Modern Polymer Spect..

40 2 Segnwital Mobility of Liquid Crystuls aid Liquidcrystalline Polymevs
6CPe
0
C H 3 - ( C H z ) e C - O D C = N
5
44.5°C 48.5OC
K - N - I
NLCP
- 0
2, H -&-O-(CHZ)~$$C O a C - N
-.
CH2
-n 3S°C 130°C
G - N - I
Figure 2-6. Sti ucture aiid transition temperatures
of the investigated low- aiid highmolecular
weight LC systems I6CPB and
NLCP).
polymers and liquid-crystalline guest-host system) in switch-on/switch-off experiments,
and on the other hand we will describe the structural consequences incurred
by a ferroelectric side-chain polymer in a poled electric field.
Nematic liquid-crystalline molecules, which consist of a rigid inesogenic unit with
an attached flexible part (sequence of methylene units, for example) (Figure 2-6),
are among the most characteristic thermotropic systems. FTIR spectroscopy has
been applied to study the mechanisiii of orientation of the liquid crystals in an
electric field; however, at present the experimental data are not always unequivocal,
and conclusions obtained are not in complete agreement with each other. T~us,
some authors did not detect any differences in orientational rates of the rigid and
the flexible part of the LC molecule [16, 51-54], which is in accordance with the
commonly accepted mechanism of the orientation of an LC as a cooperative
motion of a molecular ensemble [l, 2, 381. Other authors reported, that the motion
of the flexible part precedes that of the inesogen in both orientation and relaxation
[29, 55-57], and some investigators suggested that the motion of the mesogen precedes
the motion of the flexible part [58, 591. With respect to a more detailed exploitation
of the spectroscopic data in terms of the synchronization of the segmental
motions of different functionalities of the investigated liquid-crystalline systems,
2D-correlation analysis [ 16, 30, 60, 611 has proved an extremely valuable tool. In
view of the detailed discussion of this mathematical technique in Chapter 1 of this
book; however, this topic will not be treated here, although several results extracted
from the presented spectroscopic data are based on the application of the 2Dcorrelation
formalism.
2.5.1 Materials
To study the addressed phenomena, a commercial sample of p-cyanophenyl p-nhexylbenzoate
(hereafter abbreviated as 6CPB) (Figure 2-6) was obtained from
Roche (Basel, Switzerland), and used without further purification [53, 541. The
transition temperatures of this LC are also presented in Figure 2-6. It should be
mentioned, that the nematic phase will be retained during cooling down to 7 "C
below the crystal-nematic transition point. To demonstrate the reduction of