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

•Structural evolution of initially isotropic β - iPP during uniaxial stretching is investigated by in-situ WAXS and SAXS.•An interesting orientation of β - crystal with molecular chains perpendicular to the tensile direction is identified.•A deformation mechanism of β - iPP based on correlation among crystal transition, orientation and cavitation is proposed.

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

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