Chemistry Reference
In-Depth Information
Chapter 6
Physical Properties and Applications
of Liquid Crystalline Polymers
6.1.
ELASTIC PROPERTIES OF LIQUID
CRYSTALLINE POLYMERS
According to the elastic continuum theory of liquid crystals which was intro-
duced in Chapter 1, the three kinds of deformations can be described by
three elastic constants,
K
11
(splay),
K
22
(twist) and
K
33
(bend). In the case
of small molecular mass liquid crystals, the three constants are mainly
determined by the chemical composition of the liquid crystalline molecules.
Among them,
K
22
is the smallest while the other two are approximately
close. All three elastic constants are of the order of 10
−
12
N. The elastic con-
stants of some important liquid crystals are listed in Table 6.1. Each kind
of liquid crystals is a mixture of R5-pentyl and R6-hexyl homologues in the
ratio of 40 : 60. The data are obtained at the temperature of
T
=
T
c
−
10
◦
C
where
T
c
is the clear temperature.
For liquid crystalline polymers, the elastic constants are determined not
only by the chemical composition but also by the degree of polymeriza-
tion,
i.e
., the length of the molecular chain. One main aim of this section
is to address the effects of molecular chain length on the elastic constants
of liquid crystalline polymers. Figure 6.1 shows the three typical deforma-
tions of nematic liquid crystalline polymers. The length and flexibility of
liquid crystalline polymers make the elastic constants of liquid crystalline
polymers quite different from those of monomer liquid crystals.
de Gennes (1977) argued that a splay deformation is unlikely to occur
in the rod-like liquid crystalline polymers because of the diculty of main-
taining the splay configuration while keeping the molecules from conflicting
and overlapping. The molecular chain ends of liquid crystalline polymers in
285
