Morphological evolution and orientation development of stretched iPP Films: Influence of draw ratio

Varying the processing conditions of semicrystalline polymers can produce different morphologies of crystallization, which leads to different properties. There have been extensive studies of flow-induced crystallization on isotactic polypropylene (iPP) using predominantly shear flow. A stretching method, deduced from extrusion, was introduced to study the morphological evolution of elongation-induced shish-kebab crystallization. Different morphologies of the resultant samples with different draw ratios (DRs) were carefully investigated and characterized via differential scanning calorimetry, polarizing light microscopy, scanning electron microscopy, atomic force microscopy, and 2D small-angle X-ray scattering. In addition, the degree of orientation of the samples with different DRs was also investigated using the 2D wide-angle X-ray scattering technique. The results indicate that the elongation-induced morphology is strongly dependent on the effective stretching flow expressed in terms of the DR, which is defined as the ratio of rates between take-up and the extrusion. The spherulite is dominant at low DRs, but it starts to deform along the stretching direction with increasing DR. The shishkebab structure in the stretched film, composed of stretched chains (shish) and layered crystalline lamellae (kebabs), increases gradually with an increase in the DR, whereas the spherulites gradually decreased. Furthermore, the overall orientation of α-phase crystals, expressed by the Hermans orientation parameter, is also found to increase dramatically with the DR, and the rate of increase strongly depends on the DR. The different crystal morphologies are attributed to crystallization induced by different elongations of the stretched iPP films.