1

Example of PDLC architectural window

2

Polymer Dispersed Liquid Crystal (PDLC)

3

Liquid Crystals

Thermotropic Lyotropic

High molecular(molar) mass [ polymers]

Low molecular(molar) mass

Main-chain polymers

Side-chain polymers

Rod-like orlath-likemolecules

Calamitic

Disc-likemolecules

Discotic

Single or multicomponent systems

Homo- or co-polymers

Figure 9.1 The liquid crystal family tree.

4

Chapter 2. Polymer Liquid Crystals

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).

2.1. Liquid Crystalline Phases

5

CNC5H11

OC4H9C5H11

N

NC8H17C11H23O

F

F

C5H11CH2CH2C3H7

a. Calamitic

b. Discotic

O

O

O

O

O

O

C

C

CC

C

C O

R

R

O

ROO

R

O

R

OR

Hexa-substituted benzenes

RR

R

RR

R

Triphenylenes

R

R

R

R

R

R

Truxenes

N

N

N

N

N

NN

N

H

H

RR

R

R

RR

R

R

Phthalocyanines

(a) (b)

Fig 9.3 The structure of (a) nematic and(b) Columnar discotic phases.

Fig 9.2 The structure of molecules which form (a) calamitic and (b) discotic liquid crystalline

SOLID MESOPHASEISOTROPICLIQUID

3D order

1or 2D order

NONE

MesogenSmectogenNematogenLamellarNon-lamellar

6

7

OC

O

C8H17

F

C4H9

Gives a nematic phase

OC

O

C8H17

F

CH3CH2CHCH2

H3C

*

Gives a cholesteric phase * Chiral center

pitch

8

Molecular structure: rigid, long lathe-like shapes with a high length to breadth ratio, or disc shapes.

Chemical structure: composed of a central core comprising aromatic or cycloaliphatic units by rigid links and having either polar or flexible alkyl and alkoxy terminal groups.

Molecular design of liquid crystalline polymers (1) Incorporation of mesogens into chain-like structures by linking

them together through both terminal units to form main-chain liquid crystalline polymers. (2) Attachment of mesogens through one terminal unit to a polymer backbone to produce a side chain comb-branch structure. (3) A combination of both main and side chain structures.

9

Thermotropic LC Molecule

10

11

The mesogenic units in a polymer can form ordered structures characterized by long-range orientational order, with the long axes of the mesogenic groups arranged in one preferred direction of alignment called the director.

The lateral forces between the molecules in the smectic phases are stronger than the forces between the layers.

In the nematic phase, while the directional ordering of the mesogen long axes is maintained, the centers of gravity are no longer confined in layers, but are distributed randomly in the phase.

A chiral center imparts a twist to each successive layer in the cholesteric phase where the orientation of the director changes regularly from layer to layer, forming a helical arrangement of the directors in three dimensional space.

12

In main chain liquid crystal polymers, there is usually a transition from crystal to a mesophase, whereas in more amorphous systems when a glass transition is present. The mesophase may appear after this transition has occurred.

In multiple transition thermotropic systems, the increase in temperature leads changes from the most ordered to the least ordered states.

k: crystal, g: glass, i: isotropicS: smectic (S*: chiral S), N: nematic (N*: chiral N)Ch: cholesteric

2.2 Identification of the Mesophases

By polarized optical microscope (POM) Nematic phases: e.g. Schlieien texture, threaded texture, and marbled texture.

13

Higher order Lower order

14

Smectic phases: e.g. focal conic and fan texture, mosaic texture, and broken focal conic texture.

Cholesteric phases: e.g. Grandjear texture.

By differential scanning calorimeter(DSC) Widely used for detecting temperatures of thermotropic mesophase transitions. First-order transitions.

By X-ray diffraction (XRD) For the least ordered phases, one diffuse halo is seen at large diffraction angles. At smaller diffraction angles, a diffuse inner ring is formed in nematic samples. When the smectic phases are present, one or more sharp inner rings, arising from the more ordered lamellar structure, are seen.

15

 Nematic Schlieren texture

Mosaic texture of a hexatic SmB phase

SmA fan-shaped texture SmA focal conic texture

SmC Schlieren texture and focal conic

Nematic Phase (threaded)

Nematic Phase (marbled)

16

17

18