Chemistry Reference
In-Depth Information
successful molecular engineering, an integrated analysis of these and certain
other factors are required. In polymer systems, the molecular weight is of
special importance and is accounted for in Section 3.3.
3.3. MOLECULAR WEIGHT AND MOLECULAR WEIGHT
DISTRIBUTION
Polymers, including liquid crystal polymers, distinguish themselves from
low mass compounds by their high molecular weight and the existence of
a distribution of molecular weights. In contrast to low mass compounds,
polymers usually have limited solubility, slow chain motion and high vis-
cosity, extensive chain entanglements, slow relaxation process, retarded and
non-equilibrium phase transitions, less perfect crystallization and a lower
degree of crystallinities, as well as the existence of the glassy state and
glass transitions. Thus, a thermotropic liquid crystal polymer may be non-
crystalline and forms a liquid crystal phase in the temperature region above
Tg
(the glass transition temperature) and below
Ti
(the isotropization
temperature). On the other hand, the polymer may be a semi-crystalline
two-phase system that has such a sequence of the transition processes: the
two-phase state (amorphous plus crystalline) transfers at
Tg
to the sec-
ond two-phase state (liquid crystalline plus crystalline) that transfers at
the melting temperature
Tm
to the one-phase liquid crystalline state that
in the end transfers at
Ti
to the one-phase isotropic liquid state. This
is actually a simplified picture of the process. For example, above the
glass transition there may also be a crystallization process. Yet because
of the distribution of molecular weights and other reasons, the melting as
well as the isotropization process are usually not sharp, but happen in a
broad temperature range in which at least two phases coexist. The situ-
ation becomes even more intricate if the structure and property of each
phase and the multiphase state are to be considered. The consequence
brought about by high molecular weights and by the molecular weight dis-
tributions are profound but only a few remarks are made in this short
account.
For lyotropic liquid crystals, the relationships between the chain length
(molecular weight) and the formability of liquid crystals have been formu-
lated. According to Onsager and Flory (Chapter 2), for any solutions of
rigid rods there is a critical concentration
ν
(the volume fraction of rods in
the solution) determined by the length-to-diameter ratio (
L/d
) of the rods
