Modern Polymer Spect..

2.5 Electric Field-IwrElrced Orientation 47
period of several milliseconds after application of the electric field. This phenomenon
is well described in the literature [46]. The intensities of the 2230, 1605 and
1503 cni-' bands decrease, whereas that foi- the 757 cm-' band increases M,hen the
field is switched on. This indicates that the mesogens 1-eorient with their long axes in
the direction of the applied field (Figure 3-7c). The process of orientation at the
given experimental conditions takes about 25 ms. After switching off the field, thz
relaxation to the initial homogeneous state (Figure 2-7b) takes place. The intensity
changes of the 2929 and 2858 cnir' bands reflect the orientation and relaxation
process of the flexible part of the LC molecule. For an exact quantitative comparison
of the orientational and relaxational rates of the rigid and flexible segments of
the LC molecule, the order parameters of the different bands had to be calculated
as a function of time. However, this is possible only for the case of a pure homogeneous
orientation (it., the director of the LC monodomain and, hence) the axis of
uniaxial orientation is perpendicular to the IR beam propagation) or for the case of
a pure homeotropic orientation (where the axis of uniaxial orientation is parallel
to the beam propagation). In intermediate states the orientation of the director is
not constant. Thus, near the surface, due to strong interactions, the molecules are
oriented perpendicular to the electric field whereas at the center of the cell they are
oriented parallel to the electric-field direction [38]. For a qualitative compai-ison
of the orientational rates of the rigid and the flexible parts of the investigated LC
molecule a normalized intensity A,, was utilized which has been calculated from:
where At is the band peak absorbance at time t, A0 is the value before the npplication
of the electric field, and A,,, is the corresponding absorbance value at t = 25
ms. The result for the 2929 and 2230 cm-* bands are presented for a temperature of
41 "C in Figure 2-14a. According to these data there are no detectable differences in
A, both, during the orientation and relaxation process, for the rigid and the flexible
segments of the LC molecule. This means that under the given experimental conditions
and with the evaluatioii by the usual one-dimensional technique, the different
segments of the LC molecule orient and relax at comparable rates. The same
conclusion can be derived from Figure 2-14b where the corresponding A,,-values
are presented for 45°C. Due to the small temperature interval, the diRerence in
orientational behavior is not significant. However, it is not possible to perform
the experiment in a wider temperature range because of the narrow temperature
interval of the investigated mesophase.
The results of similar rates for the reorientation of the flexible and rigid segments
of the 6CPB molecule in an electric field are in accordance with the coninioiily
accepted mechaiiisni of orientation by a cooperative motion of the LC molecules [ l~
2, 381. However, in recent investigations it has been reported, that the response of
the mesogens and the flexible part on the applied field may be different [29, 55-59].
If these observations are correct they could be the basis of a principally new
description of LC orientation on a molecular level. Two explanations for thc Jisagreement
of o~iresults with these views can be given: