Crystallization, melting, and morphology of syndiotactic polypropylene fractions .3. Lamellar single crystals and chain folding

Highly faceted, regular, lathlike lamellar single crystals of syndiotactic polypropylene (s-PP) fractions have been investigated through transmission electron microscopy (TEM), atomic force microscopy (AFM), and electron diffraction (ED). Single crystals of s-PP over 1 mu m in size can be grown from the melt in thin films. ED results obtained from the s-PP single crystals indicate a unit cell III with a = 1.450 nm, b = 1.120 nm, and c = 0.740 nm as proposed by Lovinger and Lotz. At high crystallization temperatures, relatively low molecular weight s-PP fractions can grow lamellar single crystals with microsectors. The polyethylene decoration method has been used to identify the chain folding direction, and no preferred orientation has been observed on the nonsectorized lamellar crystals. Sectorized lamellar single crystals show two different regions. In the sectors along the long axis (the b-axis), the chain folding is found to be parallel to the 010 direction. In the sectors along the short axis (the a-axis), Little preferred orientation can be found. The deformation method of nonsectorized, high molecular weight s-PP single crystals on a plastic film has also been utilized to determine the chain folding direction. Microfibrillar structures can be observed in the cracks of the single crystals along both the a- and b-axes after deformation. This indicates that the folding direction in these nonsectorized, high molecular weight s-PP single crystals may be either along the (110) planes or a combination of the (100) and (010) microfolding and microsectoring. Zigzag-shaped edges on the deformed single crystals along the a-axis are also observed, and the sliding planes can be identified as the (110) planes.