Modern Polymer Spect..

2.7 Orientation of Liquid Crjvtals Under Mechuniccil Forces 77
2 00
f-
I50
0 50
0 25
7000 6500 6000 5500 5000 4500 4000 3500 3100
wavenumlxrs [cin-l]
I 00
0 50
0 00
2300 2200 2100
NIR MIR
Figure 2-46. FTIR/FTNIR polarization spectra of the undrawn LSCE system (the radiation
was polarized parallel and perpendicular to the subsequent drawing direction of the rheo-optical
experiment).
dichroic effects of the polarization spectra taken before the elongation procedure of
(Figure 2-46). In contrast, no dichroisin can be observed in the undrawn reference
polymer (see below).
Due to the large thickness of the samples, the spectrometer had been modified to
collect NIR spectra extending into the mid-infrared region ( 10 000-2000 cm-’ )
(tungsten-halogen source, CaF1-beamsplitter, InSb-detector). Despite an extensive
band-overlap in the NIR-region, several absorptions in the spectrum of Figure 2-
46 could be assigned by spectral comparison with low-molecular weight reference
compounds [36]. Here, however, only the data derived from the v(C=N) absorption
will be discussed in some detail.
2.7.2.5 Results and Discussion
In Figure 2-47 the stress-strain diagram of an elongation-recovery cycle up
to 43% strain at 27°C is shown for the reference elastomer. Further elongationrecovery
cycles do not reflect significant differences. The change in order parameter
of the v(C-N) absorption band during the mechanical treatment is shown in Figure
2-48. Throughout the whole experiment the drawing direction was the polarization
reference direction. Due to the crosslinking conditions, no orientation (S = 0) can
be observed for the original sample and uniaxial elongation leads to a linear increase
of the order parameter up to S = 0.45 at the maximum strain of 43%. During
recovery, an almost linear decrease of S can be observed with a small residual
orientation of the mesogens upon return to the original starting position. This is in
agreement with the hysteresis effect and the permanent elongation obserL ed in
the stress-strain diagram (Figure 2-47). When the structural absorbance A0 of
the v(C-N) absorption is plotted as a function of elongation 1361, an almost linear
decrease is observed which is in good agreement with the theoretically calculated