Effect of annealing on microstructure and tensile properties of polypropylene cast film

In this work, the effect of annealing on microstructure and tensile properties of polypropylene (PP) cast film was investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), SEM, etc. The SAXS data showed that the elevating annealing temperature led to the increase of thickness of lamellae phase and amorphous phase. The orientation of lamellae along MD direction after annealing was proved with the SAXS data. Moreover, these annealed samples had an extra low-temperature melting peak in the DSC curves and it moved to a higher temperature obviously with increasing the annealing temperature. More importantly, the relationship between the yield strength and the lamellar thickness was built through testing the tensile properties and calculating the Thomson-Gibbs equation, respectively. It revealed that the yield strength first increased rapidly and grew slowly when the lamellar thickness increased to a certain extent. A detailed physical model was also proposed to depict the change of microstructure during annealing.

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