Real time synchrotron SAXS and WAXS investigations on temperature related deformation and transitions of β-iPP with uniaxial stretching

In-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) were carried out to investigate the uniaxial drawing-induced deformation and structure transitions of β form isotactic polypropylene (iPP) at varying temperatures (30 °C, 60 °C, 80 °C, 100 °C and 120 °C). The WAXS results indicated that the initial strain for the strain-induced β–α transformation decreases with the tensile temperature according to the engineering stress–strain curves. The SAXS data showed that the long period increased along the direction perpendicular to the tensile force and changed little along the tensile direction with increasing strain in the elastic deformation stage before the yield point. The analysis of the obtained scattering results indicated that the angle between parent and daughter lamella rotates from initial 40° or 140° to close to 90° accounts for the lateral expansion of the samples with tension, which matches the essential auxetical behavior. A structure deformation and transition mechanism was proposed for β form iPP with uniaxial drawing. The initialization of the crystalline structure transition is after the yield point, then the mechanical loading-induced β–α transition seems to be a gradual process with lamella slippage and breaking which triggers the β–α polymorphic transition.