Chemistry Reference
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mean-strain measurements, 169 microcapsule dispersions, 170 nanoclay-
modified PDMS copolymers, 171 and the effects of strain history. 172
5.4 PERMEABILITY
Siloxane polymers have much higher permeability to gases than most other
elastomeric materials. For this reason, they have long been of interest for
gas separation membranes, the goal being to vary the basic siloxane struc-
ture to improve selectivity without decreasing permeability. Polysiloxanes
for pervaporation separations have also been of considerable interest. 173 -178
The techniques employed in this area include NMR and fluorescence cor-
relation spectroscopy 179 and inverse gas chromatography. 180
The repeat units of some of the polymers which have been investi-
gated 181-185 include [-Si(CH 3 ) -RO-], [-Si(CH 3 ) -XO-], [-Si(C 6 H 5 ) -RO-],
[-Si(CH 3 ) 2 - (CH 2 ) m -], [-Si(CH 3 ) 2 - (CH 2 ) m -Si(CH 3 ) 2 -O-], and [-Si(CH 3 ) 2 -
(C 6 H 4 ) m -Si(CH 3 ) 2 -O-], where R is typically an n -alkyl group and X is an
n -propyl group made polar by substitution of atoms such as Cl or N. Poly-
mers with oligo-organosiloxane side chains 186 and co-networks of PDMS
with poly(ethylene glycol) 187 have also been studied for possible use as
oxygen-permeable membranes. Unfortunately, structural changes that
increase the gas-diffusion selectivity are generally found to decrease the
permeability, and vice versa. Interest has not focused entirely on gases; for
example, liquids such as polychlorinated biphenyls and polycyclic aro-
matic hydrocarbons have also been studied. 188
In some cases, the polysiloxane was in the form of a composite—for
example, with sulfonated cross-linked polystyrene particles, 189 carbon
black, 190 acrylate latexes, 191 or sodium dodecyl sulfate. 192 Counterintui-
tively, the addition of impenetrable nanofillers can actually increase the
permeability of a membrane. 193 Also, siloxane-imide copolymers have
shown some interesting properties in membrane separations, 194 as have
polysiloxanes containing poly(ether amine) groups. 195
The interdiffusion of two samples of PDMS of different molecular
weights has also been of interest. 196
In addition to numerous experiments, there have been attempts at mod-
eling 197 using solubility parameters 198 and Flory-Huggins interaction pa-
rameters 199 to predict separation characteristics and permeability of
polysiloxane membranes. 200-202 Simulations indicate that at least the asym-
metrically substituted polysilanes [-SiRR'-] have gas permeabilities compa-
rable to that of PDMS. 203 The permeability, P , is the product of the solubility,
S , of the gas in the polymer and its diffusivity, D . 204 Values of P for the
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