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

Abstract

Abstract Morphology development during isothermal crystallization in equal molecular weight isotactic polypropylene ( i PP), syndiotactic polypropylene ( s PP), and i PP/ s PP blends was studied with time‐resolved simultaneous small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) with synchrotron radiation. The s PP melting point is 15–20 °C below that of the i PP component, and s PP multiple melting is not affected by blending for 50–100 wt % s PP compositions. SAXS and WAXD (at 115 and 137.5 °C) show that s PP crystallizes more slowly than i PP. The s PP long spacing is larger than that of i PP 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 i PP crystallizing first and nearly to completion before s PP in a 50:50 i PP/ s PP blend. At 115 °C, i PP crystals nucleate s PP in a 50:50 blend and modify the s PP lamellar spacing. The nucleation does not overcome the large difference in the i PP and s PP rates at 137.5 °C. Before s PP crystallization in a 50:50 blend (115 °C), the i PP long spacing is not affected by molten s PP. The i PP long spacing is slightly expanded by molten s PP, and the WAXD induction time is delayed at 137.5 °C. The observed i PP long spacing in the presence of molten s PP is consistent with previously reported results for i PP/atactic polypropylene ( a PP) blends of similar molecular weight. Quantitative differences between the two types of blends are consistent with previously reported thermodynamic rankings. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1876–1888, 2001