MORPHOLOGY AND THERMAL-PROPERTIES OF FULLY SYNDIOTACTIC POLYPROPYLENE

The morphology of syndiotactic polypropylene (sPP) containing >99% racemic diads has been investigated following controlled crystallization from the melt in thin films. At high growth temperatures, large rectangular single crystals are obtained with preferred growth along the crystallographic b-axis, while at lower temperatures these branch out to yield dendritic, axialitic, and eventually spherulitic aggregates. These crystals exhibit two very unusual properties: transverse fractures and ripples. Both are quasi-periodic, the fractures with a spacing of 3-7 mu m and the ripples generally 1 mu m or less. The fractures are ubiquitous in samples crystallized both at low and high undercooling and are expected to have severe negative consequences on the mechanical properties and uses of syndiotactic polypropylene. We associate formation of these transverse cracks with an approximate order-of-magnitude greater thermal expansion (and contraction) along the crystallographic b-axis vs the a-axis, which we have documented using variable-temperature X-ray diffractometry. This technique also demonstrated that samples quickly cooled to ambient undergo a heating-induced crystallographic transformation to the fully antichiral packing (cell III) that begins at ca. 40 degrees C below the melting point. Using surface decoration with evaporated polyethylene, we found that the (200) sectors have much more uniform amorphous-surface structure than the (020) sectors and that their folding is oriented along the preferred crystal-growth direction (b-axis).