Chemistry Reference
In-Depth Information
the isotropization was 0.6 kJ/mol; If n = 41, the corresponding parameters
increased to 59
◦
C, 114
◦
C, and 0.6 kJ/mol respectively. These parameters
were found to increase to 62
◦
C, 116
◦
C, and 0.9 kJ/mol with the further
increase in the degree of polymerization to n = 114.
CH
2
CH
n
3
.
13
COO
CH
2
CH
2
O
COO
OCH
3
Stevens
et al
. (1984) carried out a more detailed study on the effect of
molecular weight. Several side-group type polymers,
, with polysilox-
ane backbones and flexible spacers of different number of methylene units
were studied:
3
.
14
CH
3
n
Si
CH
3
Si
O
CH
3
Si
O
3
3
3
.
14
CH
2
CH
2
CH
2
O
COO
OCH
3
m
2
m
=
3
6.
The degree of polymerization of these polymers varied from n = 1 to
n = 100.
A few conclusions were obtained from this study. For all four systems
with m = 3 to 6, the properties change most significantly when the degree
of polymerization increases from 1 to 10. For example, for the polymer
of m = 3, the monomer and the dimer do not form liquid crystals. The
trimer forms a nematic liquid crystalline phase between the glass transition
(
Tg
= 270
◦
K) and isotropization (
Ti
= 285
◦
K). The mesophase is sta-
ble within this temperature interval. With the further increase of n to 10,
both
Tg
and
Ti
shift to higher temperatures, but the latter increases much
faster than the former so that the temperature range of the stable liquid
crystalline phase also increases with n. Above n = 10, the transition temper-
atures increase only slightly until stable values are reached at approximately
n = 100. For the polymer of m = 3 and n = 95,
Tg
and
Ti
were found
to be 290
◦
K and 356
◦
K respectively. The other polymers with different m
have similar behavior. However, the polymer with the longest spacer in the
series, m = 6, forms a smectic phase in addition to a nematic phase. The
influence the flexible spacer has on the property of the side-group type liquid
crystalline polymers will be discussed in Section 3.5. Stevens and coworkers
