Chemistry Reference
In-Depth Information
The strength of a siloxane bond has been estimated,
272
using both den-
sity functional theory and single-molecule measurements.
273
Ab initio
methods have been applied to gain a better understanding of siloxane-
functionalized ionic liquids, with the specific focus being on ion pairs in
1-methyl-3-pentamethyldisiloxymethylimidazolium chloride.
274
Phase diagrams show miscibility and phase separation among various
polysiloxanes, including cyclics.
275-277
Related to phase separation is the
segregation of siloxanes to surfaces in multiphase systems because of the
low surface energies of these polymers.
55,
278-280
As a result, a blend or
block copolymer having only a few percent of siloxane can have surfaces
consisting almost entirely of this minor constituent.
There is also much interest in applying various types of liquid-state
theory, to PDMS in particular.
10,
11
5.8 CRYSTALLINITY
The crystallization of PDMS in the undeformed state has long been of in-
terest, including unfilled elastomers
23,
80,
82
and block copolymers rein-
forced with silica.
81
Of greater interest, however, is PDMS, generally in the
filled state, in elongation
79,
84,
85,
281
where strain-induced crystallization
provides considerable reinforcement of the elastomer. Strain-induced
crystallization is of practical as well as fundamental importance.
282,
283
Theoretical models have been developed to characterize this type of crys-
tallization.
86,
87
Similar experimental studies of strain-induced crystallization have been
carried out on stereoregular poly(methyl-3,3,3-trifluoropropylsiloxane).47-50
47-50
5.9 SOME ADDITIONAL UNUSUAL PROPERTIES OF PDMS
Table 5.4 summarizes some of the unusual physical properties exhibited
by PDMS.
113,
240, 268, 271,
284
Atypically low values are exhibited for the char-
acteristic pressure
206,
207
(a corrected internal pressure, which is used in
the study of liquids), the bulk viscosity
η
, and the temperature coefficient
of
η
.
252
Also, entropies of dilution and excess volumes on mixing PDMS
with solvents are much lower than can be accounted for by theory.
268, 271
Finally, as has already been mentioned, PDMS has a surprisingly high
permeability.
Another striking feature of siloxane polymers is their unusual surface
properties.
54, 285-290
Fluorosiloxane polymers
291
have been studied most
