Temperature dependent crystallization of iPP: a combined SIST-AFM study
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The crystallization process of semi-crystalline polymers can be divided in three stages: nucleation, growth and perfection of the crystal structure. At homogeneous and/or heterogeneous nuclei the polymer chains begin to organize and are folding in parallel segments, so called lamellae. The growth occurs radially and results finally in spherulites. This structure formation especially the thickness and perfection of lamellae is influenced strongly by temperature. Stepwise isothermal segregation technique (SIST) is a suitable method to determine the perfection of the lamellae - forming und thickening - during various temperature stages of semi-crystalline polymers like isotactic polypropylene. The method requires several isothermal crystallization steps. Isotactic polypropylene samples with different molecular weight distributions were examined with five isothermal crystallization steps in a range from 145°C to 105°C. The obtained melting endotherms were converted in lamellae thickness distributions using the Gibbs-Thomson equation. For validation the lamellar thicknesses were determined by scanning force microscopy. The comparison between samples with different molecular weight distribution (MWD) and crystallization temperatures show that samples with broader MWD form thinner lamellae than in the samples with smaller MWD.