Spontaneous crystallization process of the planar zigzag form at 0 degreesC from the melt for syndiotactic polypropylene

The spontaneous crystallization process of the planar zigzag form (form III ) of syndiotactic polypropylene has been investigated at 0 degrees C for sa mples quenched from the melt by real-time wide-angle X-ray diffractometry a nd high-resolution solid-state C-13 NMR spectroscopy. The X-ray diffraction profile just after quenched from the melt is in good accord with that of t he melt, indicating that the sample is in the noncrystalline state. With in creasing crystallization time, broad diffraction peaks assignable to form I II are found to evidently appear for the diffraction profile obtained after the subtraction of the noncrystalline contribution. The spontaneous crysta llization of form III is also confirmed by high-resolution solid-state C-13 NMR spectroscopy; for example, the CH3(I) line assigned to the tt conforma tion really increased in intensity with increasing crystallization time in concomitancy with the decrease of the CH3(II) line ascribed to the tg confo rmation. The line shape analysis of these CH3 lines reveals that the degree of crystallinity rapidly increases up to about 0.10 in 5 h, but this value is still as low as 0.13 even after 4500 h. The additional crystallization of form I with the ttgg conformation is also found to be highly hindered fo r the sample kept at 0 degrees C for a longer period when it is annealed at room temperature, although this form is readily produced for the sample he ld at 0 degrees C for a shorter period. On the basis of these results, a st ructural model for the crystallization of form III at 0 degrees C is propos ed by considering the possible formation of aggregates composed of the segm ental chains with the trans-rich conformations in the noncrystalline region .