The document discusses liquid crystals and liquid crystal polymers. It notes that liquid crystals have properties between solids and liquids, with some positional and orientational order. They can exist in nematic, smectic, and cholesteric phases. Liquid crystal phases are important in biological systems like cell membranes and the brain. Liquid crystal polymers are highly resistant to heat and chemicals. They have applications in displays, body armor like Kevlar, and as heat sensors.

1. Liquid Crystal and
Liquid Crystal Polymers
By,
Saurav Chandra Sarma
Int. Ph.D. 4th Sem.

2. 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

3.  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.
Introduction

4. 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?

5. 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
calories/gram.
 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.

6. 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

7. Liquid Crystals
Thermotropic Lyotropic
High molecular
(molar) mass
[ polymers]
Low molecular
(molar) mass
Main-chain
polymers
Side-chain
polymers
Rod-like or
lath-like
molecules
Calamitic
Disc-like
molecules
Discotic
Single or multicomponent
systems
Homo- or co-polymers
Figure 9.1 The liquid crystal family tree.

8. 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
‘Director’ n.
 This phase has a symmetrical axis C∞ along
the director
 Point Group D∞h.
 It has thread like structure when seen
under polarizing microsope.

9. 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 next
 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.

10. 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.

11. Lyotropic Liquid Crystal
 Lyotropic LCs are two-component systems where an amphiphile is dissolved in a
solvent.
 Lyotropic mesophases are concentration and solvent dependent.

12. Thermotropic Liquid Crystal
 The transitions to the liquid crystalline state are induced thermally

13. 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
elongated shape.
Divided into 2 groups:
Nematic and Smectic
2. Discotic LCs:

14. Order Parameter
 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
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.
S=(1/2)<3Cos2q -1>
Nematic LC

15. 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
electric field.
The effect of magnetic field is analogous to the electric
field.
 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.

16. 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
difference

17. 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
following diagram

18. 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
disclination.
Defects in Liquid Crystal

19. Calamitic vs. Discotic Nematics

20. Experimental Identification of Liquid
Crystals
 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
be seen.

21. Experimental Identification of Liquid
Crystals
 X-ray Crystallography: This can be used to study the extent of
translational or positional order, and thus infer the type of liquid crystal
phase
 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

22. Applications of liquid crystals
 Display application of liquid
crystals: The most common
application of liquid crystal
technology is liquid crystal displays
(LCDs.)
 Thermal mapping and non-
destructive testing
 Medicinal Uses: Cholesteric liquid
crystal mixtures have also been
suggested for measuring body skin
temperature, to outlines tumours
etc.
 Optical Imaging and Liquid Crystal
Interactions with Nanostructure
 Liquid Crystal in
Chromatography
 Liquid Crystal as Solvents in
Spectroscopy

23. Characteristics:
• 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)

24. Polymer Liquid Crystals

25. Advantage of Liquid Crystal Polymer (LCP)

26. Advantage of LCP
 High heat resistance
 Flame retardant
 Chemical resistance
 Dimensional stability
 Mold ability
 Heat aging resistance
 Adhesion
 Low viscosity
 Wieldable
 Low cost
Disadvantage of LCP
 Form weak weld lines
 Highly anisotropic properties
 Drying required before
processing
 High Z-axis thermal expansion
coefficient

27. • Soap
• Conducting foams
• Heat Sensitive cameras use liquid crystal screens
that respond to heat.
Applications

28. • Kevlar, the most widely used body armor is made up of
intertwined liquid crystal polymers.
Applications