Chemistry Reference
In-Depth Information
slope. The two lines cross in such a way that the predicted value of
T
i
for
PDMS lies
below
the melting temperature,
T
m
, meaning that PDMS does not
show a liquid-crystalline phase because it crystallizes before it gets to
T
i
.
99
Another interesting feature of the table pertains to the melting points
for polymers having
m
= 6-9. The side chains are now long enough to crys-
tallize. Such side-chain crystallization has also been seen in polysilane
homopolymers
169
and chemical copolymers of polyethylene,
170 -171
(so-
called linear low-density polyethylene). In any case, such crystallization
presumably could interfere with the mesophases that might otherwise
form.
99
Some polysiloxanes have cyclic groups in the backbone, typically cyclics
of -Si(CH
3
)
2
-O- units of various sizes, or such siloxane units mixed with
some carbosiloxanes (with additional -CH
2
- sequences).
16 4, 172-176
he
cyclic portions can add considerable stiffness, resulting in isotropization
temperatures above the decomposition temperatures.
One aspect of the mechanical properties of these polymers is the in-
crease in the isotropization temperatures with increasing elongation.
177-179
Interesting examples are the elongation results reported for PDES as a
function of mesomorphic structure.
148
The tensile curves differ greatly de-
pending on the amount of mesophase present.
99
Specifically, for low meso-
phase content, the isotherms are similar to those of natural rubber, with
upturns in force at high elongations. In the case of larger amounts, the
curves show yield points akin to those shown by partially crystalline poly-
mers. Crystallinity may be either present initially or induced by the defor-
mation. Elongation-retraction curves demonstrate that formation of a
second phase leads to irreversibility in the stress-strain isotherms.
148
he
larger the elongation during the deformation, the larger the irreversibility
(hysteresis) upon retraction.
In some cases, smectic ordering has been observed.
180
There is also interest
in phenylpyrimidine liquid-crystalline hosts
181
and ferroelectric organosilox-
anes.
182
Results on chain dynamics have also been reported for polysiloxanes
containing
p
-phenyleneterephthalate,
183
or polyimide mesogens.
184
3.4.2 Side-Chain Liquid-Crystalline Elastomers
The units giving rise to the liquid-crystalline behavior can be in the side
chains.
99,
185-187
Although most of the polymers studied have been linear,
there has been some work on hyperbranched
188
or comb-like structures.
189
The orientation of the mesogenic groups is important.
190-191
Frequently back-
bones include siloxanes and acrylates,
192-195
but a variety of other structures
