Three representative types of WAXD/SAXS patterns to establish the bimodal structure concept of stacked lamellae in isotactic polypropylene spherulites

Abstract

Two types of lamellar stacking model were proposed for the inner structure of the melt-grown spherulites of isotactic polypropylene (iPP): the edge-on structure (flat lamellar planes stand on the spherulite plane, and a*-axis // growth direction of spherulite, b-axis ┴ spherulite plane, and c-axis // spherulite plane) and the flat-on structure (flat lamellar planes lay on the spherulite plane, a*-axis // growth direction, b-axis // spherulite plane, and c-axis ┴ spherulite plane). After a literature review was performed, the observed X-ray scattering data were not high quality and could not be used to establish these bimodal structures; moreover, the X-ray data analyses were not performed satisfactorily. As a result, the derivations of actual structures were still debatable. We performed simultaneous time-resolved WAXD/SAXS measurements using a synchrotron X-ray microbeam technique. The collected data were classified into three sets of completely different WAXD/SAXS patterns. Detailed quantitative analysis enabled the determination of the spatial orientations of the stacked lamellae and related crystallographic axes. Although the bimodal structure of iPP spherulites was considered to be a well-known and already-established concept for long years, the bimodal structure was successfully confirmed for the first time in the present study, making the concrete discussions on the growth mechanism of the iPP spherulites possible. The in situ wide-angle (WAXD) and small-angle X-ray scattering (SAXS) measurements have been performed using a synchrotron microbeam technique for the melt-isothermally-growing iPP spherulites. The thus-collected data were found to be classified into three sets of totally different WAXD/SAXS patterns, from which the three different orientation modes of the stacked lamellae and related crystallographic axes were deduced. These structural information allowed us to discuss the growth mechanism of stacked lamellae in the iPP spherulites from the microscopic point of view.