In situ FT-IR spectroscopic study on the conformational changes of isotactic polypropylene in the presence of supercritical CO2

The conformational changes of isotactic polypropylene (iPP) under supercritical CO2 condition with different pressure and temperature have been carefully studied by in situ Fourier-transform infrared spectroscopy (FT-IR). Analysis of the corresponding spectra shows that the conformational ordering by supercritical CO2 results in the intensity enhancement of the regularity bands of iPP. Due to the high CO2 concentration and strong intermolecular interaction, iPP can reach an equilibrium state in a short time at high CO2 pressure. The equilibrium time increases with soaking temperature. After supercritical CO2 treatment, two mechanisms, the formation of short helix from amorphous phase and the extension of short helix into long one, happen simultaneously. The latter mechanism undergoes quickly at the beginning of induced conformational changes and then slows down, resulting in the slight increase of crystallinity. At the same time, the conformational ordering in amorphous phase happens continuously until a thermodynamic equilibrium. In summary, in the presence of supercritical CO2, the conformational ordering of iPP chains occurs exclusively in the amorphous region, with no impact on the crystal part.