Investigation of the Crystallization Process of Syndiotactic Polypropylene Quenched at 0^oC from the Melt or Concentrated Solutions by Solid-State ¹³C NMR Spectroscopy

 (Ryukoku Umiv.) Takahiko Nakaoki,^* (Kyoto Univ.) Yasumasa Ohira, Fumitaka Horii

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^oC.  The sPP film just after quenched at 0 ^oC from the melt is in the noncrystalline state, but when the film is left at room temperature, crystals with sPP in the t₂g₂ conformation are quickly produced.  The ¹³C NMR spectral shape of the CH₂ resonance line is similar to that of the sPP gels previously obtained.  For the gels quenched at 0^oC from the 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 no penetration of solvent molecules.  The increment of viscosity in the noncrystalline phase results in the decrease of molecular mobility.  In particular, the 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 CH₃ 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, the crystallization from solutions with appropriate concentrations produces crystals with the most stable t₂g₂ sequences, whereas form III with the planar zigzag conformation is induced around 0 ^oC in the solvent-free bulk state.