Introduction to Liquid Crystals
The study
of liquid crystals began in 1888 when an Austrian botanist named Friedrich
Reinitzer observed that a material known as cholesteryl benzoate had two
distinct melting points. In his experiments, Reinitzer increased the
temperature of a solid sample and watched the crystal change into a hazy
liquid. As he increased the temperature further, the material changed again
into a clear, transparent liquid. Because of this early work, Reinitzer is
often credited with discovering a new phase of matter - the liquid crystal
phase.
Liquid
crystal materials are unique in their properties and uses. As research into
this field continues and as new applications are developed, liquid crystals
will play an important role in modern technology.
What are Liquid Crystals?
Liquid crystal materials
generally have several common characteristics. Among these are a rod-like
molecular structure, rigidness of the long axis, and strong dipoles and/or
easily polarizable substituents.
The
distinguishing characteristic of the liquid crystalline state is the tendency
of the molecules (mesogens) to point along a common axis, called the director.
This is in contrast to molecules in the liquid phase, which have no intrinsic
order. In the solid state, molecules are highly ordered and have little
translational freedom. The characteristic orientational order of the liquid
crystal state is between the traditional solid and liquid phases and this is
the origin of the term mesogenic state, used synonymously with liquid crystal
state.
It is
sometimes difficult to determine whether a material is in a crystal or liquid
crystal state. Crystalline materials demonstrate long range periodic order in
three dimensions. By definition, an isotropic liquid has no orientational
order. Substances that aren't as ordered as a solid, yet have some degree of
alignment are properly called liquid crystals.
To
quantify just how much order is present in a material, an order parameter (S)
is defined. Traditionally, the order parameter is given as follows:
S = (1/2)<3cos2q-1>
where
theta is the angle between the director and the long axis of each molecule. The
brackets denote an average over all of the molecules in the sample. In an
isotropic liquid, the average of the cosine terms is zero, and therefore the
order parameter is equal to zero. For a perfect crystal, the order parameter
evaluates to one. Typical values for the order parameter of a liquid crystal
range between 0.3 and 0.9, with the exact value a function of temperature, as a
result of kinetic molecular motion.
The
tendency of the liquid crystal molecules to point along the director leads to a
condition known as anisotropy. This term means that the properties of a
material depend on the direction in which they are measured. For example, it is
easier to cut a piece of wood along the grain than against it. The anisotropic
nature of liquid crystals is responsible for the unique optical properties
exploited by scientists and engineers in a variety of applications.
Characterizing Liquid Crystals
The
following parameters describe the liquid crystalline structure:
- Positional Order
- Orientational Order
- Bond Orientational Order
Each of these parameters
describes the extent to which the liquid crystal sample is ordered. Positional
order refers to the extent to which an average molecule or group of molecules
shows translational symmetry. Orientational
order, represents a measure of the tendency of the
molecules to align along the director on a long-range basis. Bond Orientational
Order describes a line joining the centers of nearest-neighbor molecules without
requiring a regular spacing along that line. Thus, a relatively long-range
order with respect to the line of centers but only short range positional order
along that line.
Most
liquid crystal compounds exhibit polymorphism, or a condition where more than
one phase is observed in the liquid crystalline state. The term mesophase is
used to describe the "subphases" of liquid crystal materials.
Mesophases are formed by changing the amount of order in the sample, either by
imposing order in only one or two dimensions, or by allowing the molecules to
have a degree of translational motion.
Liquid Crystal Phases
The
liquid crystal state is a distinct phase of matter observed between the
crystalline (solid) and isotropic (liquid) states. There are many types of liquid
crystal states, depending upon the amount of order in the material.
Nematic Phases
The nematic liquid crystal phase
is characterized by molecules that have no positional order but tend to point
in the same direction . In the following diagram, notice
that the molecules point vertically but are arranged with no particular order.
Liquid
crystals are anisotropic materials, and the physical properties of the system
vary with the average alignment with the director. If the alignment is large,
the material is very anisotropic. Similarly, if the alignment is small, the
material is almost isotropic.
A special
class of nematic liquid crystals is called chiral nematic. Chiral refers to the
unique ability to selectively reflect one component of circularly polarized
light. The term chiral nematic is used interchangeably with cholesteric.
Smectic Phases
The word "smectic" is
derived from the Greek word for soap. This seemingly ambiguous origin is
explained by the fact that the thick, slippery substance often found at the
bottom of a soap dish is actually a type of smectic liquid crystal.
The smectic state is another
distinct mesophase of liquid crystal substances. Molecules in this phase show a
degree of translational order not present in the nematic. In the smectic state,
the molecules maintain the general orientational order of nematics, but also
tend to align themselves in layers or planes. Motion is restricted to within
these planes, and separate planes are observed to flow past each other. The
increased order means that the smectic state is more "solid-like"
than the nematic.
Many
compounds are observed to form more than one type of smectic phase. In the smectic-A mesophase, the
director is perpendicular to the smectic plane, and there is no particular
positional order in the layer. Similarly, the smectic-B mesophase orients with
the director perpendicular to the smectic plane, but the molecules are arranged
into a network of hexagons within the layer. In the smectic-C mesophase,
molecules are arranged as in the smectic-A mesophase, but the director is at a
constant tilt angle measured normally to the smectic plane.
As in the nematic, the smectic-C
mesophase has a chiral state designated C*. Consistent with the smectic-C, the
director makes a tilt angle with respect to the smectic layer. The difference
is that this angle rotates from layer to layer forming a helix. In other words,
the director of the smectic-C* mesophase is not parallel or perpendicular to
the layers, and it rotates from one layer to the next.
Cholesteric Phases
The cholesteric (or chiral
nematic) liquid crystal phase is typically composed of nematic mesogenic
molecules containing a chiral center which produces intermolecular forces that
favor alignment between molecules at a slight angle to one another. This leads
to the formation of a structure which can be visualized as a stack of very thin
2-D nematic-like layers with the director in each layer twisted with respect to
those above and below.
In the fabrication of films,
since putting chiral nematic liquid crystals directly on a black background
would lead to degradation and perhaps contamination, the crystals are
micro-encapsulated into particles of very small dimensions. The particles are
then treated with a binding material that will contract upon curing so as to
flatten the microcapsules and produce the best alignment for brighter colors.
An application of a class of chiral nematic liquid crystals which are less
temperature sensitive is to create materials such as clothing, dolls, inks and
paints.
The wavelength of the reflected
light can also be controlled by adjusting the chemical composition, since
cholesterics can either consist of exclusively chiral molecules or of nematic
molecules with a chiral dopant dispersed throughout. In this case, the dopant
concentration is used to adjust the chirality and thus the pitch.
Columnar Phases
Columnar
liquid crystals are different from the previous types because they are shaped
like disks instead of long rods. This mesophase is characterized by stacked
columns of molecules. The columns are packed together to form a two-dimensional
crystalline array. The arrangement of the molecules within the columns and the
arrangement of the columns themselves leads to new mesophases.
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