Modern Polymer Spect..

2.5 Elrc tric Field-Iiidircwl Orim tri tiori 57
INFRARED BEAM
Figure 2-24. Polai-ization geometry and
mesogen alignment of the investigated
FLCP in the polarity-switched electric field
IA: polarity +. B: polarity -J.
POLARIZATION
polymer into the cell through the gap between the electrodes by capillary forces.
After filling the cell, it was slowly cooled (O.S"C/min) and an AC-voltage of 30
Vpp, IOHz, was applied during cooling with the aim to reduce defects in orientation.
The FLCP was investigated in the SmC*-phase where two surface-stabilized
orientational states exist, depending on whether the mesogens incline to the right or
to the left relative to the rubbing direction of the polyimide support (Figure 2-24).
The switching between these two states is possible by changing the polarity of the
applied field. The propagation direction of the polarized 1R beam was perpendicular
to the plane of the Ge-windows. The largest absorption intensity changes during
the switching process were obtained by fixing the polarizer at 45" relative to the
rubbing direction of the polyimide coating. All spectra were collected with a spectral
resolution of 8 cm-' and a time resolution of 0.1 ms. For a more detailed
characterization of the reorientation mechanism, the experiments were carried out
at different temperatures (28 "C, 42 "C and 47 "C ( f 0.1 "C)). At the lower temperature
only rapid-scan measurements were required during orientation at 20 V (DC),
whereas at the two higher temperatures the step-scan technique had to be applied
due to the much faster segmental movement. Here, for the switching between the
two orientational states a bipolar rectangular signal with a voltage of 20 Vpp and a
frequency of 20 Hz and 100 Hz was applied at 42 "C and 47 "C, respectively (Figure
2-25). The spectral data collection was synchronized with the negative edge of the
electric signal.
2.5.5.3 Results and Discussion
Polarization spectra of the preoriented FLCP are shown in Figure 2-26. For the
characterization of the mesogen and the flexible spacer, the 1607 cm-' ( Y(C=C),,)
and 2926 cm-' ( vas(CH?)) absorptions, respectively, were chosen. Unfortunately,
no bands free of overlap can be attributed to the backbone. The band at 1020 cm-'
(if( Si-0-Si)) is strongly overlapped by the i!,(C-O-C) absorption of the ester group
and the band near 807 cm-' is also superimposed in a rather complex manner.
For the characterization of the methyl substituents of the siloxane-backbone, the
h'(Si-CH3) vibration at 1260 cm-' was chosen, although it contains a contribution
from the I*~,,(C-O-C) vibration related to the mesogen. The 1607 and 1260 cm-'
bands have parallel dichroism and the 2926 cm-' band has perpendicular dichro-