Three series of amorphous cross-linked poly(ethylene oxide) (XLPEO) rubbers were prepared by photopolymerization of prepolymer solutions containing poly(ethylene glycol) diacrylate (PEGDA) crosslinker and one of three structurally similar short chain acrylate co-monomers: 2-hydroxyethyl acrylate (2-HEA), ethylene glycol methyl ether acrylate (EGMEA) and 2-ethoxyethyl acrylate (2-EEA). Copolymerization with the mono-functional acrylates led to the insertion of short side branches along the network backbone, terminated by either hydroxy, methoxy or ethoxy functional groups. Permeability measurements for the copolymers (35 degrees C) are presented for CO2, H-2, CH4, O-2 and N-2; corresponding solubility and diffusivity data are presented for CO2 and CH4. The side branches significantly influenced the thermal and gas transport properties of the polymers: methoxy and ethoxy terminated side-branches modestly increased fractional free volume and gas permeability. This effect was more pronounced by the introduction ofethoxy-terminated 2-EEA, while the presence of hydroxy terminated 2-HEA side-branches reduced free volume, resulting in a strong decrease in gas permeability with increasing co-monomer content. In each case, gas diffusivity correlated with polymer fractional free volume, while gas solubility was a function of both free volume characteristics and the corresponding concentration and accessibility of polar groups within the networks. The resulting selectivity ratios reflected variations in penetrant affinity and size discrimination as a function of copolymer composition. This study highlights the significant changes in various XLPEO properties that can be achieved by making small changes to its network structure. (c) 2008 Elsevier B.V. All rights reserved.