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have also been studied, including amphiphilics. 141, 196 -209 Side chains have
included chiral groups 210-212 and groups capable of hydrogen bonding. 213
The side chains in these structures can rearrange either parallel or per-
pendicular to the deformed chain backbone, 214 as illustrated in figure 3.2.
The outcome depends on the nature and length of the flexible spacer con-
necting the mesogenic groups to the chain backbone. As expected, the
physical properties can become strongly anisotropic. 215
Side-chain liquid-crystalline materials can be oriented by imposing an
electric or magnetic field.216 216 The chains can also be aligned when deformed
(generally in elongation but also in some cases in compression) and cross
linked into network structures. The focus in these experiments was how
the mechanical deformation affected the nature of the mesophase (in par-
ticular its axial direction relative to the direction of the strain) and its
isotropization temperature. The studies generally involved measurements
of both stress and birefringence as a function of strain, and the ratio of
the former to the latter (the “stress-optical coefficient”). The mesogenic
behavior of such networks obviously depends strongly on their structures.
The effects of degree of cross linking, and composition in the case of copo-
lymers, for example, have been documented. 217
The phase transitions depend significantly on spacer length, as has
been demonstrated for oligo-oxyethylene spacers. 218 Closer coupling be-
tween the mesogenic groups and the polymer backbone tends to make the
system more sensitive to the mechanical deformation, at least in the case
of methylene groups in the spacer. 219
Relatively high degrees of cross linking can be introduced without de-
stroying the liquid crystallinity. 222 Cross linking can generally be induced
Figure 3.2:
Approximately parallel and perpendicular arrangements of mesogenic side groups on a
chain backbone stretched in the vertical direction.
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