Crystal structure and melting behavior of homo-polypropylene and heterophasic ethylene–propylene copolymer after long time heat treatment

The homo-polypropylenes and heterophasic ethylene–propylene copolymers (EP-HECO) were prepared by the non-isothermal and isothermal crystallization methods. The crystal structure and crystal phase were detected by the wide-angle X-ray diffraction (WAXD). The morphology of the spherulite growth process was observed under a polarized optical microscope (POM) with a hot stage. The melting behavior was performed by the differential scanning calorimetry (DSC). It was found that the restricted ethylene segment in the long propylene sequences could act as a heterogeneous nucleating agent of β phase only when the crystallization temperature was very high. PP with long molecular chain had more polymer sequences entering crystal lattice to form large lamellar thickness. Nucleation and spherulitic growth occurred at the same time. Both the final spherulite size and the crystallinity of iPP were related to the melt flow rate. When samples crystallized isothermally at a very high temperature, shorter molecular chain would crystallize into some small-size defect crystals, and this resulted in a multiple melting behavior in the DSC endotherm.

► Isothermal crystallization is used to study crystallization and melt behavior. ► Ethylene segment acted as a heterogeneous β-nucleating agent in EP-HECOs. ► Small-size defect crystal led to multiple melt behavior. ► The final spherulite size and the crystallinity were related to the melt flow rate.