Mechanism of form I′ formation in polybutene-1/polypropylene blends

Isotactic polybutene-1 (PB) can crystallize into form I′ polymorph directly in its blends with isotactic polypropylene (iPP), but the mechanism remains controversial. In this work, by using Fourier-transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC), we investigated systematically cold crystallization of PB in its blends with iPP and with atactic polypropylene (aPP), one crystallizable and the other amorphous. It was found that form I′ was produced in both PB/iPP and PB/aPP blends, the higher the PP content, the greater the form I′ fraction, and iPP was found to be more effective in promoting form I'. However, in a PB/iPP blend containing 10 wt% iPP with the iPP fully crystallized at 120 °C, the PB crystallized into form II exclusively in the cold crystallization, at a faster rate than in neat PB. These experimental results indicate that in PB/iPP blends both confined nucleation mechanism and surface nucleation mechanism operate, but the former governs the PB polymorph formed. Under the confinement only the smaller form I′ nuclei are allowed, and the PP crystals then expedite the process via the surface nucleation mechanism.