Modern Polymer Spect..

2.7 Orieritation of Liquid Ciysttrls Under Mechanictrl Forces 71
Figure 2-40. Experimental set-up for the shearing experiment
combined with POM iiieasureinents (A: microscope,
B: goniometer, C: fixed NaCl crystal, D: shearing NaCl
crystal, E: temperature control).
polarizer for 90" the spectrum with perpendicular polarization relative to the
shearing direction was recorded.
2.7.1.3 Results and Discussion
The question to be clarified by the FTIR spectroscopic polarization studies was
whether:
the interaction between the polymer chain and the side-on linked mesogenic dye
functionalities dominates and the chroniophores orient parallel to the polyi~ier
chain [ 107, 1081 and thus perpendicular to the non-chroniophoric mesogens
(Figure 2-41a); or
the interaction between the different mesogens (non-chromophoric and chromophoric)
is dominating and leads to their parallel alignment (perpendicular to the
chain direction) (Figure 2-41b).
In order to determine the orientational behavior of the main chain, the spacer and
the mesogenic and chroniophoric side groups relative to the shearing direction,
detailed band assignments of the monomers, homopolymer and copolymers preceded
the dichroic measurements of the sheared polymers [36, 1061. Based on these
investigations, the polarization spectra of the sheared copolymers I and I1 (Figure
2-42) yielded the surprising qualitative result, that the orientation of the side-on
fixed chroniophores is doiniiiated by the interaction with the mesogenic groups and
leads to the structure represented in Figure 2-41b. Thus, the dichroisiii of specific
absorptions indicates only low orientation effects for the main chain parallel to the
shearing direction, whereas the mesogens and the spacers clearly orient preferentially
perpendicular to the direction of shear. The o-dichroisni of the v,(NO?)-
absorption (Figure 2-42) supports the perpendicular alignment of the chromophore
long axis relative to the shearing direction, and thus the validity of the model shown
in Figure 2-4 1 b [36]. These dye-containing copolymers luay therefore not be LIS~~LI~
for nonlinear optics [ 1061.