Modern Polymer Spect..

main chain
rt-
YH2
spacer rnesogeri
+A 0
H-C-C-O-CH2 ) -O-O-$-O-O-CN
95 Figure 2-4. Structure and transition temperatures
of the investigated side-chain liquid-crystalline
polyacrylates.
2.4.2 Results
Despite identical sample pretreatment, gradually decreasing dichroic effects could
be observed for the LC-polymers with decreasing spacer length. The FTIR polarization
spectra of tlie P6CN- and P2CN-systen1, measured with radiation polarized
parallel and perpendicular to the rubbing direction of the polyimide surface,
respectively, are shown in Figure 2-5. From these data the coupling effect of the
spacer between the main chain and the mesogen is clearly evident. Thus, the hesamethylene-spacer
almost completely decouples the mesogen from the main chain
and allows the induction of an extremely high orientation of the rigid mesogen
(S(v(C3N)) = 0.62) (see also insert in Figure 2-5). The short diniethylene-spacer,
on the other hand, strongly links the mesogen to the polymer backbone and no
orientation of the mesogen on the anisotropic surface can be observed. The spectra
of the system with the tetramethylene-spacer (not shown here) display intermediate
effects (S(i(C-N) = 0.46). A detailed analysis of the dichroic effects in the polarization
spectra of the whole series in terms of the relative orientation of the mesogen,
spacer and main chain has been given in [36].
2.5 Electric Field-Induced Orientation and Relaxation
of Liquid-Crystalline Systems
The reorientation of liquid crystals in an external electric field is the basis of their
technical application in liquid-crystal displays (LCDs) and has been treated in
numerous books and reviews [I-3, 12, 38,46-501. This section is intended to denionstrate
tlie application of time-resolved FTIR spectroscopy for a better understanding
of tlie electro-optical performance of liquid-crystalline systems. Due to tlie
reversible character of the orientational and relaxational motions of liquid crystals
under tlie perturbation of an electric field, the time-resolution of the FTIR measurements
can be extended down to the microsecond range by the implementation
of tlie step-scan technique (see Section 2.2). Thur, an insight on ;I molecular level