Chain Diffusion in Ultralong n-Alkane Crystals Studied by 13C NMR

Abstract

13C NMR progressive saturation measurements were used to investigate longitudinal relaxation in the crystal phase of semicrystalline alkanes. Novel monodisperse, ultralong n-alkanes were melt crystallized in such a way that they comprised crystals containing extended chains or once-folded chains. The crystalline morphology was confirmed using DSC, SAXS, and WAXS. Quantifying the longitudinal relaxation is critically dependent on an accurate measure of the crystallinity of the sample. Crystallinities were measured using X-ray scattering, calorimetric, and 1H broadline NMR techniques. The longitudinal relaxation is interpreted via a solid-state chain diffusion process. Very high crystallinity was shown to suppress the diffusion. A one-dimensional diffusion model with a single reptation time was shown to represent successfully the relaxation of crystals comprising extended chains. The addition of chain folds was shown to result in a decrease in the diffusion coefficient, although the addition of branches at ...