Transcript chemistry of LC - Electrical and Computer Engineering

Liquid Crystal Materials
N
C
Broad Classification
Lyotropics
amphiphilic molecules, polar and non-polar
parts form liquid crystal phases over certain
concentration ranges when mixed with a
solvent
hydrophobic
non-polar tail
+
-
hydrophilic
polar head
Thermotropics
molecules consisting of a rigid core and
flexible tail(s) form liquid crystal phases
over certain temperature ranges.
flexible tail
rigid core
The Lyotropic Phases
micelle
cross section
reverse
micelle
cross section
The Thermotropic Liquid Crystal Molecule
Chemist’s
View
Physicist’s
Engineer’s
View
• Shape Anisotropy
• Length > Width
The molecule above (5CB) is ~2 nm × 0.5 nm
Geometrical Structures of
Mesogenic Molecules
Low Molecular Weight
High Molecular Weight
(polymers)
disk-like
(
rod-like
(
most practical applications
)n
)n
The Liquid Crystal Phase
n
Crystal
Nematic LC Isotropic
Temperature
The Nematic Director n
n
director
The local average axis
of the long molecular axis
Other Liquid Crystal Phases
z
n
q
Smectic C
n
Smectic A
Temperature
n
Nematic
Chirality
The methyl group on the 2nd carbon atom
on the alkyl chain of the molecules extends
out of the plane of the paper and the hydrogen atom extends into the plane of the paper.
Therefore the 2nd carbon can be thought of
as a right or left handed coordinate system
left-handed right-handed H H H H H
mirror images
C N
H-C-C-C-C-C
H H H H H
non-chiral
H
H H H
H
C N
H-C-C-C-C-C
non-superimposable
CH3
H H H
H
chiral (RH)
The Chiral Nematic
Ordinary Nematic
CN
Chiral Nematic
CN
director
n
pitch
P
The Chiral Doped Nematic
You can create a cholesteric material by doping a conventional
nematic with a chiral dopant.
1
 HTP
Pc
Chiral Dopant
S-811
IS-4651
For dilute solutions
HTP (mm)-1
-14
-13.6
- indicates left twist sense
For a 10% doping of S-811
1
P
HTP  c
1

 0.71m m
1
14 mm   0.1
The Chiral Smectic C: Ferroelectrics
C10H21 O
N
CH3
COO CH2 CH C2H5
q
m
Eye- dipole moment m
fin - chiral
ferroelectric LC has a
dipole moment perpendicular to its long
axis, and is chiral.
The Chiral Smectic: TGB
Twisted Grain Boundary (TGB)
A twisted grain boundary smectic A phase (frustrated) - TGBA*
R
Discotic Liquid Crystal
C
C
R
O
O
C
R
O
C
O
O
O
C
R
O
O
O
example: R=OCOC11H23
O
C
R
C
O
R
Discotics Liquid Crystals
n
n
Columnar, columns of molecules
in hexagonal lattice
Nematic discotic phase
Polymer Liquid Crystals
Combining the properties of liquid crystals and polymers
Main Chain
mesogenic moieties are
connected head-to-tail
rigid
semi-flexible
Side Chain
mesogenic moieties
attached as side chains
on the polymer backbone
Polymer Liquid Crystals
forming nematic liquid crystal phases
n
side-chain
main-chain
Example of Side-Chain Polymer LCs
R1
-(-CH2-C-)XO C-O-(CH2)n-O
•
•
•
•
•
•
O
C-O
R2
Too slow for display applications (switching times ~ 0.5-1 s
Useful for other applications such as:
Optical filters
Optical memory
Alignment layers for low molecular weight LCs
Non-linear optic devices (optical computing)
The Order Parameter
n
1
2
S  P2 (cos q )  (3 cos q  1)
2
q
cos q 
2
2
cos
 q d

 d
1

no order
3

n
cos2 (q  0o )  1
perfect order
S  P2 (cos q )  1
perfect crystal
S  P2 (cos q )  0
isotropic fluid
Maier-Saupe Theory - Mean Field Approach
Interactions between individual molecules are represented by a
potential of average force
n
V  cosq   vP2  cosq   P2 
q
• {V: minimum} when phase is ordered (-P2(cosq))
• {V: V=0} when phase is disordered (<P2(cosq)>)
• factor for intermolecular strength ( n)
y
f
From Statistical Mechanics (Self Consistency)
1
 P  cosq  exp   vP  cosq   P )d  cosq  
2
 P2 
2
2
0
1
 exp   vP  cosq   P   d  cosq 
2
0
2
=(kT)-1
Maier-Saupe Theory - Mean Field Approach
n
Order Parameter, S
1.0
Isotropic
Fluid
0.0
Nematic
Liquid
Crystal
n
-0.6
Temperature
Landau-de Gennes Theory
1 2 1 3 1 4 1
2
f  f 0  aS  bS  cS  L(S )  GS ( z )
2
3
4
2
a=ao(T-T*), ao, b, c, T*, L are phenomenological constants
Order Parameter, S
G is a surface interaction strength
Good near NI transition
Temperature
Predicts order near
surface
The Order Parameter:
How does it affects display performance ?
The order parameter, S, is proportional to a number of important
parameters which dictate display performance.
Parameter
Elastic Constant
Birefringence
Dielectric Anisotropy
Magnetic Anisotropy
Viscosity Anisotropy
Nomenclature
Kii
Dn
De
Dc
Dh
proportional to

S2
S
S
S
S
Example: Does the threshold switching voltage for a TN increase
or decrease as the operating temperature increases.
K
S2
Scales as the square root of S
VTH 

 S
De
S
therefore lowers with increasing temperature
Anisotropy: Dielectric Constant
Off-axis dipole moment, angle  with molecular axis


NhFS 
m 2F
2
De 
Da 
3cos   1


eo 
2kBT

N:
h,f:
S:
Da:
m:
kB:
T:
number density
reaction field, reaction
cavity parameters
order parameter
anisotropy in polarizability
molecular dipole moment
Boltzman constant
Temperature
For values of the angle 54.7o, the
dipolar term is positive, and for
values 54.7o, the dipolar term is
negative, and may result in a
materials with an overall -De.
Anisotropy: Dielectric Constant
positive
++
+++
E
e
- ---
De  e  e
e
0
E
negative
De  e  e  0
E
all angles in
the plane 
to E are
possible for the
-De materials
Anisotropy: Duel Frequency
low frequency, De>0
high frequency, De<0
MLC-2048 (EM Industries), Duel Frequency Material
Frequency (kHz)
0.1
1.0
10
50
Dielectric Anisotropy (De)
3.28 3.22 0.72 -3.0
100
-3.4