Segmental dynamics in a blend of alkanes: Nuclear magnetic resonance experiments and molecular dynamics simulation

The segmental dynamics of a model miscible blend, C24H50 and C6D14, were investigated as a function of temperature and composition. The segmental dynamics of the C24H50 component were measured with C-13 nuclear magnetic resonance T-1 and nuclear Overhauser effect measurements, while H-2 T-1 measurements were utilized for the C6D14 component. Use of low molecular weight alkanes provides a monodisperse system in both components and allows differentiation of dynamics near the chain ends. From these measurements, correlation times can be calculated for the C-H and C-D bond reorientation as a function of component, position along the chain backbone, temperature, and composition. At 337 K, the segmental dynamics of both molecules change by a factor of 2 to 4 across the composition range, with the central C-H vectors of tetracosane showing a stronger composition dependence than other C-H or C-D vectors. Molecular simulations in the canonical and isobaric-isothermal ensembles were conducted with a united-atom force field that is known to reproduce the thermodynamic properties of pure alkanes and their mixtures with good accuracy. With a minor change to the torsion parameters, the correlation times for pure tetracosane are in good agreement with experiment. For pure hexane and its mixtures with tetracosane, the simulated dynamics are faster than experiment. (C) 2002 American Institute of Physics.