Crystallization and orientation development in fiber and film processing of polypropylenes of varying stereoregular form and tacticity

We investigated the crystallization and orientation development in melt spinning and tubular blown film extrusion of several different types of polypropylenes, including conventional high tacticity isotactic polypropylenes (iPP) and metallocene catalyst low tacticity iPPs and syndiotactic polypropylenes (sPP). The fiber and film samples were characterized by wide-angle X-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). In melt spinning iPP, we found that the mesomorphic structure of iPP is more readily formed in lower tacticity fibers, and significant amounts of hexagonal β-form crystals are found in low tacticity iPP fibers spun at high draw-down ratios. Low tacticity iPP fibers exhibited a significant decrease in the crystalline chain-axis orientation at high draw-down ratios, resulting from increased epitaxially branched lamellae. Melt-spun sPP fibers exhibit Form I helical structure at low spinning speeds and Form III zigzag all trans structure at high spinning speeds. We found that the level of spinline stress is the governing factor for this structural change. Melt-spun sPP fibers exhibit much higher chainaxis (c-axis) orientation factors ([f.sub.c]) and lower birefringence than iPP fibers spun at the same spinline stresses. In tubular blown sPP films, the a-axis of Form I unit cell tends to orient perpendicular to the film surface, while the b-axis of monoclinic α unit cell does so in iPP blown films.
INTRODUCTION Stereoregular crystalline polypropylene was first synthesized in 1955 by Natta and his co-workers (1, 2). It was the isotactic form (iPP), which has asymmetric carbon atoms with all the [...]