Effect of Postdrawing Temperature on Structure, Morphology, and Mechanical Properties of Melt-Spun Isotactic Polypropylene Tapes

Structure, morphology, and mechanical properties of melt-spun and postdrawn isotactic polypropylene (iPP) tapes are analyzed to study the effect of postdraw temperature applied. For affine drawing conditions, i.e., no effective relaxation of the molecules occurs during postdrawing, the Young's modulus is uniquely determined by the applied draw ratio. Only above a certain postdrawing temperature, the tensile modulus starts to decrease, indicating the onset of a nonaffine deformation, i.e., the occurrence of effective molecular relaxation during postdrawing. However, the overall mechanical behavior strongly depends on the postdrawing temperature used. For fixed draw ratios, a significant drop in yield stress can be observed with decreasing drawing temperature. Structural and morphological characterization indicates that, beside affine and nonaffine drawing regimes, mainly formation of the mesomorphic phase during solid-state drawing yields to the different mechanical behavior.