Liquid crystals : The fourth state of matter

Liquid crystals : The fourth state

of matter

Thierry Chuard, LLB, Universität Bern

Abstract

 What is a liquid crystal ?

 Some applications of liquid crystals being a liquid

crystal ?

 How can we identify a compound

What is a liquid crystal ?

Crystal

Highly organized solid

Liquid

Non-organized fluid

Liquid crystal

Organized fluid (mesophase)

heat

heat heat

The liquid crystalline state is a hybrid state between the liquid and and the solid phase. Liquid crystals possess both the fluidity of liquids and organization found in solids.

This is an anisotropic phase.

Liquid crystals

Depends on

- the temperature - the solvent

- the concentration Depends only

on the

temperature

Thermotropic Lyotropic

Which kind of molecules can

show liquid crystalline state ? _Calamitics

Rigid and rod-shaped

Rigid and

disc-shaped Discotics

A calamitic compound

4-methoxybenzilidene-4'-butylaniline

Crystal Liquid Liquid

crystal

20 °C 47 °C

OCH₃ CH

N

How do calamitic molecules

organize ?

The nematic phase

Directo r

+ +

+ +

+ +

+ +

+ -

- -

- -

- - -

-

+ - + +- +

+ + +

+ +

+ - -

-

- - - -

-

Random orientation

of the dipoles

No polarity of the

mesophase

The nematic phase

The less organized mesophase is the most fluid

 

The cholesteric phase

A cholesteric phase is a chiral nematic phase

The cholesteric phase

p

The wavelength  of the light reflected

by a cholesteric material is

proportional to the helical pitch p and

to the mean

refractive index n :

 = p ∙ n Bragg’s law

Smectic A Smectic C

The smectic (lamellar)

phases

Smectic B

The smectic ^(lamellar) phases

hexagonal

phase

N S_A

S_C

S H₂₁C₁₀O

C₅H₁₁ O

The same compound can show different mesophases

Cryst al

63 °C

60 °C 80 °C

Liqui d 86 °C

A discotic compound

Liquid crystal

Crystal

Liquid

H₁₁C₅O

H₁₁C₅O OC₅H₁₁

OC₅H₁₁ H₁₁C₅O OC₅H₁₁

How do discotic molecules organize ?

Hexagonal Rectangular Tetragonal

 Nematic phase

 Columnar phases

How can we identify a compound

being a liquid crystal ?

 X-Ray diffraction

 Miscibility

 Differential scanning calorimetry

 Polarized transmitted-light

microscopy

20 40 60 80 100

Temperature (°C)

20 40 60 80 100

Temperature (°C)

Differential scanning calorimetry

Temperat ure

Control

Heat flow Heat flow

Differential scanning calorimetry

20 40 60 80 100

Temperature (°C)

Thermogram = difference between heat flows

20 40 60 80 100

20 40 60 80 100

Temperature (°C) Temperature (°C)

Differential scanning calorimetry

Temperat ure

Control

1-2 mg

Heat flow Heat flow

Differential scanning calorimetry

20 40 60 80 100

Temperature (°C) Melting point

Thermogram = difference between heat flows

Thermogram of a non liquid-crystalline compound

Liquid

Solid

Thermogram of a liquid-crystalline compound

Solid Liquid

Crystal Liquid

X-Ray diffraction

 Determination of the layer thickness

in smectic (lamellar) phases

 50 - 100 mg samples

 Identification of the mesophase

 Determination of the angle in non orthogonal smectic

phases

polarized light

Polarized transmitted-light

microscopy

The Schlieren texture

Nematic phase

The focal conic fan texture

Smectic A phase

The focal conic fan texture

Columnar hexagonal

phase

Some applications of liquid crystals

 Displays

 Materials (polymers)

 Thermometry

 Pigments (cars)

 Specific oils

 Adjustable Tinted Window

LC displays

 different technologies

 Switching time always faster

 Switching voltage always lower

 Screen size

 Resolution

 Colors quality and greyscale

Twisted nematic (TN) cell

Front glass

Rear glass Contacts

Electrode

Counter-electrode

Alignment layers

LC

Twisted nematic (TN) cell

Front glass

Rear glass LC

Polarizer

Polarizer

TN cells switching

Front glass

Rear glass Electrode

Counter-Electrode Alignment layer

Alignment layer Polarizer

Polarizer

The plane of the polarized light is turned by 90°

by the LC The light can go through the

2nd polarizer

Only the polarized Light in the plane

of the polarizer goes through

The plane of the polarized light

is not turned by the LC

The light cannot go through the

2nd polarizer



How can we obtain a white pixel on a black

background with the

same disposal ?

The plane of the polarized light is turned by 90°

by the LC

The light cannot go through the

2nd polarizer

The plane of the polarized light

is not turned by the LC The light can go through the

2nd polarizer



ITO electrodes

ITO electrodes

Conclusio

n