Chemistry Reference
In-Depth Information
Si
O
O
Si
θ ´
CH 3
CH 3
Si
θ ˝
CH 3
CH 3
CH 3
CH 3
Figure 2.1:
Sketch of the PDMS chain, showing some structural information relevant to its high flex-
ibilit y. 25 Because of the difference in bond angles ( θ' = 37°, θ” = 70°). The all-trans configu-
ration closes to Figure 1.1 after 360°/(70°-37°) = 11 repeat units. The torsional barrier
for rotations about the skeletal bonds is very low, which accounts for the high dynamic
flexibility and low glass transition temperature. Reproduced by permission of John Wiley
and Sons.
Si-O-Si bond angle of ~143 o is much more open than the usual tetrahe-
dral bonding (occurring at ~110 o ), and can “invert” 102 through the linear
(180 o form) with little cost in energy. Similarly, torsional rotations can
occur without serious increases in energy. These structural features have
the effect of increasing the dynamic flexibility of the chain 25-27 ; they also
increase its equilibrium flexibility, which is the ability of a chain to take
on a compact shape when in the form of a random coil. Flexibility is gen-
erally measured, inversely, by the mean-square end-to-end distance or
radius of gyration of the chain in the absence of excluded-volume
effects.
2.5 ELASTOMER TECHNOLOGY
Pure siloxane polymers are only rarely appropriate for use in technology.
Numerous additives are incorporated in order to improve their properties.
A typical formulation contains the siloxane polymer, plus some or all of the
following ingredients: reinforcing fillers, extending (nonreinforcing) fill-
ers, processing aids, heat-aging additives, pigments, and curing agents (e.g.,
end-linking agents with associated catalysts, or organic peroxides). 103-107
The siloxane polymer usually has a rather high molecular weight, and
may have reactive ends for end-linking or vinyl side chains for peroxide
curing, as described in chapter 7.
The preferred reinforcing filler is high surface area silica, particularly
that made by the fume process, which gives the greatest reinforcement,
and, because of its high purity, yields excellent electrical insulation prop-
erties. 108-109 Silicas obtained from aqueous solutions 110 impart moder-
ately good reinforcement but, because of the presence of water on the
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