Effect of annealing on phase structure and mechanical behaviors of polypropylene hard elastic films
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The effect of annealing temperature on phase structure of polypropylene hard elastic (HEPP) films was investigated. The morphology and orientation of crystalline phase were examined by scanning electron microscope (SEM) and Fourier transform infrared (FT-IR). It was found that the content of defects in crystals was decreased and the orientation of crystalline phase was increased after annealing. A shoulder endothermic peak was observed below the melting peak in DSC melting curve of annealed HEPP film, and the peak of this shoulder peak increased to a higher temperature with the increase of annealing temperature. The integral area of this shoulder peak decreased when tensile strain increased from 0 to 2.0, and it almost disappeared when the film stretched to the strain of 2.0. The shoulder endothermic peak in DSC melting curve could be explained by the formation of microcrystals in the amorphous between primary lamellae during annealing process. The microcrystals acted as inter-lamellar entanglements during tensile process, which improved the strength of amorphous phase and elastic recovery of HEPP films.