Modern Polymer Spect..

Figure 2-12. Stack-plot of spectra measured during the electric field-induced orirnration and duriiip
part of the relaxation of thc 6CPB molecules (41 -C'.
Figure 2-13. Relative absorbance/time-plot during orientation and relaxation of 6CPB (41 C!
for selected absorption bands: (0) 2929 cni-'. (7) 2858 cm-', (012230 cin-l. I + ) 1605 cm-'. I x
1503 cm-'. (+) 757 cni.~'.
approximately 1.5 Vpp. In the homeotropic type of orientation, the long axis of the
LC molecule is normal to the electrode surface and parallel to the IR beam propagation.
According to geometrical considerations [62] a reorientation to this honieotropic
state thus leads to intensity changes of the Ail-component only. This component
asymptotically approaches the Al intensity value at sufficiently high fields.
For this reason, all time-resolved measurements have been performed with light
polarized parallel to the polyimide-layer rubbing direction.
A stack-plot of time-resolved spectra recorded during the honiogeneous-homeotropic
transition and part of the relaxation process is presented in Figure 2-12. A
good signal-to-noise-ratio is observed for all spectral regions (except when the
absorbance exceeds a value of 2). Relative absorbance changes A, for selected
absorption bands were calculated from:
where A, is the absorbance at time t and A0 is the absorbance just before reorientation.
These relative intensity changes of selected absorption bands are presented
in Figure 2-13 and remarkable changes are observed only after a definite induction