Crystallization and Orientation Development in Melt Spinning Isotactic PP of Varying Tacticity

摘要

We investigated the effects of tacticity on crystallization and orientation development in melt spinning isotctic polypropylene (iPP) through air. Melt-spun fibes were characterized by wide-angle X-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). The melt-spun fibers usually exhibited the monoclinic alpha-crystalline form, but the mesomorphic form was found under certain processing conditions. The mesomorphic structure was formed more readily in low tacticity fibers. We found certain amounts of hexagonal beta-crsytalline form in all of the alpha-form fiber samples. The amounts were generally negligibly small, but the low tacticity fibers spun at high draw-down ratios exhibited significant amounts of beta-form crystals. At low spinline stress levels, low tacticity fibers exhibited much higher birefringence and chain-axis orientation levels than expected. This was attributed to a reduction in epitaxial lamellar branching (a-axis oriented group in bimodal orientation). However, the extent of lamellar branching in low tacticity fibers appeared to increase again at higher stress levels. This led to a significant decrease in birefringence and crystalline orientation.