Investigation of the crystallization process of syndiotactic polypropylene quenched at 0°C from the melt or concentrated solutions by solid-state 13C NMR spectroscopy

Abstract

The crystallization process of syndiotactic polypropylene (sPP) quenched from the melt or concentrated solutions has been investigated by high-resolution solid-state 13C NMR spectroscopy in order to make clear the formation of the planar zigzag form at 0°C. The sPP film just after quenching at 0°C from the melt is in the noncrystalline state, but when the film is left at room temperature, crystals with sPP in the t 2g 2 conformation are quickly produced. The 13C NMR spectral shape of the CH 2 resonance line is similar to that of the sPP gels previously obtained. For the gels quenched at 0°C from concentrated solutions, T 1C and T 2C measurements reveal that segmental mobility remarkably decreases with increasing polymer concentration in the noncrystalline phase, whereas it stays unchanged in the crystalline phase as a result of lack of penetration of solvent molecules. Increase of viscosity in the noncrystalline phase results in the decrease of molecular mobility. In particular, molecular mobility is extremely restricted in the noncrystalline phase for the solvent-free sample, compared with the case of the gels. A line shape analysis of the CH 3 resonance line indicates that the trans fraction of the noncrystalline component is significantly increased above about 70 wt% concentration. These results lead to the conclusion that the molecular mobility in solutions below about 70 wt% is fast enough to take the almost random chain conformation as expected, but the sPP chains in solutions above about 70 wt% including in the bulk state tend to take trans-rich conformations, probably due to some kind of intermolecular interaction. As a result, crystallization from solutions with appropriate concentrations produces crystals with the most stable t 2g 2 sequences, whereas form III with the planar zigzag conformation is induced around 0°C in the solvent-free bulk state.