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

摘要

The crystallization process of syndiotactic polypropylene (sPP) quenched from the melt or concentrated solutions has been investigated by high-resolution solid-state ~(13)C NMR spectroscopy in order to make clear the fonnation of the planar zigzag fonn at 0degC. The sPP film just after quenching at DoC 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 ~(13)C NMR spectral shape of the CH_2 resonance line is similar to that of the sPP gels previously obtained. For the gels quenched at 0degC 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 foithe 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 70wt% 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 confonnation as expected, but the sPP chains in solutions above about 70wt% including in the bulk state tend to take trans-ricb 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 confonnation is induced around 0degC in the solvent-free bulk state.