Influences of the chemical defects on the crystal thickness and their melting of isothermally crystallized isotactic polypropylene

Abstract

Ziegler-Natta and Metallocene Catalysis isotactic polypropylene with different chemical defects were isothermally crystallized at various crystallization temperatures. The crystal thickness and their corresponding melting behavior were studied using small angle X-ray scattering, atomic force microscopy, optical microscopy, and differential scanning calorimetry. The equilibrium melt temperature of the samples was calculated from the Hofmann-Weeks extrapolation for the supercooling. Two lamellar populations were distinctly observed in all cases during the crystallization process. Relatively thicker and stable lamellar crystals which melt at higher temperatures were observed with lowering the supercooling and found catalysis dependence in these crystals. During melting, no significant recrystallization of the samples has been detected for higher crystallization temperature where recrystallization processes enhance the lamellae thickness. The melting of the crystals has found strong dependence with the crystallization temperatures, the catalysis process and the nature of the defects present in the isotactic polypropylene. The increase of the crystal lamellae thickness and their melting temperature might be presumably related with the chain folding mechanism as well as the stability of the crystals formed during the isothermal crystallization process. A combined plot of SAXS and DSC results is demonstrated for the equilibrium melting temperature followed by critical analysis of the results.