Modern Polymer Spect..

48 2 Segliientril Mobility qf Liquid Crystals arid Liquid-Cr.~~stiillirie Polymers
ORIENTATION c+
RELAXATION
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I0 20 30 40 5n
TIME lms
n 10 213 310 40 50
TlMElms
Figure 2-14. Normalized absorbaiice/time-plot during orientation and relaxation of 6CPB at two
different temperatures for selected absoiptioii bands: (0) 2929 cm-', (0) 2230 cin-'.
1. The effect of different response of the flexible and the rigid part of the LC
molecule towards application of an electric field is not general. This is supported
by results [56], where this phenomenon is detected under certain experimental
conditions only. Moreover, up to now most experiments have been performed
on one LC (4-n-pentyl-4'-cyanobiphenyl, 5CB) only, which has a different
structure compared to the LC molecule investigated here.
2. The effect is too small to be detected under the given experimental conditions
with conventional data analysis. To detect this effect, Urano et al. [55, 561 have
applied tiine-resolved spectroscopy by using a modified dispersive instrument.
With this instrument, absorbance changes down to lop6 a.u. have been detected.
However, the time requirement for the experiment in the step-scan inode
depends on the period of the event (0.25 s in our case). To reach such a high
signal-to-noise-ratio we had to average the data of many more events at the
individual retardation points. For example, the data collection for 2368 retardation
points over 100 events in each point (compared with our five events) would
require approximately 17 h of experimental time. Due to uncontrolled changes
in the environmental conditions and drifts of electronics during such a time
period, the main requirement of the step-scan technique, i.e., the reproducibility
of the periodical event, could be violated.
To extract more information, the data were processed with the newly developed
method of two-dimensional spectroscopy [60, 6 11. This technique is especially useful
for the detection of small differences in the orientational rates of different molecular