Chemistry Reference
In-Depth Information
Table 5.3.
SOME GAS PERMEABILITY INFORMATION
a
(
)
PP
ON
2
Polymer Repeat Unit
Gas
10
8
P
b
2
[-Si(CH
3
)
2
O-]
O
2
6.0
1.9
“
N
2
3.1
—
[-C(Si(CH
3
)
3
) = C(CH
3
)-]
O
2
72.
1.7
“
N
2
42
—
a
Reproduced by permission of the American Chemical Society.
b
Units of cm
3
(STP)cm/(cm
2
s cm Hg).
polysilanes studied seem to be high because of very high gas solubility in the
polysilanes. It is interesting to note that these simulations indicate a much
lower permeability for poly(dimethylsilmethylene) [-Si(CH
3
)
2
-CH
2
-]. he
presence of a trimethylsilyl group [-Si(CH
3
)
3
] as a side chain in an acetylene
repeat unit increases the permeability of the polymer to a value above that
of PDMS. The specific polymer is poly[1-(trimethylsilyl)-1-propyne]
205-213
;
Table 5.3 gives some comparisons between it and PDMS.
113
Remarkably, its
permeability,
P
, is about an order of magnitude higher than that of PDMS
without much decrease in selectivity (as measured by the ratio of the
P
values for oxygen and nitrogen). The greatly increased values of
P
are appar-
ently due to the unusually high solubility of gases in this polymer.
206
Studies
of the effects of substituting the trimethylsilyl group onto other polymer
backbones are in progress. Membranes containing a liquid-crystalline
PDMS have also been studied.
214
Another type of membrane designed as an artificial skin coating for burns
also exploits the high permeability of siloxane polymers.
215,
216
he inner
layer of the membrane consists primarily of protein and serves as a template
for the regenerative growth of new tissue. The outer layer is a sheet of silicone
polymer that not only provides mechanical support but also permits outward
escape of excess moisture while preventing ingress of harmful bacteria.
Soft contact lens prepared from PDMS provide a final example, as
shown in figure 5.7.
113
The oxygen required by the eye for its metabolic
processes must be obtained by inward diffusion from the air rather than
through blood vessels.
217,
218
PDMS is ideal for such lenses
215
because of its
high oxygen permeability, but it is too hydrophobic to be adequately
wetted by the fluids covering the eye. Poor wetting means the lens does
not “feel right” and can also cause adhesion of the lens to the eye itself.
One way to remedy this problem is to graft a thin layer of a hydrophilic
polymer to the inner surface of the lens. Because of the thinness of the
coating the high permeability of the PDMS is essentially unaffected.
