The effect of branching on chain diffusion in ultralong N-alkane crystals observed by 13C NMR

¹³C NMR progressive saturation measurements are used to investigate solid state chain diffusion in semicrystalline alkanes. Monodisperse, ultralong n-alkanes, of 198 and 191 carbon atoms in length, are characterized and prepared in such a way that they comprise crystals containing chains which are exclusively of once-folded conformation. This preparation is confirmed with DSC and SAXS. The progressive saturation experiments show that the longitudinal relaxation of magnetisation is consistent with a solid state chain diffusion process. Reptation and one-dimensional diffusion models are demonstrated to satisfactorily represent the data. The addition of branches to the alkane chains is shown to result in a decrease in the diffusion coefficient. The obtained diffusion coefficients range from 0.0918 nm²s⁻¹ for the linear chain to 0.016 nm²_s⁻¹ for a chain with a branch 4 carbons in length. These diffusion coefficients are consistent with those previously obtained for polyethylenes.