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Subdiffusive dynamics of a liquid crystal in the isotropic phase
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 r...
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Published in: | The Journal of chemical physics 2008-05, Vol.128 (19), p.194501-194501-11 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | 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
β
-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
T
c
are
244
±
6
and
258
±
6
K
, respectively, the latter is some
10
K
below the corresponding experimental values. The different values of
T
c
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
. |
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ISSN: | 0021-9606 1089-7690 |
DOI: | 10.1063/1.2916681 |