recrystallization, some β-iPP crystals were always produced. The content of the β-iPP crystal depends strongly on the melting temperature and melting time of the iPP fibers. It was confirmed that melting the iPP fibers at relatively lower temperature, e.g. 176 °C, less amount of β-iPP crystals were observed. The content of β-iPP crystal enhances first with increasing melting temperature and then decreases with further increase of the fiber melting temperature. The β-iPP crystallization is found to be most favorable upon melting the fibers at 178 °C for 2 min. This demonstrates the requirement of a certain chain or chain segment orientation for generating β-iPP crystallization on the one hand, while higher orientation of the iPP chains or chain segments encourages the growth of iPP crystals in the α-form on the other hand. This has been further confirmed by varying the melting time of the fiber at different temperatures, since relaxation of the iPP molecular chains at a fixed temperature is time dependent. Moreover, the complete transformation of α-iPP fibers in some local places into β-iPP crystals implies that the αβ-transition may not be required for the orientation-induced β-iPP crystallization.