Liquid crystals are found to be birefringent, due to their anisotropic nature. That is, they demonstrate double refraction (having two indices of refraction). Light polarized parallel to the director has a different index of refraction (that is to say it travels at a different velocity) than light polarized perpendicular to the director. In the following diagram, the blue lines represent the director field and the arrows show the polarization vector. Thus, when light enters a birefringent material, such as a nematic liquid crystal sample, the process is modeled in terms of the light being broken up into the fast (called the ordinary ray) and slow (called the extraordinary ray) components. Because the two components travel at different velocities, the waves get out of phase. When the rays are recombined as they exit the birefringent material, the polarization state has changed because of this phase difference.
The birefringence of a material is characterized by the difference, Dn, in the indices of refraction for the ordinary and extraordinary rays. To be a little more quantitative, since the index of refraction of a material is defined as the ratio of the speed of light in a vacuum to that in the material, we have for this case, ne = c/V| | and no = c/V^ for the velocities of a wave travelling perpendicular to the director and polarized parallel and perpendicular to the director, so that the maximum value for the birefringence, Dn = ne – no. We won’t deal here with the general case of a wave travelling in an arbitrary direction relative to the director in a liquid crystal sample, except to note that Dn varies from zero to the maximum value, depending on the direction of travel. The condition ne > no describes a positive uniaxial material, so that nematic liquid crystals are in this category. For typical nematic liquid crystals, no is approximately 1.5 and the maximum difference, Dn, may range between 0.05 and 0.5. The length of the sample is another important parameter because the phase shift accumulates as long as the light propagates in the birefringent material. Any polarization state can be produced with the right combination of the birefringence and length parameters. It is convenient here to introduce the concept of optical path in media since for the above two wave components travelling with different speeds in a birefringent material, the difference in optical paths will lead to a change in the polarization state of the wave as it progresses through the medium. We define the optical path for a wave travelling a distance L in a crystal as nL so that the optical path difference for the two wave components mentioned above will be L (ne – no) = LDn. The resultant phase difference between the two components (the amount by which the slow, extraordinary component lags behind the fast, ordinary one) is just 2p LDn/lv where lv is the wavelength in vacuum. The following simulation demonstrates the optical properties of a birefringent material. A linearly polarized light wave enters a crystal whose extraordinary (slow) index of refraction can be controlled by the user. The length of the sample can also be varied, and the outgoing polarization state is shown. The concept of optical path difference and its influence on polarization state can also be explored here. This leads to a discussion of optical retardation plates or phase retarders, in the context of the simulation.
Liquid Crystal and Liquid Crystal Polymer
Liquid Crystal and
Liquid Crystal Polymers
Saurav Chandra Sarma
Int. Ph.D. 4th Sem.
Liquid Crystal and Life
Liquid crystals are also fundamentally
important to life. DNA and cell
membranes have liquid crystal phases.
Our brains are around 70% liquid
crystal, and liquid crystals are also
found in muscles, the amazing
iridescent colours of some insects, and
also slug slime!
Liquid crystals are beautiful and mysterious; I am fond of them for both reasons.
- P.G. De Gennes
Liquid crystals (LCs) are matter in a state that has properties between
those of conventional liquid and those of solid crystal.
For instance, an LC’s may flow like a liquid, but its molecules may be
oriented in a crystal like way.
There are many different type of LC phases, which can be distinguish
by their different “optical properties” (such as birefringence.
Which viewed under a microscope using a polarized light source,
different liquid crystal phases will appear to have distinct textures.
Positional Order + Orientational Order = Crystal Phase
Positional Order + No Orientational Order = Plastic Phase
Varying Positional Order + Orientational Order = Liquid Crystal Phase
No Positional Order + No Orientational Order = Isotropic Phase
Liquid crystals are classified in terms of following criterion:
(1) Translational order/ Positional Order
(2) Bond orientational order
(3) Correlation between smectic layers
(4) With chirality?
(5) Cubic structure?
Liquid Crystal-Is it a Solid or Liquid..???
The amount of energy required to cause the phase transition is called latent
heat of the transition and is useful to measure of how different the two phases are.
In the case of cholesteryl myristate, the latent heat of solid to liquid crystal is 65
calories/gram,while the latent heat for liquid crystal to liquid transition is 7
The smallness the latent heat of liquid crystal to liquid phase transition is evidence
that liquid crystal are more similar to liquids than they are to solids.
Mesophase: a phase lying between solid (crystal)
and isotropic (liquid) states.
Liquid crystals: fluid (l) but also show birefringence (c);
have properties associated with both crystals and liquids.
Thermotropic: liquid crystalline phase is formed
when the pure compound is heated.
Lyotropic: liquid crystalline phase forms
when the molecules are mixed with a solvent (solution).
Liquid Crystalline Phases
Single or multicomponent
Homo- or co-polymers
Figure 9.1 The liquid crystal family tree.
No translational order—Nematics
The word “Nematic" is derived from the
Greek word for thread-like structure.
It is the only liquid crystal phase with no
long range translational order.
It is the least ordered mesophase
Preferred Orientation is denoted by the
This phase has a symmetrical axis C∞ along
Point Group D∞h.
It has thread like structure when seen
under polarizing microsope.
One-dimensional translational order—Smectic
The word "Smectic" is derived from the Greek word for soap
Liquid-like motion of the rods in each layer
No correlation of the molecular positions from one layer to
The layers can easily slide
In the smectic A phase, molecules tend to be perpendicular to
the smectic layers
In the smectic C phase, the molecules in the layers are parallel
and tilted in arrangement with respect to the normal of the
layers by a tilt angle θ.
Chiral Liquid Crystal- Cholesteric
Also known as “Chiral nematic”
Molecules have non-symmetrical carbon
atoms and thus lose mirror symmetry
Shows a helical structure.
In general the helical pitch of cholesteric
liquid crystals is of the order of visible light’s
wavelength—about a few hundreds nm and
so shows different color.
Lyotropic Liquid Crystal
Lyotropic LCs are two-component systems where an amphiphile is dissolved in a
Lyotropic mesophases are concentration and solvent dependent.
Thermotropic Liquid Crystal
The transitions to the liquid crystalline state are induced thermally
Thermotropic Liquid Crystal
The essential requirement for a molecule to be a thermotropic LC is a structure
consisting of a central rigid core (often aromatic) and a flexible peripheral moiety
(generally aliphatic groups). This structural requirement leads to two general
classes of LCs:
1. Calamitic LCs: Calamitic or rod-like LCs are
those mesomorphic compounds that possess an
Divided into 2 groups:
Nematic and Smectic
2. Discotic LCs:
To quantify just how much order is present in a
material, an order parameter (S) is defined.
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
For a perfect crystal, the order parameter evaluates
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
External influences on Liquid Crystals
External perturbation can cause significant changes in the macroscopic properties of
the liquid crystal system. The order of liquid crystals can be manipulated by
mechanical, electric or magnetic forces.
Electric and Magnetic field effect:
Due to the effect of electric field permanent electric
dipole results which aligns the director along the
The effect of magnetic field is analogous to the electric
Surface Preparations: It is possible, however, to force the director to point in a
specific direction by introducing an outside agent to the system.
For example, when a thin polymer coating (usually a polyimide) is rubbed in
a single direction,on a glass substrate, with a cloth, it is observed that liquid crystal
molecules in contact with that surface align with the rubbing direction.
Birefringence in Liquid Crystals
When light enters a birefringent material, such as a nematic liquid crystal
sample, the process is modeled in terms of the light being broken up into the
fast (called the ordinary ray) and slow (called the extraordinary ray)
components. Because the two components travel at different velocities, the
waves get out of phase. When the rays are recombined as they exit the
birefringent material, the polarization state has changed because of this phase
Liquid Crystal Textures
The term texture refers to the orientation of liquid crystal molecules in the vicinity
of a surface. Each liquid crystal mesophase can form its own characteristic
textures,which are useful in identification. We consider the nematic textures here.
If mesogenic materials are confined between closely spaced plates with rubbed
surfaces (as described above) and oriented with rubbing directions parallel, the
entire liquid crystal sample can be oriented in a planar texture, as shown in the
Defects Under the Microscope:
• The abrupt changes in brightness seen in the pictures signal a rapid change in
director orientation in the vicinity of a line or point singularity known as a
disclination. A disclination is a region where the director is undefined. The
following is a diagram that shows the orientation of the director around a
Defects in Liquid Crystal
Experimental Identification of Liquid
Differential Scanning Calorimetry (DSC): It
provides valuable information like the exact
transition temperatures and the enthalpies of
the different phases
Polarizing Microscope: When a liquid crystal
material is placed on a microscope slide with a
cover slip and the slide is heated and viewed
using a polarizing microscope, textures
characteristic of each type of liquid crystal can
Experimental Identification of Liquid
X-ray Crystallography: This can be used to study the extent of
translational or positional order, and thus infer the type of liquid crystal
Extended X-ray absorption fine structure spectroscopy
(EXAFS): EXAFS was used to investigate the local structure of the polar
spines of metal ion soaps in the columnar liquid crystalline state
Applications of liquid crystals
Display application of liquid
crystals: The most common
application of liquid crystal
technology is liquid crystal displays
Thermal mapping and non-
Medicinal Uses: Cholesteric liquid
crystal mixtures have also been
suggested for measuring body skin
temperature, to outlines tumours
Optical Imaging and Liquid Crystal
Interactions with Nanostructure
Liquid Crystal in
Liquid Crystal as Solvents in
• These are a class of aromatic polymer.
• Extremely unreactive and inert.
• Highly resistant to fire.
Liquid crystallinity in polymers can be obtained :
By dissolving in a solvent. (Thermotropic)
By heating above melting transition point. (Lyotropic)
Liquid Crystal Polymer (LCP)