Modern Polymer Spect..

2.5 Electric Field-Induced Orientlition 51
Figure 2-17. Normalized
absorbaace/time-plot during
orientation and relaxation of
NLCP at dif'rerent temperatures
for selected absorption
bands: (x) 2926 ciii-l, i+)
2229 cm-' .
0 1 2 3 4 5 0 5 1 0
m / r m / r
between their transition moment and the molecular axis of the LC molecule. In
order to coinpare the rates of molecular motion for the differents segments, the
normalized absorbance A, was used. The A, values for bands characterizing the
niesogen and the flexible part are shown in Figure 2-17. As is evident from this
figure, no significant differences of their intensities can be detected during the orientation
and relaxation process. Thus, it must be assumed, that the mesogen and
the flexible part reorient as a rigid unit and the application of the electric field does
not lead to conformational changes in the flexible spacer. This conclusion, based on
the direct monitoring of the movement of different molecular segments, is in agreement
with the results of [63] derived from the measurement of rotational viscosity of
the same NLCP in a magnetic field and from theoretical modeling. However, it has
been reported that the application of the electric field led to a decrease of trans
conformers in a flexible spacer of an NLCP similar to that studied here. This conclusion
was based on the analysis of the 1490-1410 cni-' region which is sensitive
to the conformation of the polymethylene sequence. As has been shown recently
for a set of side-chain liquid crystalline polymers with different length of the polymethylene
spacer, this region also contains absorption bands of the aromatic ring
[64]. In our opinion, the conformational analysis based on the absorbance in this
region is thus very complex and ambiguous.
If the main chain takes part in the reorientation, it is expected that its motion will
be, at least partially, shifted in phase with respect to the side chain (spacer and
rnesogen). A detailed 2D analysis [30] in the region of the in(CH2) bands (2936 and
2863 cni-' ) does not reveal any significant asynchronicity. Hence, we may conclude
that the main chain does not take part in the reorientation, or it moves only slightly.