Variations of regular conformation structures in melt of syndiotactic polypropylene

Regular structures in polymer melt play important roles during crystallization and subsequently influence performances of polymer materials. In the present work, differential scanning calorimetry (DSC) and variable temperature Fourier transform infrared spectroscopy (VT-FTIR) were used to characterize the variations of regular structures in melt of syndiotactic polypropylene (sPP) during heating. It was found that during heating, the structural transition in sPP melt undergoes four stages: (1) destruction of long regular structures; (2) formation of short regular structures; (3) transition from short regular structures to isotropic state; and (4) isotropic state. These regular structures in sPP melt have distinct memory effects on the crystallization behaviors. By using Flory's RIS model, it was found that in final isotropic state, the most common conformers are ttgg and tttt, corresponding to helical and planar-zigzag conformations, respectively. These findings are helpful to deeply understand the essence of structures in sPP melt and microstructure variations during melting.