Chemistry Reference
In-Depth Information
where
Dn
eff
0
¼ n
x
0
n
y
0
. The samples satisfy the condition for this simplification:
Dn
eff
0
is of the order of 10
2
whereas the refractive index normal to the long axis
process of the director from 0
toward 90
.
3.3 Director Rotation Characterized by Polarized FTIR
The director-rotation behavior under electric fields was also characterized by
polarized FTIR [
33
]. Polarized FTIR has been used to characterize mesogen
reorientation and mobility in monodomain LCEs in response to external mechani-
the polarizer angle reflects the average orientation of the bond:
A
is proportional to
hð
m
2
denotes the molecular transition dipole moment, vector
E denotes the electric field of the IR beam, and the bracket represents averaging
over all molecules in the measured region. In usual optical birefringence measure-
ments, the information obtained is only the difference of the contributions in two
directions such as (
n
x
n
y
). Polarized FTIR provides further information about the
mesogen reorientation, because this technique gives information about the contri-
bution in each direction separately.
Almost the same observation geometry used in the optical birefringence mea-
with ITO electrodes that are transparent to infrared beams were used. The absorp-
tion band of the stretching vibration of the terminal cyano group parallel to the long
axes of the mesogens (A-6OCB) and solvent (5CB) at around 2,225 cm
1
was used
to characterize the director rotation. The absorbances of the incident polarized light
parallel and normal to the initial director axis (denoted by
A
x
0
and
A
y
0
, respectively)
were evaluated from the area of the corresponding peak as a function of
V
0
. The
absorbance of the silicone oil at around the wavelength of interest (2,225 cm
1
) was
negligible. The absorbances
A
x
0
and
A
y
0
reflect the total contributions from the
mesogens of the LCE and the solvent, and they are used to analyze the director-
rotation behavior.
E
Þ
i
where vector
m
¼A
x
0
/
[
(l
z
A
y
0
0
)] and
A
y
[
¼A
y
0
/(l
z
A
y
0
0
)], where
A
x
0
and
A
y
0
are reduced by l
z
to consider the
variation in effective thickness due to deformation, and
A
y
0
0
is the value of
A
y
0
at
V
0
¼
N bond parallel
to the long axis of the mesogens. A finite difference in
A
x
and
A
y
at
V
0
¼
0. The absorbance
A
reflects the average orientation of the C
0 (denoted
A
x
0
and
A
y
0
, respectively) reflects the initial uniaxial orientation (original director)
in the
x
-direction. As
V
0
increases, the difference in
A
x
and
A
y
becomes smaller
while
A
y
remains almost unchanged. Furthermore,
A
x
becomes nearly equal to
A
y
at
high
V
0
. When a uniform reorientation of mesogens in the specimen is assumed,
A
x
and
A
y
correspond to the long and short axes of the intersection in the
x
-
y
plane at
Search WWH ::
Custom Search