Melting behavior and structural and morphological changes of isotactic polypropylene from heat treatment

Time-resolved X-ray measurements during melting and detailed structural and morphological analyses of samples heat treated near their melting temperature for a short time revealed a structural change of isotactic polypropylene (iPP) in which the melt-crystallized at a cooling rate (CR) of 20 K/min. During melting at a heating rate (HR) of 5 K/min, we observed the appearance of a peak with a higher melting temperature (Tm); however, we could not clearly observe −231, −161 reflections that are typical for an α2 crystal. The α2 crystal was a minor component of the heat-treated sample with elevated Tm in which reorganization was almost completed during the heat treatment. These results suggest that lamellar thickening of the α1 crystal is the main mechanism of reorganization. Lamellar thickening was accompanied by an increase in central diffuse scattering of small-angle X-ray scattering (SAXS), suggesting that thickening occurred at randomly distributed positions and resulted in the disordering of lamellar stacking observed based on observations with a transmission electron microscope (TEM) for the heat-treated samples. The drastically increased long period (L) above 160 °C, which can be attributed to the partial melting of thin lamellae, was maintained after the heat treatments by the aforementioned reorganization.