Liquid Crystal and Liquid Crystal Polymer

Liquid Crystal and

Liquid Crystal Polymers

By,

Saurav Chandra Sarma

Int. Ph.D. 4th Sem.

Liquid Crystal and Life

Liquid crystals are also fundamentallyimportant to life. DNA and cellmembranes have liquid crystal phases.Our brains are around 70% liquidcrystal, and liquid crystals are alsofound in muscles, the amazingiridescent colours of some insects, andalso 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.

Introduction

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

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.

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

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.

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.

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.

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’swavelength—about a few hundreds nm andso shows different color.

Lyotropic Liquid Crystal

Lyotropic LCs are two-component systems where an amphiphile is dissolved in asolvent. 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 arethose mesomorphic compounds that possess anelongated shape.

Divided into 2 groups:Nematic and Smectic

2. Discotic LCs:

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 longaxis of each molecule

The brackets denote an average over all of themolecules in the sample.

In an isotropic liquid, the average of the cosine termsis zero, and therefore the order parameter is equal tozero.

For a perfect crystal, the order parameter evaluatesto one

Typical values for the order parameter of a liquidcrystal range between 0.3 and 0.9, with the exactvalue a function of temperature, as a result of kineticmolecular motion.

S=(1/2)<3Cos2q -1>

Nematic LC

External influences on Liquid CrystalsExternal perturbation can cause significant changes in the macroscopic properties ofthe liquid crystal system. The order of liquid crystals can be manipulated bymechanical, electric or magnetic forces.

Electric and Magnetic field effect:Due to the effect of electric field permanent electricdipole results which aligns the director along theelectric field.The effect of magnetic field is analogous to the electricfield.

Surface Preparations: It is possible, however, to force the director to point in aspecific direction by introducing an outside agent to the system.

For example, when a thin polymer coating (usually a polyimide) is rubbed ina single direction,on a glass substrate, with a cloth, it is observed that liquid crystalmolecules 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 crystalsample, the process is modeled in terms of the light being broken up into thefast (called the ordinary ray) and slow (called the extraordinary ray)components. Because the two components travel at different velocities, thewaves get out of phase. When the rays are recombined as they exit thebirefringent material, the polarization state has changed because of this phasedifference

Liquid Crystal Textures

The term texture refers to the orientation of liquid crystal molecules in the vicinityof a surface. Each liquid crystal mesophase can form its own characteristictextures,which are useful in identification. We consider the nematic textures here.If mesogenic materials are confined between closely spaced plates with rubbedsurfaces (as described above) and oriented with rubbing directions parallel, theentire liquid crystal sample can be oriented in a planar texture, as shown in thefollowing diagram

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 adisclination. A disclination is a region where the director is undefined. Thefollowing is a diagram that shows the orientation of the director around adisclination.

Defects in Liquid Crystal

Calamitic vs. Discotic Nematics

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.

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 crystalphase

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 liquidcrystals: The most commonapplication of liquid crystaltechnology is liquid crystal displays(LCDs.)

Thermal mapping and non-destructive testing

Medicinal Uses: Cholesteric liquidcrystal mixtures have also beensuggested for measuring body skintemperature, to outlines tumoursetc.

Optical Imaging and Liquid CrystalInteractions with Nanostructure

Liquid Crystal in Chromatography

Liquid Crystal as Solvents in Spectroscopy

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)

Polymer Liquid Crystals

Advantage of Liquid Crystal Polymer (LCP)

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

• Soap

• Conducting foams

• Heat Sensitive cameras use liquid crystal screens that respond to heat.

Applications

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

Applications