Effects of ultrasound on the conformation and crystallization behavior of isotactic polypropylene and β-isotactic polypropylene

The effects of ultrasonic irradiation on conformation and crystalline structure of isotactic polypropylene (iPP) and β-isotactic polypropylene (β-iPP) were studied by means of Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results demonstrate that ultrasonic irradiation decreases the helical conformation order and changes the crystalline structures: for iPP, Tm and Tonset decrease, Tm − Tonset becomes larger; for β-iPP, the βc, (1 − λ)β and kβ increase, the intensities of α(040) plane and α(130) plane in WAXD profiles decrease evidently. For both iPP and β-iPP, the crystallinity decreases, d-spacing increases, the crystallite size L decreases and ultrasound shows a selective effect on the growth of β-crystal. Furthermore, DSC and WAXD were employed to observe the effect of ultrasound on the “melting memory effect” of β-iPP. The results indicate that ultrasonic irradiation destructs the existing “locally ordered structure” in β-iPP melt, as a result greatly inhibits the βα-recrystallization of β-iPP samples during heating.