Chemistry Reference
In-Depth Information
Table 5.2.
EXAMPLES OF LIQUID-CRYSTALLINE
POLYSILOX A NES
a
Polymer Repeat Unit
~
T
, °C
Transition
[-Si(CH
3
)
2
O-]
(Reference polymer)
-40
Cryst →Isotropic
[-Si(C
2
H
5
)
2
O-]
-60
Cryst →Cryst'
“
0
Cryst' → Mesomorphic
“
40
Mesomorphic → Isotropic
[-Si(
n
-C
3
H
7
)O-]
-55
Cryst →Cryst'
“
60
Cryst' →Mesomorphic
“
205
Mesomorphic → Isotropic
a
Reproduced by permission of the American Chemical Society.
class of semi-inorganic polymers, the polyphosphazenes, are also rela-
tively flexible, and show similarly interesting behavior.
17,
133
Some polysi-
loxanes form liquid-crystalline phases because of the presence of relatively
stiff side chains.
134-141
This group has been studied with regard to the
effect of deformation of the elastomeric polysiloxane phase on the meso-
morphic behavior exhibited by the side chains.
Although the polysiloxanes are much more flexible than their organic
counterparts, the polysilanes seem to display intermediate properties. It
is instructive to compare a polysilane with its hydrocarbon analogue in
terms of chain flexibility. For example, relevant conformational energy
calculations have been carried out on polysilane itself [-SiH
2
-]
x
.
142-143
Energy maps
143
for two consecutive results skeletal rotation angles
φ
sug-
gest that the lowest energy conformation should be a sequence of
gauche
states (
φ
= ± 120°) of the same sign.
142, 143
This conclusion is in contrast to
polyethylene [-CH
2
-]
x
, which has a preference for
trans
states. Such pref-
erences generally dictate the regular conformation chosen by a polymer
chain when it crystallizes. Polyethylene crystallizes in the all-
trans
planar
zig-zag conformation
16,
20,
21
It would be interesting to determine whether
polysilane crystallizes in the predicted helical form generated by placing
all of its skeletal bonds in
gauche
states of the same sign. The calculations
also predict that polysilane should have a higher equilibrium flexibility
than polyethylene.
143
Solution characterization techniques could be used
to test this expectation. Dynamic flexibility, can also be estimated from
such energy maps by determining the barriers between energy minima.
Relevant experimental results could be obtained by a variety of dynamic
techniques.
144
