Computer simulation of ordering and dynamics in liquid crystals in the bulk and close to the surface
Creators: name: Pizzirusso, Antonio <1980>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2009
Subjects: CHIM/02 Chimica fisica

description: The aim of this PhD thesis is to investigate the orientational and dynamical properties of liquid crystalline
systems, at molecular level and using atomistic computer simulations, to reach a better understanding of
material behavior from a microscopic point view. In perspective this should allow to clarify the relation
between the micro and macroscopic properties with the objective of predicting or confirming experimental
results on these systems.
In this context, we developed four different lines of work in the thesis.
The first one concerns the orientational order and alignment mechanism of rigid solutes of small dimensions dissolved in a nematic phase formed by the 4-pentyl,4 cyanobiphenyl (5CB) nematic liquid crystal.
The orientational distribution of solutes have been obtained with Molecular Dynamics Simulation (MD) and
have been compared with experimental data reported in literature. we have also verified the agreement
between order parameters and dipolar coupling values measured in NMR experiments. The MD determined
effective orientational potentials have been compared with the predictions of Maier­Saupe and Surface tensor
models.
The second line concerns the development of a correct parametrization able to reproduce the phase
transition properties of a prototype of the oligothiophene semiconductor family: sexithiophene (T6).
T6 forms two crystalline polymorphs largely studied, and possesses liquid crystalline phases still not well
characterized, From simulations we detected a phase transition from crystal to liquid crystal at about 580 K,
in agreement with available experiments, and in particular we found two LC phases, smectic and nematic.
The crystal­smectic transition is associated to a relevant density variation and to strong conformational
changes of T6, namely the molecules in the liquid crystal phase easily assume a bent shape, deviating from
the planar structure typical of the crystal.
The third line explores a new approach for calculating the viscosity in a nematic through a virtual exper-
iment resembling the classical falling sphere experiment. The falling sphere is replaced by an hydrogenated
silicon nanoparticle of spherical shape suspended in 5CB, and gravity effects are replaced by a constant force
applied to the nanoparticle in a selected direction. Once the nanoparticle reaches a constant velocity, the
viscosity of the medium can be evaluated using Stokes' law. With this method we successfully reproduced
experimental viscosities and viscosity anisotropy for the solvent 5CB.
The last line deals with the study of order induction on nematic molecules by an hydrogenated silicon
surface. Gaining predicting power for the anchoring behavior of liquid crystals at surfaces will be a very
desirable capability, as many properties related to devices depend on molecular organization close to surfaces.
Here we studied, by means of atomistic MD simulations, the flat interface between an hydrogenated (001)
silicon surface in contact with a sample of 5CB molecules. We found a planar anchoring of the first layers of
5CB where surface interactions are dominating with respect to the mesogen intermolecular interactions. We
also analyzed the interface 5CB­vacuum, finding a homeotropic orientation of the nematic at this interface.