Transcript Document 7177449

Example of PDLC architectural window
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Polymer Dispersed Liquid Crystal (PDLC)
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Liquid Crystals
Lyotropic
Thermotropic
Low molecular
(molar) mass
High molecular
(molar) mass
[ polymers]
Main-chain
polymers
Side-chain
polymers
Homo- or co-polymers
Rod-like or
lath-like
molecules
Disc-like
molecules
Calamitic
Discotic
Single or multicomponent
systems
Figure 9.1 The liquid crystal family tree.
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Chapter 2. Polymer Liquid Crystals
2.1. Liquid Crystalline Phases
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).
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a. Calamitic
C5H11
(a)
CN
(b)
F
C5H11
OC 4H9
N
C11H23O
C8H17
N
F
C3H7
CH 2CH 2
C5H11
Fig 9.3 The structure of (a) nematic and
(b) Columnar discotic phases.
b. Discotic
R
C
O
O
R
O
O
R
C
R
O
O
C
O
O
C
R
O
O
O
C
C
O
R
R
R
R
R
R
Hexa-substituted benzenes
R
Triphenylenes
R
R
R
R
R
R
N
N
R
R
N
N
H
3D
order
1or 2D
order
ISOTROPIC
LIQUID
NONE
R
R
N
R
R
R
MESOPHASE
N
H
N
N
SOLID
R
Truxenes
Phthalocyanines
Fig 9.2 The structure of molecules which form (a) calamitic and
(b) discotic liquid crystalline
Mesogen
Smectogen
Nematogen
Lamellar
Non-lamellar
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F
C4H9
F
O
OC
C8H17
Gives a nematic phase
H3C
CH 3CH 2CHCH 2
*
O
OC
C8H17
Gives a cholesteric phase
* Chiral center
pitch
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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.
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Thermotropic LC Molecule
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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.
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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: isotropic
S: 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.
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Higher order
Lower order
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 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.
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Nematic Schlieren texture
Nematic Phase (threaded)
Nematic Phase (marbled)
SmA fan-shaped texture
SmA focal conic texture
Mosaic texture of a hexatic SmB phase
SmC Schlieren texture and focal conic
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