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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The segmental relaxation characteristics of UV cross-linked networks based on poly(ethylene glycol) diacrylate [PEGDA] copolymerized with either poly(ethylene glycol) methyl ether acrylate [PEGMEA] or poly(ethylene glycol) acrylate [PEGA] were investigated using dynamic mechanical analysis. The molecular weights of the PEGDA cross-linker and acrylate comonomers were selected to obtain rubbery, amorphous membrane materials with constant ethylene oxide content. The introduction of PEGMEA or PEGA in the reaction mixture was used to control cross-link density and led to the insertion of flexible oligomeric branches within the resulting networks. For both copolymer series, the introduction of acrylate comonomer led to a decrease in glass transition temperature (T g) and a systematic reduction in cross-link density; the downward shift in Tg was much more pronounced for the PEGDA/PEGMEA copolymers. Time-temperature superposition was used to construct modulus master curves across the glass transition, and these could be satisfactorily fit using the Kohlrausch-Williams-Watts (KWW) function. The KWW curve fits indicated a narrowing of the glass-rubber relaxation with reduced crosslink density that correlated with a decrease in fragility for the networks. Gas transport measurements revealed a strong sensitivity to copolymer composition for the PEGDA/PEGMEA networks (-OCH 3 branch end group) as compared to the PEGDA/PEGA networks (-OH end group), with CO2 permeability and CO2/H2 selectivity increasing with increased branch content in the PEGDA/PEGMEA membranes. Both the observed glass transition and transport behavior correlated with measured variations in fractional free volume for these networks.

The viscoelastic relaxation characteristics of ultraviolet crosslinked networks based on poly(ethylene glycol) diacrylate [PEGDA] have been investigated by dynamic mechanical methods. Effective crosslink density in the networks was varied via the use of PEGDA prepolymers of different molecular weight, or by the introduction of controlled amounts of water in the reaction mixture. In all cases examined, fully amorphous networks were obtained. Time-temperature superposition was applied to obtain master curves of storage modulus versus frequency across the glass transition, and these could be satisfactorily described using the Kohlrausch-Williams-Watts relaxation function. The glass transition temperature (T g ), relaxation breadth, and fragility of the segmental relaxation were correlated with the effective crosslink density obtained in the networks. Gas permeation measurements on the PEGDA/water networks indicated only a very modest variation in gas transport properties, despite the sizeable variation in apparent crosslink density achieved in these materials. This result suggests that the controlling structural factor for gas transport in the networks is not simply crosslink density, and that attempts to correlate gas transport to network structure must necessarily consider the broader relationships between crosslink density, segmental mobility, and fractional free volume. Figure 6. Time-temperature master curve for PEGDA (n ¼ 14) network; T REF ¼ À40 8C. Solid curve is KWW best-fit. Inset: log(shift factor, a T ) versus temperature (8C). 2064 KALAKKUNNATH ET AL.

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