Modern Polymer Spect..

70 2 Segrnentrrl Mobility of Liquid Crystals and Liquirl-Ci.vstalliiit1 Po1jwiei.r
r-
-45 % -47 70 -8 %
- [ .I-[ 7- f - .}
4H2 41
C=O
c=o
I
?.
H 3C-FH
C=O
I
9
CH2 -CH3
chiral mesogen
phase transition
main chain + mesogen
copolymer
98
(- 8”h) Figure 2-39. Chemical structure, coiiiposition
and phase-transition temperatures of the
different investigated copolymers.
+ ChreNphore n
introduction of chromophores with extended n-conjugation it was attempted to
combine the ferroelectric properties of the mesomorphic phase with the NLOproperties
of the chromophore. The synthesis has been described in detail 11061, and
Figure 2-39 shows the structure and composition of the investigated homopolymer
(containing no chromophore) and the modified copolymers alongside their corresponding
phase-transition temperatures.
2.7.1.2 Experimental
Oriented polymeric films were obtained in a mechanical force field by applying
a shear stress. The NaCl plates used as sample supports were coated with a thin
polyiinide layer to improve the surface properties. The shearing process along a
defined direction was first performed in the isotropic state above the clearing temperature
and then at the isotropic/SmC*-transition. Afterwards, the temperature
was gradually decreased to room temperature. The mechanical process was simultaneously
analyzed using polarization optical microscopy. The experimental set-up
is shown in Figure 2-40. For the subsequent dichroic measurements the samples
were mounted in the spectrometer so that the shearing direction and the polarization
direction of the incoming radiation were initially parallel. By rotating the