A series of cross-linked polymers, XLPEGDA, was prepared by photopolymerizing poly(ethylene glycol) diacrylate (PEGDA) in the presence of varying amounts of water or monofunctional
poly(ethylene glycol) methyl ether acrylate (PEGMEA) to vary cross-link density. All of the polymers
had essentially the same chemical composition but displayed a systematic variation in cross-link density
as estimated from water swelling experiments, models such as the Flory−Rehner and modified Flory−Rehner theories, and dynamic mechanical testing. Cross-link density decreased with increasing water or
PEGMEA content in the prepolymer solutions. Interestingly, gas solubility, diffusivity, and permeability
were essentially independent of cross-link density for the series of materials prepared from PEGDA and
water. The polymer density, fractional free volume, glass transition temperature, and polymer d spacing
were also constant when water was used to vary cross-link density. On the basis of this result, it appears
that cross-link density does not necessarily affect gas diffusion and permeation, the polymer glass
transition temperature, or the fractional free volume in network polymers, and ascribing changes in these
properties to changes in cross-link density alone, as is seen commonly in the literature, should be done
with great care.
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