Chemistry Reference
In-Depth Information
Figure 3.1:
Liquid-crystalline polymers in which the mesomorphic sequences occur in the chain
backbone, in the side chains, or in both.
poly(diethylsiloxane) (PDES) [-Si(OC
2
H
5
)
2
O-],
99
which forms a nematic
mesophase. Relevant studies have include (i) heat capacity,
144
(ii) dielectric
relaxation,
145
(iii) nuclear magnetic resonance (NMR) characterization,
146
(iv) structural changes during transitions,
147
(v) effects of stretching on
the transitions,
148
(vi) optical properties such as refractive indices and op-
tical rotations,
149
(vii) thermoelastic (force-temperature) properties,
150
(viii) mechanical properties in general,
151
(ix) end linking of the chains
into “model networks,”
152-153
(x) effects of molecular weight,
154
(xi) charac-
terization using atomic force microscopy
155
or x-ray diffraction,156
156
(xii)
thermopolarization effects (cooling a heated sample in an electric field),157
157
(xiii) solution properties
26
and swelling behavior,
153
(xiv) segmental orien-
tation,
158
(xv) responses of guest chains in deformed PDES elastomers
159
and the properties of blends.
160
One item of great interest with regard to these materials is the temper-
ature at which the nematic liquid-crystalline phase becomes isotropic.
99
Table 3.2 lists such
isotropization
(“clearing”) temperatures Ti
i
for a variety
of symmetric polysiloxanes polymers having repeat units [Si((CH
2
)
m
CH
3
)
2
-
O-]. Although the focus is on PDES (
m
= 1), PDMS with
m
= 0 is included
for comparison. The table includes polymer molecular weights (M) where
important, and some relevant melting points, T
m
. PDMS is very different
from PDES in that it does not show a liquid-crystalline phase. This situa-
tion is explained in connection with table 3.2. PDES shows values of
T
i
that decrease with decreasing in molecular weight. In fact, the liquid-
crystalline phase
does not form at all
if
M
is below approximately 25,000 g
mol
-1
.
99
This situation is in sharp contrast to the behavior of low molecular
weight liquid-crystalline molecules; many cholesterol molecules that show
mesophases have molecular weights down in the hundreds.
161
