The effects of stretching on the mechanical properties of neat and nucleated isotactic polypropylene and its enhancement mechanism

Abstract

Both stretching and adding nucleating agent can improve the mechanical properties of isotactic polypropylene (iPP), but their combined effects and mechanisms remain unclear. In this paper, α-PP and β-PP were induced by different nucleating agents and the effects of stretching on their mechanical properties and their mechanisms were studied. According to the results, stiffness and toughness of iPP can be enhanced simultaneously through stretching. After stretching, the flexural modulus, tensile strength, and impact strength of α-PP were increased by 145%, 379% and 816% while those of β-PP were increased by 208%, 486% and 1316% individually at 23 °C. Typically, the mechanical properties of iPP will significantly decrease at low temperature. The impact strengths of α-PP and β-PP were reduced by 51.0% and 75.2% at -20 °C respectively. However, after stretching, their toughness hardly decreased. Then the enhancement mechanism was investigated by differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA), Raman laser spectrometer (RLS), scanning electron microscope (SEM) and micro computed tomography (micro-CT). The results indicated that the cavities were generated during stretching, which can absorb energy when fractured and improve the toughness of iPP. The increases in crystallinity, the interphase iPP content and orientation result in the improved stiffness. The amorphous phase almost disappeared, which contributes to the improvement of mechanical properties of iPP at low temperature. Additionally, stretched β-PP exhibited a higher orientation and more obvious cavitation phenomenon than stretched α-PP, leading to its superior mechanical performance. This paper not only provides a new method to improve the stiffness and toughness of iPP, especially the toughness at low temperature, but also elucidates the enhancement mechanisms.