Modern Polymer Spect..

2.5 Electric Field-Itzduced Orientation 45
Figure 2-11. FTIR polarization
spectra of 6CPB (41 "C) taken
with radiation polarized parallel
(upper spectrum) and perpendicular
(lower spectrum) to the
rubbing direction of the polyimide
surface.
3300 3000 2750 2500 2250 2000 1750 1500 1250 1003 600
Wavenuiber cm-'
Table 2-1. Band assignment and waveiiuiiiber position of selected absorption bands for 6CPB (as =
antisymmetric; s = symmetric; ar = aromatic; oop = out-of-plane).
Band assignment Wavenumber (cm- ' )
2929
2558
2230
1742
1605
1503
1415
1464
1264
1207- 101 7
843
757
with light polarized parallel and perpendicular to the rubbing direction of the polyimide
layer, respectively). This means, that the long axis of the mesogenic unit is
oriented predominantly parallel to the rubbing direction (Figure 2-7b). The 757
cni-' band (ring CH out-of-plane defoimation) exhibits a perpendicular dichroism
with Al > A,,, since the transition moment for this band is perpendicular to the
ring plane and hence, perpendicular to the long axis of the mesogen. These four
bands are characteristic of the mesogenic units. For the characterization of the
flexible part of the LC-molecule the 2929 cm-' and 2858 c1n-l bands (antisymmetric
and symmetric v( CH2)-stretching vibrations) were evaluated. All the selected
bands had absorbance values < 1 for the given cell thickness.
When a voltage higher than the threshold value is applied to the electrodes, the
LC molecules undergo a transition from the homogeneous (parallel to the rubbing
direction) to the honieotropic (parallel to the electric field) orientation. According
to our data for 6CPB, the threshold voltage to induce such a reorientation is