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 chain‐axis (c‐axis) orientation factors (fc) 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.

A comparative study of the crystallization and orientation development in injection molding isotactic and syndiotactic polypropylenes was made. The injection molded samples were characterized using wide angle X‐ray diffraction (WAXD) techniques and birefringence. The injection molded isotactic polypropylene samples formed well‐defined sublayers (skin, shear and core zones) and exhibited polymorphic crystal structures of the monoclinic α‐form and the hexagonal β‐form. Considerable amounts of β‐form crystal were formed in the shear and core zones, depending on the injection pressure or on the packing pressure. The isotactic polypropylene samples had relatively high frozen‐in orientations in the skin layer and the shear zone. The injection molded syndiotactic polypropylene exhibited the disordered Form I structure, but it did not appear to crystallize during the mold‐filling stage because of its slow crystallization rate and to develop a distinct shear zone. The core zone orientation was greatly increased by application of high packing pressure. The isotactic polypropylene samples exhibited much higher birefringence than the syndiotactic polypropylene samples at the skin and shear layers, whereas both materials exhibited similar levels of crystalline orientation in these layers.

A basic investigation of the development of crystallization and orientation during the melt spinning of syndiotactic and isotactic polypropylene is described. Both polymers were melt spun through ambient air or into ice water at various draw-down ratios and melt temperatures. The melt spun ®laments were characterized by wide angle x-ray diffraction (WAXD), birefringence and differential scanning calorimetry (DSC). The polymorphic behavior of isotactic polypropylene during melt spinning is well known. The ®laments melt spun through ambient air exhibit the monoclinic a-form while the ®laments melt spun into ice water form a smectic structure at low draw-down ratios. Syndiotactic polypropylene also exhibited polymorphic behavior during melt spinning. Disordered Form I with (t 2 g 2 ) 2 helices was found at low spinline stresses and Form III with all trans chain conformation was found at high spinline stresses. The criterion of the spinline stress for this transition was found to be about 6±9 MPa for all samples of various molecular weights, melt temperatures and cooling rates. Syndiotactic polypropylene crystallized much more slowly than isotactic polypropylene and did not exhibit a smectic mesomorphism under ice-water quenched conditions. The melt spun ®laments of syndiotactic polypropylene exhibited lower birefringences and amorphous orientations but higher crystalline orientations than those of isotactic polypropylene, compared at the same spinline stresses.

The differences in behavior of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP) in tubular film extrusion are qualitatively described. The crystalline form and orientation in the films were characterized using wide-angle X-ray diffraction (WAXD) patterns, pole-figure analysis and birefringence. The sPP films had the crystalline form of the disordered Form I and the a-crystallographic axis was found to be preferentially oriented in the film normal direction (ND) under the conditions of biaxial stresses. High transverse orientations were developed in the sPP films. In the iPP films, the monoclinic crystalline form was found and the bcrystallographic axis was preferentially oriented in the ND. The birefringence of the fims showed trends very similar to the crystalline orientations characterized by WAXD in both iPP and sPP films.

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.