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investigated, see Fig. 38 and Table 1. This family of materials were found to
exhibit only smectic A phases, however, as the aliphatic chain length of the
spacer was increased, the mesophase stability increased. All of the family ex-
hibited glassy phases on cooling the smectic A phase, interestingly the glass
transition temperatures were below room temperature [84].
The number of mesogenic groups can be increased in the periphery by the
hydrosilylation of the first-generation hexadecavinyl octasilsesquioxane den-
drimer, see Fig. 39, with cyanobiphenyl mesogens containing terminal Si-H
groups, thereby affording the liquid crystal multipede
39
, see Fig. 40, which
contains 16 cyanobiphenyl groups attached to the dendritic core [86]. For
this material, the
1
H NMR spectrum showed well-resolved resonances for
all the protons of the cyanobiphenyl mesogenic groups, indicating complete
conformational freedom of the mesogenic units in solution. Complementary
29
Si NMR spectroscopy shows that the reaction of the cyanobiphenyl meso-
gens with the vinyl units was incomplete, and although the distribution was
much sharper than normally found for polymers, nevertheless, the material
showed a degree of dispersity and is not a unique compound. The molecular
weight of
39
was under-estimated by SEC, an effect frequently encountered in
Fig. 38
Effect of spacer chain length on the liquid crystal properties of dendrimers of gen-
eral structure
38
Table 1
Transition temperatures (
◦
C) as a function of methylene spacer length (
n
)for
octamers
38
Spacer chain length (
n
)
g to SmA
SmA to Iso Liq
4
11.0
93.9
6
0.3
116.5
11
-7.0
128.5
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