Structural evolution of β - iPP during uniaxial stretching studied by in-situ WAXS and SAXS

Polymorphism and crystal transition are of great significance for property mediation in polymer materials. Isotactic polypropylene (iPP) with beta - crystal has been widely utilized for the preparation of high performance plastics or films. In the present work, the structural evolution of initially isotropic beta - nucleated iPP (beta - iPP) during uniaxial stretching at different temperatures was investigated by in situ X - ray scattering using synchrotron radiation. The wide - angle X - ray scattering (WAXS) results confirmed that the beta - crystal transformed either to the mesophase at lower temperature (30 degrees C) or to the alpha - crystal at higher temperature (60,100 and 120 degrees C) during stretching. An interesting orientation of beta - crystal with molecular chains perpendicular to the tensile direction was identified. As revealed by small - angle X - ray scattering (SAXS), cavitation took place in beta - iPP stretched at temperatures lower than 120 degrees C. The size and shape of the cavities were observed by scanning electron microscope. A deformation mechanism of beta - iPP combining the crystal transition, cavitation and orientation was proposed. (C) 2014 Elsevier Ltd. All rights reserved.