Modern Polymer Spect..

50
w
CJ 1.50
z
3 1.00
2
s
0. so
w
5 0.00
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g -0.50
ORIENTATION
RELAXATION
0 25 SO 75 100 0 100 200 300 400
TIME / s
TIME / s
$ 1.50
z
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f 0. 50
*
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k-
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Fl -O. 50
0.0 I. 5 3.0 4. 5 0
TIME / s
5 10 1s 20
TIME / s
Figure 2-16. Relative absorbance/time-plot during orientation and relaxation of NLCP at different
temperatures for selected absorption bands: (0) 2926 cm-', (0) 2229 cm-', (*) 757 cm-I.
for this band is also nearly prependicular to the traiis-polymethylene chain, this
indicates that the spacer is oriented preferably parallel to the rubbing direction of
the polyimide support and the mesogen. We used the 2130 and the 757 an-' bands
for the characterization of the movement of the mesogen and the 2916 cm-' band
for that of the spacer. Unfortunately, the bands which characterize the backbone
are heavily overlapped with absorptions of the spacer and they are much less
intense than the corresponding spacer bands (a monomeric unit contains one
methylene group in the iiiaiii chain und six in the spacer). Therefore, in the case
of this polymer, we cannot follow the inoveiiient of the backbone. Owing to the
experimental geometry, the reorientation of NLCP led only to changes in intensity
of light polarized parallel to the rubbing direction. For this reason, as before, we
used only polarized radiation to monitor the molecular motion of the NLCP.
The relative intensity changes during orientation and relaxation at different temperatures
for selected absorptions bands are shown in Figure 2-16. As can be seen
from this figure, the orientation and relaxation rates incrcased more than 20-fold
when the temperature increased from 80 to 120 "C. At temperatures close to the
glass transition, the orientation times increase to a few tens of minutes. This is
apparently related to the decrease of viscosity with increasing temperature. At each
temperature the relaxation takes more time than the reoreination. The A, values
change less at higher temperatures than at lower temperatures (Figure 3-16). This is
the consequense of the decrease in the order parameter of the NLCP with increasing
temperature. The values of A, for different absorption bands depend on the angle