Sistemi liquido cristallini complessi: simulazioni al calcolatore e studi ESR
Creators: name: Miglioli, Isabella <1982>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2010
Subjects: CHIM/02 Chimica fisica

description: The aim of this PhD thesis was to study at a microscopic level different liquid crystal (LC) systems, in
order to determine their physical properties, resorting to two distinct methodologies, one involving computer
simulations, and the other spectroscopic techniques, in particular electron spin resonance (ESR) spectroscopy.
    By means of the computer simulation approach we tried to demonstrate this tool effectiveness for calculating anisotropic static properties of a LC material, as well as for predicting its behaviour and features. This
required the development and adoption of suitable molecular models based on a convenient intermolecular
potentials reflecting the essential molecular features of the investigated system.
    In particular, concerning the simulation approach, we have set up models for discotic liquid crystal dimers and
we have studied, by means of Monte Carlo simulations, their phase behaviour and self­-assembling properties,
with respect to the simple monomer case. Each discotic dimer is described by two oblate Gay­Berne ellipsoids
connected by a flexible spacer, modelled by a harmonic "spring" of three different lengths. In particular we
investigated the effects of dimerization on the transition temperatures, as well as on the characteristics of
molecular aggregation displayed and the relative orientational order.
    Moving to the experimental results, among the many experimental techniques that are typically employed
to evaluate LC system distinctive features, ESR has proved to be a powerful tool in microscopic scale
investigation of the properties, structure, order and dynamics of these materials. We have taken advantage
of the high sensitivity of the ESR spin probe technique to investigate increasingly complex LC systems
ranging from devices constituted by a polymer matrix in which LC molecules are confined in shape of nano-
droplets, as well as biaxial liquid crystalline elastomers, and dimers whose monomeric units or lateral groups
are constituted by rod-like mesogens (11BCB).
    Reflection-mode holographic-polymer dispersed liquid crystals (H-PDLCs) are devices in which LCs are
confined into nanosized (50­-300 nm) droplets, arranged in layers which alternate with polymer layers, forming
a diffraction grating. We have determined the configuration of the LC local director and we have derived
a model of the nanodroplet organization inside the layers. Resorting also to additional information on the
nanodroplet size and shape distribution provided by SEM images of the H-PDLC cross-section, the observed
director configuration has been modeled as a bidimensional distribution of elongated nanodroplets whose
long axis is, on the average, parallel to the layers and whose internal director configuration is a uniaxial quasi-
monodomain aligned along the nanodroplet long axis. The results suggest that the molecular organization is
dictated mainly by the confinement, explaining, at least in part, the need for switching voltages significantly
higher and the observed faster turn-off times in H-PDLCs compared to standard PDLC devices.
    Liquid crystal elastomers consist in cross-linked polymers, in which mesogens represent the monomers
constituting the main chain or the laterally attached side groups. They bring together three important aspects: orientational order in amorphous soft materials, responsive molecular shape and quenched topological constraints.
In biaxial nematic liquid crystalline elastomers (BLCEs), two orthogonal directions, rather than the one of
normal uniaxial nematic, can be controlled, greatly enhancing their potential value for applications as novel
actuators. Two versions of a side-chain BLCEs were characterized: side­-on and end­-on. Many tests have
been carried out on both types of LCE, the main features detected being the lack of a significant dynamical
behaviour, together with a strong permanent alignment along the principal director, and the confirmation of
the transition temperatures already determined by DSC measurements. The end­-on sample demonstrates a
less hindered rotation of the side group mesogenic units and a greater freedom of alignment to the magnetic
field, as already shown by previous NMR studies. Biaxial nematic ESR static spectra were also obtained
on the basis of Molecular Dynamics generated biaxial configurations, to be compared to the experimentally
determined ones, as a mean to establish a possible relation between biaxiality and the spectral features.
This provides a concrete example of the advantages of combining the computer simulation and spectroscopic
approaches.
    Finally, the dimer α,ω-bis(4'-cyanobiphenyl-4-yl)undecane (11BCB), synthesized in the "quest" for the
biaxial nematic phase has been analysed. Its importance lies in the dimer significance as building blocks
in the development of new materials to be employed in innovative technological applications, such as faster
switching displays, resorting to the easier aligning ability of the secondary director in biaxial phases. A
preliminary series of tests were performed revealing the population of mesogenic molecules as divided into
two groups: one of elongated straightened conformers sharing a common director, and one of bent molecules,
which display no order, being equally distributed in the three dimensions. Employing this model, the
calculated values show a consistent trend, confirming at the same time the transition temperatures indicated
by the DSC measurements, together with rotational diffusion tensor values that follow closely those of the
constituting monomer 5CB.