The interpretation of simultaneous small and wide-angle X-ray scattering data collected during quiescent crystallisation

Small and wide-angle X-ray scattering patterns have been calculated from randomly oriented stacks of lamellar crystals of isotactic polypropylene, at various stages during a hypothetical growth process. When the crystals have a small lateral width, it is possible to detect a long-period peak in the small-angle pattern when the crystalline wide-angle trace is too weak to be detected above the background noise. This finding has important implications for the interpretation of simultaneous small and wide-angle X-ray data collected during quiescent crystallization experiments. In particular, it challenges the suggestion that such measurements provide evidence for crystal growth through spinodal decomposition.