Modern Polymer Spect..

2.7 Orientatioiz of Liquid Crystals Under Mechanical Forces 79
Figure 2-49. Stress-strain diagram
of an elongation-recovery cycle of
the LSCE at 27 “C.
0 10 20 30 40
STRAIN [“/.I
I). so-
Figure 2-SO. Order parameter/
strain-plot for the vjC_N)
absorption corresponding to
the elongation-recovery cycle
shown in Figure 2-49 (see
text).
s 0.40-
0.30.
0. 20-
0.10~
S’I’RALN [%.I
During interval I the stress increases linearly with strain and no significant
changes in the mesogen orientation can be observed.
When the external stress (a,) reaches the internal stress (0,) of the LSCE systein
(induced by the second crosslinking procedure) [lll] at about 15% strain, the
stress-strain curve levels off and siinultaneously the order parameter decreases
rapidly towards zero (interval 11) (Figures 2-49 and 2-50).
During interval 111, the reorientation of the niesogens in the direction of the
external field takes place until maximum elongation. This procedure is accompanied
by a slight decrease in stress due to the cooperativity of the reorientational
motion after exceeding the internal stress of the LSCE.
Depending on the experimental conditions (temperature, niaxiinuni strain), the
described phenomena are completely reversible upon recovery (Figure 2-51).
Due to the intrinsic anisotropy in the original LSCE network, the spectroscopically
monitored inversion point of the order parameter (S = 0) could principally
be interpreted by the three different mesogen alignments shown schematically in
Figure 2-52 [36]. For the elucidation of this question the structural absorbance A,,