Chemistry Reference
In-Depth Information
by gamma radiation or chemical means.
220-221
IR (infrared) spectroscopy
and stress-strain measurements in extension indicate that relatively low
strains can frequently induce significant organization. One relevant ex-
periment involved two cross-linking procedures: the first cross linking
produced a network in which the mesogenic units could be oriented
223
; the
second subsequently locked in the network anisotropy. Samples were clear
and x-ray diffraction patterns were characteristic of a highly ordered nem-
atic material. Transitions from the isotropic phase to the nematic phase
caused significant increases in sample length. Using one or two cross-link-
ing agents in the presence of a magnetic field could also be used to prepare
“monodomain” nematics in which the director alignment is claimed to be
macroscopically uniform.
192
The thermoelastic behavior of these materials has also been reported.
224
Such experiments resolve the thermodynamics of the nematic to isotropic
transition into entropic and enthalpic contributions.
193
Also in this category are side-chain cholesteric liquid-crystalline poly-
siloxanes containing groups of varying rigidity,
225
thermotropic liquid-
crystalline polyimides with siloxane linkages,
226
and fluorine-containing
liquid-crystalline polysiloxanes having cholesteryl cinnamate mesogens
and trifluoromethyl-substituted mesogens.
227
Although experimental studies have been of the greatest interest by far,
there has been some molecular modeling of side-chain liquid-crystalline
polysiloxanes.
228
Two of the most recent applications of these materials are in the areas
of electrorheology
229
and holography.
230
3.5 CYCLICS
3.5.1 Introduction
The synthesis of polysiloxanes generally produces significant cyclics as well
as linear chains. A number of studies report equilibrium cyclic concentra-
tions under various conditions.
231-241
Cyclic polysiloxanes have also been
prepared in dilute solutions by cyclodepolymerization of linear dihydroxy-
PDMS,
242
or ring-closing dehydrocoupling of dihydroxy-PDMS and di-
hydro-PDMS.
243
Diels-Alder reactions have also proved useful for the
production of high-purity cyclics,
244
as have been polyfunctional cage oli-
gosilsesquioxanes by thiol-ene additions.
245
Considerable information on polysiloxane cyclics is available in reviews
of cyclic polymers in general.
246 -251
Considerable attention has been paid to
cyclics of poly(dimethylsiloxane),
252
poly(phenylmethylsiloxane),
253
and
