In situ FTIR spectroscopic study of the conformational change of syndiotactic polypropylene during the isothermal crystallization

The isothermal crystallization of syndiotactic polypropylene (sPP) was investigated by in situ Fourier transform infrared spectroscopy (FTIR) and wide angle X-ray diffraction (WAXD). It was found that the ordered helical structure developed during the induction period of the isothermally crystallization of sPP. Moreover, the normalized intensity profiles of the characteristic FTIR absorption bands of the sPP helical conformation at 1005, 867 and 812 cm−1 are not synchronized during the induction period of the sPP crystallization, which suggest that these bands should be corresponding to the helical chain with various persistence lengths. The non-zero value of the initial normalized intensity for the band at 867 cm−1 indicates that there are sPP chains in the short helical conformation in the initial amorphous state. However, the helical chains with longer persistence length can only be observed with increasing the annealing time in the induction period as suggested by the intensity changes of the bands at 1005 and 812 cm−1. Particularly, the intensity of the characteristic absorbance band at 1005 cm−1 starts to increase at an earlier time than that of 812 cm−1. These observations are discussed in terms of the critical length by Doi theory. It can be estimated that the sPP melt system is stable when the persistence length of helical sequences is less than 16 monomer units. The results could be helpful on the understanding the pathway of polymer chains packing in the early stage of the crystallization of semi crystalline polymers.