Segmental dynamics in a blend of alkanes: Nuclear magnetic resonance experiments and molecular dynamics simulation
J. Chem. Phys. 116, 8209 (2002); doi:10.1063/1.1464538
Issue Date: 8 May 2002
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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 13C nuclear magnetic resonance T1 and nuclear Overhauser effect measurements, while 2H T1 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 CH and CD 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 CH vectors of tetracosane showing a stronger composition dependence than other CH or CD vectors. Molecular simulations in the canonical and isobaricisothermal 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. ©2002 American Institute of Physics.
| History: | Received 7 December 2001; accepted 4 February 2002 |
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