Morphology development during isothermal crystallization. I. Isotactic and atactic polypropylene blends

Morphology development during isothermal crystallization in equal molecular weight isotactic polypropylene (iPP) and atactic polypropylene (aPP) blends was studied with time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering methods with synchrotron radiation. The final long period obtained after crystallization at 115 °C was nearly independent of blend composition up to 50 wt% aPP but showed an increase in the 80 wt% aPP blend. At a high crystallization temperature (137.5 °C), the increase in the final long period with aPP content was significant, and the evolution of iPP crystallinity was also affected. However, at low crystallization temperatures, the additive decrease of the crystallinity and the constant melting point with increasing aPP content suggest that the crystallizability and crystal morphology of iPP is not a strong function of aPP. The iPP/aPP blends showed a strong low-angle SAXS upturn as a function of composition, which suggests the segregation of aPP on size scales larger than the lamellar spacing. A detailed analysis of the SAXS patterns indicates that aPP disrupts the ordering within the lamellar stacking. The results are generally consistent with predominantly interfibrillar incorporation of the aPP diluent within the microstructure, with only modest interlamellar incorporation dependent on the crystallization temperature. The findings can be attributed to the partial miscibility/mixing of the aPP and iPP components in the blend before crystallization, depending on the crystallization undercooling.