Chemistry Reference
In-Depth Information
samples of higher molecular weights. This and related findings have ren-
dered partial explanations for the molecular weight effect on the stability
of the polymeric liquid crystal phase.
Very few reports have been given on the effect of molecular weight
distribution. In the restudy on Ober's polymer mentioned above in this
section, we see also the influence of the molecular weight distribution on
the thermal stability of the liquid crystal phase. The unfractionated sam-
ple has a broader molecular weight distribution and a broader temperature
range of the nematic phase (148
◦
C to 170
◦
C). On the other hand, the
fractionated sample with a little higher viscosity (0.39) has narrower dis-
tribution and a narrower temperature range of the mesophase (149
◦
Cto
159
◦
C). However, in another study on a side group type liquid crystalline
polymethacrylate, the two samples with very close values of molecular
weight (
Mn
= 9860 and 10300) but significantly different molecular weight
distributions (
Mw/Mn
=4
.
21 and 1.70) were found to have the same
Tg
(= 206
◦
C) and the same
Ti
(= 344
◦
C).
For polymers, not only is the thermal stability of the mesophases impor-
tant, the consequence of molecular weight and its distribution on the process
and application properties is also of primary significance. For instance,
higher polymers result in higher melt viscosity and slower response to
applied physical fields. Thus, for certain applications where fast responses
are essential, high molecular weight liquid crystals may not have any supe-
riority. On the other hand, higher molecular weight polymers usually result
in superior mechanical properties. It is known that, e.g., the strength of the
fiber spun from 2-phenyl-1,4-phenylene terephthalate with the molecular
weight of 5000 is 0.6 Gpa, only about one sixth of that with a molecular
weight of 40000, or that from the polymer of m.wt. 30000 is only one third of
the one from m.wt. 300000 (Jackson, 1988). High molecular weight is thus
required for polymers used as high-strength and high-modulus materials.
However, things are more complicated even in this case. In a study on the
copolymers from 2,6-naphthalenedicarboxylic acid (20 mol%), terephthalic
acid (20 mol%), 1,4-phenylene diacetate (40 mol%) and 4-acetoxybenzoic
acid (60 mol%), Jackson (1992) found that the tensile strength and flexural
properties of the injection-molded liquid crystalline polymer first increased
with molecular weight as would have been expected, but decreased markedly
when the absolute molecular weight was over 40000. Light scattering data
indicate that this polymer behaves as a rigid rod at lower molecular weights.
It becomes worm-like and forms chain entanglements at the higher molecu-
lar weights. The measurement of the Hermans orientation factors through
