Morphology development during isothermal crystallization. II. isotactic and syndiotactic polypropylene blends

Morphology development during isothermal crystallization in equal molecular weight isotactic polypropylene (iPP), syndiotactic polypropylene (sPP), and iPP/sPP blends was studied with time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) with synchrotron radiation. The sPP melting point is 15-20 °C below that of the iPP component, and sPP multiple melting is not affected by blending for 50-100 wt% sPP compositions. SAXS and WAXD (at 115 and 137.5 °C) show that sPP crystallizes more slowly than iPP. The sPP long spacing is larger than that of iPP at both crystallization temperatures, exhibits a broader distribution, and changes to a greater extent during crystallization. Differential scanning calorimetry (DSC) cooling and SAXS/WAXD measurements show iPP crystallizing first and nearly to completion before sPP in a 50:50 iPP/sPP blend. At 115 °C, iPP crystals nucleate sPP in a 50:50 blend and modify the sPP lamellar spacing. The nucleation does not overcome the large difference in the iPP and sPP rates at 137.5 °C. Before sPP crystallization in a 50:50 blend (115 °C), the iPP long spacing is not affected by molten sPP. The iPP long spacing is slightly expanded by molten sPP, and the WAXD induction time is delayed at 137.5 °C. The observed iPP long spacing in the presence of molten sPP is consistent with previously reported results for iPP/atactic polypropylene (aPP) blends of similar molecular weight. Quantitative differences between the two types of blends are consistent with previously reported thermodynamic rankings.