Effect of compressed CO2 on the melting behavior and βα-recrystallization of β-form in isotactic polypropylene

This work is aimed at studying the effect of compressed CO2 on the melting behavior and βα-recrystallization of β-form in isotactic polypropylene (β-iPP). The melting behavior of β-iPP under compressed CO2 was investigated using high-pressure differential scanning calorimetry (DSC). The result showed that the melting temperatures of β-form and the recrystallized α-form in iPP (α-iPP) decreased linearly with increasing CO2 pressure at a nearly same slope of −1.2 °C/MPa, indicating the helix conformation but not the crystal structure in iPP played an important role in determining the plasticization effect of CO2 on iPP. In situ high-pressure Fourier transform infrared (FTIR) was also applied to detect the melting process of β-iPP under atmospheric N2 and compressed CO2. It was revealed that the melting process of β-iPP was comprised of three stages, i.e., the melting of β-iPP, βα-recrystallization and melting of recrystallized α-iPP. The βα-recrystallization was shortened with increasing CO2 pressure due to the gradually increased βα-recrystallization rate, and disappeared at the CO2 pressure of 6 MPa. The effect of the annealing process under atmospheric N2 and high pressure CO2 on the βα-recrystallization at temperatures ranging from 140 to 155 °C was also investigated using wide-angle X-ray diffraction (WAXD) and DSC. It was also shown that the high pressure CO2 significantly increased the βα-recrystallization, and β-iPP could recrystallize completely into α-iPP even at the annealing temperature of 140 °C under 12 MPa CO2.