Investigation of the melt-crystallization of polypropylene by a temperature slope method

To investigate the in-situ ordering process of isotactic polypropylene (iPP) from a melt state, a stationary growth front was prepared by the temperature slope crystallization (TSC) method. During the melt-crystallization, iPP was crystallized into the a-phase or beta-phase depending on the crystallizing conditions. The mechanism of the meltcrystallization at the growth front was precisely observed by wide-angle and small-angle x-ray scattering (WAXS and SAXS) using a strong synchrotron beam. In the TSC apparatus, the sample was crystallized in between a heater, controlled to 220degreesC, and a cooler, cooled by water to 25degreesC. We define the z-axis parallel to the temperature gradient. A-lamellae and B-lamellae are also defined as those whose lamellar normal are perpendicular and parallel to the z-axis, respectively. In a sample-stop (SS) stage before the TSC, the original a-phase lamellae became thicker, approaching to the melt-solid boundary by annealing. The annealing process showed that the a-phase B-lamellae remained and the SAXS reflection was stronger on the meridian near the melt-solid boundary in the SS stage. In the beginning of the TSC, the a-phase B-lamellae developed as a primary crystallization. During secondary crystallization under high supercooling, the SAXS cross pattern appeared showing that the a-phase developed both A- and B-lamellae. As the growth direction of A-lamellae is parallel to the z-axis, A-lamellae grow faster than B-lamellae. By the self-epitaxial mechanism on the side surface of the A-lamellae, the B-lamellae grow on the base of the A-lamellae. Following appearance of a spontaneous beta-nucleus, the beta-phase lamellae grew preferentially, excluding the alpha-phase, and occupied the whole area of the sample. In this case also, A-lamellae are advantageous to grow because of the growth direction parallel to the z-axis. As a result, the SAXS beta-phase reflection appeared on the equator.