A possible explanation to the structure change of isotactic polypropylene occurring at about 135°C

The in situ Fourier transform infrared spectroscopic study of isotactic polypropylene (iPP) shows that a structure change occurs at about 135°C. Some experimental results suggest that this structure change may result from the disordering of helical sequences in the non-crystalline region, which can be further confirmed by the in situ infrared observation of chloride polypropylene (Cl-PP). If the iPP sample is drawn, the transition temperature is elevated. According to the change of infrared absorbance with temperature between 135 and 155°C, the various regularity bands of iPP can be rearranged in terms of the order degree from high to low: 940, 1220, 1167, 1303, 1330, 841, 998, 900, 808, 1100 and 973 cm−1. This sequence is identical with our recent observation based on the melting of iPP crystals.