Chemistry Reference
In-Depth Information
relative to the direction of orientation. Consequently, the absorption will
be different according to the direction of oscillation of the incident polar-
ized light. Thus the order parameter, S, can be determined from the ratio
of absorptions or the dichroic ratio
R
according to the following equations
(Zbinden, 1964):
A
A
⊥
ln(
I
0
/I
)
ln(
I
0
/I
⊥
)
2(
R
−
1)
(
R
+ 2)(3 cos
2
α
−
R
=
=
and
S
=
1)
,
where
I
0
is the intensity of the incident light;
I
and
I
⊥
are the intensities
of the transmitted light parallel and at right angles to the direction of
elongation, respectively;
α
is the angle made by the axis of orientation and
the transition moment of the absorbing group. If the transition moment is
parallel to the orientation axis,
α
is zero and
S
=(
R
−
1)
/
(
R
+ 2); If it is
perpendicular to the axis,
S
= 2(1
−
R
)
/
(
R
+ 2).
Using this technique Wen and coworkers (Wen
et al.
, 1988) studied the
molecular orientation in the nematic phase of the liquid crystalline polyester
with following repeating units:
O
O
O
O
O
C
C
O
C
C
O
O
y
x
C
37
From the dichroic ratio
R
of the phenyl ring C
−
H out-of-plane bending
bands (870 and 720 cm
−
1
), the transition moment of which is perpendicular
to the long axis of the chain and the mesogenic units, the order parameter
was found to be 0.45.
IR dichroism is also used to obtain the orientation of mesogenic units
relative to the main chain in side-chain type liquid crystalline polymers.
One example was given by Xu
et al.
(1993). The polymer studied by them
was a mesogen-jacketed liquid crystalline polymer shown below:
CH
CH
n
2
CH
3
O
CO
O
O
CO
O
CH
3
In this study two IR bands, 2905 cm
−
1
and 2939 cm
−
1
were used to discuss
the orientation of the main-chain axis of the polymer molecules. They were
