Synthesis and Alignment of Nanostructured Materials Using Liquid Crystals - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 190

7.1 INTRODUCTION

Liquid crystalline materials, combining order (crystal) and mobility (liquid) at
the molecular-nanoscale level, appear as leading candidates for the synthesis,
self-assembly, and alignment of nanoscale materials with controlled size, shape,
dimensionality, and structure (Brad et al., 2005; Wang et al., 2009). In addition,
the dynamic functional liquid crystals (LCs) are known to respond to external
fi elds (such as electric and magnetic fi elds) and interact with surfaces, thereby
infl uencing their structure and properties (Hegmann et al., 2007). As a result
of the LC intrinsic orientational order together with the molecular motion,
they are recognized in bioscience (as model systems for cell membranes) and
have been widely used in displays, sensors, optical elements (controllable
lenses and lasers), and as drug delivery vehicles (Hegmann et al., 2007; Wang
et al., 2009).
Liquid crystals are commonly divided into two general classes: thermotropic
(TLCs) and lyotropic liquid crystals (LLCs) according to the composition and
formation conditions. In general, molecular shape, microsegregation of incom-
patible parts, specifi c molecular interactions, and self-assembly are important
factors that derive the formation of both thermotropic and lyotropic liquid
phases (Bisoyi and Kumar, 2011). TLCs are defi ned as ordered fl uid phases
(based on pure compound) and exhibit diverse phases that differ in their
degree of order as a function of temperature (Douglas et al., 2008; Hyde, 2001;
Hyde et al., 1997). The molecules that tend to form TLC structures have aniso-
tropic shapes (e.g., typically rod or disk shaped) and contain a relatively rigid
core and a number of fl exible alkyl tails (Fig. 7.1). In general, the rigid cores
of these molecules promote ordered packing, whereas the fl exible tails tend
to disorder the system until a compromise is met to produce an ordered fl uid
state (Hegmann et al., 2007). The mesophase formation in this kind of thermo-
tropic LC is driven by the segregation of chemically incompatible subunits
from one another, such as the segregation of rigid aromatic cores from fl exible
alkyl tails within a molecule (Fig. 7.1).
For example, in the nematic phase, the least ordered TLC phase, the aggre-
gates solely possess orientational ordering and no positional ordering. It is
generally formed when the volume fraction of an incompatible core unit
exceeds that of the incompatible wings and may result for both rod- and disk-
shaped molecules (Fig. 7.1). In this phase, the aggregates are aligned along a
direction that is termed the director n (Fig. 7.1). The director n is oriented
either parallel to the long molecular axis for rodlike aggregates (N u , the uni-
axial nematic phase), parallel to the column axis for columnar aggregates
formed by amphiphilic or disklike molecules (N Col , the columnar nematic
phase), or parallel to the short molecular axis for disklike aggregates of amphi-
philes (i.e., platelike micelles), molecules, or particles (N D , the discotic nematic
phase) (Goodby, 2002).
In contrast to the thermotropic LC phases, the lyotropic systems contain at
least two chemical components, an organic molecule (amphiphilic molecule)