Chemistry Reference
In-Depth Information
majority of the polymers are copolyesters. Post-polymerization and trans-
esterification reactions may take place during characterization and process.
Lenz and Feichtinger (1979) found that extensive transesterifications and
structural rearrangements occurred in the melt of a thermotropic liquid
crystalline copolyester of 4-hydroxybenzoic. The rigid moieties tended to
bind together and form longer sequences of 4-oxybenzoic units. As a result,
the originally random copolymer became at last a block copolymer with the
loss of meltability and thermotropic liquid crystallinity. On the other hand,
a post-polymerization experiment described by Zhou
et al
. (1989) doubled
the molecular weight of a polyester in 90 minutes. The melting point and
clearing temperature of its liquid crystal phase also increased significantly.
The temperature range of the liquid crystal phase was broadened by 22
◦
C.
3.5. LIQUID CRYSTALLINE POLYMERS OF THE
SIDE-GROUP TYPE WITH END-ON ATTACHMENT
These types of liquid crystalline polymers have mesogenic moieties as side
groups or as a component part of side groups (Figure 3.1). They are tra-
ditionally named “side chain liquid crystal polymers.” Because the side
groups may not take the form of chains, we are advised to use the present
name. Excellent reviews on this subject are given collectively by the topic
“Side chain liquid crystal polymers” edited by McArdle (1989). For molec-
ular engineering, the readers are best referred to the chapters contributed
by Percec and Pugh (1989) and by Gray (1989).
In side group type liquid crystalline polymers, mesogenic units attach
to the main chain in two basic ways,
i.e.
, by end attachment (end-on) or by
side attachment (side-on). Most polymers studied are of the end-on type.
The side-on type is relatively new and only a few systems have been studied.
However, because the latter polymers have displayed unusual structure and
property, they are discussed separately in Section 3.6.
3.5.1. Synthesis
While main-chain type liquid crystalline polymers are made by step-wise
polymerizations, side-group type polymers have been synthesized by both
step and chain polymerizations of monomers with desired mesogenic side
groups. In addition, mesogenic side groups can be introduced to polymers
