Fractionation in mixtures of polyethylene oxide fractions during crystallization

Abstract

AbstractTwo narrow‐molecular‐weight‐fraction polyethylene oxides were mixed and studied by differential scanning calorimetry (DSC), small‐angle x‐ray scattering (SAXR), microscopy, and small‐angle light scattering (SALS). DSC measurements indicate the presence of two melting points at each composition, leading to a eutectic diagram with a eutectic point located close to the axis of the constituent with the lower molecular weight. The depression in melting point of the higher‐molecular‐weight component allows the calculation of a thermodynamic interaction parameter of −0.09 between the two fractions, indicating that they are miscible in the melt despite the fractionation process occurring during crystallization. Two SAXR long periods are also observed at each composition, indicating phase separation during crystallization. These two phases are included into large spherulites, the structure of which depends mainly upon the crystallization characteristics of the higher‐molecular‐weight component. The other component must then crystallize between the spherulite lamellae. Finally, small spherulites of the order of the micron are observed from the SALS patterns inside the large spherulites (of the order of one millimeter as seen by microscopy). The two polyethylene oxide fractions give rise to different size SALS spherulites and both types of spherulites are present in the mixtures.