Modern Polymer Spect..

2.5 Electric Field-Induced Orientatioii 53
GUEST-HOST SYSTEM
ZLI - 1695
Figure 2-19. Chemical structures and
transition temperatures of the investigated
nematic liquid-crystalline
guest-host system.
H
I
N
2-NAPHTHALDEHYDE
GCIEST
external electric field in a switching experiment. Time-resolved step-scan FTIRspectroscopy
has been applied to study how a solute follows the reorientation
motion of the nematic solvent [72].
2.5.4.1 Materials
A 10% solution of 2-naphthaldehyde in the nematic liquid-crystal mixture 4-cyano-
4'-alkylbicyclohexyls (ZLI-1695, Merck) (Figure 2- 19) was prepared for the measurements.
This nematic phase is very convenient as an anisotropic solvent because
of its weak IR spectrum, allowing one to obtain polarization spectra in practically
the whole mid-IR region.
2.5.4.2 Experimental
For the switching experiment, the cell described in Figures 2-7 and 2-8 was utilized
(here the path length was 10 pin). The spectral resolution was 16 cni-' and a function
generator supplied the electrical signal (Figures 2-9 and 2-10) needed to orient
the liquid-crystalline sample (16V,,, 1 kHz, 20 ms on, 260 ms off). The positive
edge of this signal also synchronized the start of the data acqusition at each step
during the complete step-scanning experiment. The time resolution of the experiment
was 100 ps; each spectrum contains coadded data of two orientation/relaxation
cycles.
The reorientation behavior of the nematic solvent was determined by the v(C-N)
and the v,(CH?) absorption bands at 2239 and 2857 cm-', respectively, whereas the
v(C=O) absorption at 1698 cm-' was utilized to characterize the segmental motion
of the solute (Figure 2-20).
2.5.4.3 Results and Discussion
The step-scan spectra of the studied solution, sampled at 0.0 and 19.5 ms, are pi-esented
in Figure 2-20. The observed intensity changes correspond to the reorientation
of the solute and solvent molecules from their preferential alignment in the
rubbing direction (0.0 ms) into the applied electric field axis i 19.5 nisi. The detailed
relative intensity changes of the three references bands versus time after the electric
field was switched off are shown in Figure 2-21. Here, the lower values for the rela-