A SAXS-WAXD study on the mesomorphic-alpha transition of isotactic polypropylene

The smectic-alpha transition of iPP was studied by wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). WAXD and SAXS patterns were taken at different temperatures during the transition. Two different approaches were taken in the analysis of SAXS data: the correlation function and a fitting method based on theoretical distribution models. Up to 80 degrees C just smectic phase was observed, whereas beyond that temperature alpha lamellae appeared and the two populations of lamellae were found to coexist in the sample. The new alpha phase population stemmed from within the preexistent smectic stacks, according to a lamellar insertion model. alpha lamellae thickened to a larger extent than the smectic phase, that just underwent thermal expansion. Results were consistent with a mechanism of transformation involving a rearrangement of the chains without a melting-recrystallization process.