Temperature-Dependent Crystalline-Amorphous Structures in Isotactic Polypropylene - Small-Angle X-Ray-Scattering Analysis of Edge-Bounded 2-Phase Systems

Abstract

Small-angle X-ray scattering (SAXS) experiments on isothermally crystallized isotactic polypropylene (iPP) reveal a clear temperature dependence of the crystalline-amorphous structure over a broad range of temperatures below the crystallization temperature. This phenomenon is analyzed using SAXS combined with dilatometry. A new concept for the evaluation of SAXS data-the specific length of edges L(s)-is used. It allows a quantitative interpretation of g''(0), the second derivative of the electron density correlation function at r = 0. Using this parameter, a clear identification of the relevant physical process is possible. Whereas primary crystallization leads to the formation of laterally extended crystalline lamellae, subsequent cooling generates additional small crystallites in the amorphous regions. A model relating this mode of partial crystallization and melting to the special morphology of iPP-the cross-hatched structure-is suggested.