Modern Polymer Spect..

2.5 Electric Field-nclticetl Orientntioi? 41
mobility in an external electric field when the mesogen is attached to a polymeric
backbone, the nematic liquid-crystalline side-chain polymer (NLCP), whose sidechain
is largely identical to the structure of the 6CPB (Figure 2-6), was investigated
under analogous experimental conditions.
2.5.2 Experimental
The schematic construction of the measurement cell and the principles of operation
are outlined in Figure 2-7. For the construction of this cell, two Ge-plates (approximately
10 x 10 x 2 mm3) are used as IR-transparent window material and electrodes.
The distance between the two Ge-plates is ensured by thin stripes of
poly(ethyleneterephtha1ate) film (7 pm) acting as spacer. To avoid the occurrence of
interference fringes in the spectra, the precise parallel aligninelit of the Ge-windows
was intentionally disturbed by two supplementary spacers of polycarbonate film (2
pnij on one side of this assembly (Figure 2-7a). To fill the assembled cell, the LCsample
was heated above the nematic-isotropic transition temperature and then
introduced by capillary forces. For the preparation of a prealigiied nionodoniain-
LC, the inner surface of each Ge-plate was covered with a thin layer of polyimide
and then rubbed in one direction with polyamide cloth (see also Section 2.4.1). This
pretreatment induced a homogeneous orientation of the investigated LC-molecules
or mesogenic functionalities (in the case of the polymer) along the rubbing direction
of the surface (Figure 2-7b). Upon application of an electrical voltage across the
Ge-plates, the liquid crystals rotate with the long axes of their niesogens into the
direction of the applied field (Figure 2-7c). Switching off the voltage leads to a
relaxational motion back to the original state. The accompanying intensity changes
were monitored with IR-radiation polarized parallel to the rubbing direction of the
Ge-windows (Figure 2-7b). A home-built heating cell has been used to keep the
investigated sample at a constant temperature within -1- 0.1 "C during the experiment
(Figure 3-8).
The principle of the electronic set-up for the step-scan measurements, and the
method of data acquisition relative to the experimental time-scale, are shown in
Figures 2.9 and 2.10, respectively. When the scanning mirror has stepped to a definite
retardation point, a control electronic unit coupled with a sine-wave generator
sends a 6 Vpp (peak-to-peak height) pulse with a frequency of 10 kHz and a definite
length (25 nis for the experiments presented here) to the sample cell to induce the
reorientation of the LC. After this period the voltage is switched off during 225 ms
to allow relaxation to the original state. Simultaneously to the application of the
electric field, the same electronic unit sends a trigger pulse to the spectrometer to
start the data collection. The data are sampled by the internal analog-to-digital
converter (ADC) of the instrument during 50 ms with 0.05 ms time resolution.
During this time interval 1000 data points are acquired for one fixed retardation
(the applied data-acquisition scheme is imposed by the capacity of the computer).
This cycle is repeated several times (five times for the presented results on 6CPB)
and the data points corresponding to the same time in the period of the event are
averaged to improve the signal-to-noise ratio. When the data collection at a given