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

Abstract

We investigated the effects of tacticity on crystallization and orientation development in melt spinning isotactic polypropylene (iPP) through air. Melt-spun fibers were characterized by wide-angle X-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). The melt-spun fibers usually exhibited the monoclinic -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 bcrystalline form in all of the -form fiber samples. The amouns were generally negligibly small, but the low tacticity fibers spun at high draw-down ratios exhibited significant amounts of b-form crystals. At low spinline stress levels, some low tacticity fibers exhibited lower birefringence and chain orientation than high tacticity fibers due to a less perfect crystal structure. At certain high 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.

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“…Both T g and T d of PTFS are lower than the materials reported in the literature (T g : 128 °C, T d : 408 °C). This difference may be attributed to the different tacticity of the polymers, which affects the crystallization and orientation of the polymers, [26][27][28] and further changes glass transition and thermal decomposition behavior. [29][30][31][32] In addition, the experimental test conditions may also affect the measured T g and T d .…”