Chemistry Reference
In-Depth Information
repeat units between barium, gallium, zinc, or cobalt cations, and a con-
trolled number of such cations per chain.
108-110
Groups that are chemically unreactive have also been added to PDMS
chains. For example, perylene groups have been placed at the ends of the
chains to induce vesicle formation,
111-112
as have been modified bichromo-
poric perylenes to study ordering effects.113
113
Similarly, anthracenyl groups
were introduced so that fluorescence spectroscopy could be used to inves-
tigate glass transitions and melting of polysiloxanes.
114
Related molecules
such as pyrene have been introduced by simple doping techniques.
115
he
photochemistry of polysiloxanes has been reviewed recently.
116
Polysiloxanes have also been pyrolyzed to give ceramics and organic/
inorganic hybrid materials.
117-118
The general topic of pre-ceramic poly-
mers is discussed chapter 9.
119
3.3 DENDRIMERS AND HYPERBRANCHED POLYMERS
The literature
124 -127
describes a number of dendrimers and the closely re-
lated star-like
120-123
polysiloxanes. The hyperbranched polysiloxanes
128
are the primary example of more random structures. Although the em-
phasis has been on synthesis and characterization,
129-136
modeling on hy-
perbranched polymers has also been carried out.
137
Some of the most
interesting species involve polysiloxane chains.
124-126, 138-139
Star polymers,
some with nanosized silica cores, have also been synthesized.
120, 122-123
Hyperbranched polysiloxanes have been prepared with controllable
molecular weights and polydispersities,
128, 133, 140
with epoxy terminal
groups
132
; some are UV-curable
130
and some serve as a source of molecular
silica.
129
Hyperbranched polysiloxanes have also been used in the sol-gel
preparation of polypropylene/silica nanocomposites.
134
Heavily branched polysiloxanes can be prepared by a variety of tech-
niques,
131
some of which generate hybrid mesostructures.
141
Branched and
linear chains have been compared with regard to their interactions with
solvents.
142
3.4 LIQUID-CRYSTALLINE POLYMERS
3.4.1 Main-Chain Liquid-Crystalline Elastomers
The sequences that give rise to the liquid crystallinity can be in the chain
backbone, in the side chains, or in both,
143
as shown schematically in
figure 3.1. The polymer in the category that has been the most studied is
