Subdiffusive dynamics of a liquid crystal in the isotropic phase
| Autor: | Alessandro Tani, Giacomo Prampolini, Luca De Gaetani |
|---|---|
| Rok vydání: | 2008 |
| Předmět: |
Mean squared displacement
Molecular dynamics Correlation function (statistical mechanics) Condensed matter physics Chemistry Scattering Liquid crystal SUPERCOOLED LIQUIDS Isotropy Degrees of freedom (physics and chemistry) General Physics and Astronomy Physical and Theoretical Chemistry Plateau (mathematics) |
| Zdroj: | The Journal of chemical physics. 128(19) |
| ISSN: | 0021-9606 |
| Popis: | The isotropic phase dynamics of a system of 4-n-hexyl-4'-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275-330 K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the beta-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1 ns). The time-temperature superposition principle and the factorization property predicted by the idealized version of MCT hold, again for both kinds of dynamics. The temperature dependence of diffusion coefficient and orientational relaxation time is well described by a power law. Critical temperatures Tc are 244+/-6 and 258+/-6 K, respectively, the latter is some 10 K below the corresponding experimental values. The different values of Tc we obtained indicate that ODOF freezes earlier than TDOF. This appears due to the strongly anisotropic environment that surrounds a 6CB molecule, even in the isotropic phase. The lifetime of these "cages," estimated by time dependent conditional probability functions, is strongly temperature dependent, ranging from some hundreds of picoseconds at 320 K to a few nanoseconds at 275 K. |
| Databáze: | OpenAIRE |
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