Gas transport properties of rubbery cross-linked poly(ethylene oxide) films containing short phenoxy-terminated pendant chains are reported. Poly(ethylene glycol) diacrylate (PEGDA) was UV-polymerized with poly(ethylene glycol) phenyl ether acrylate co-monomers of two different ethylene oxide repeat unit lengths: n = 2 (DEGPEA) and n = 4 (PEGPEA). Although fractional free volume increased with increasing co-monomer concentration, gas permeability did not rise accordingly. For instance, while FFV increased from 0.120 to 0.135 in both series of copolymers, CO2 permeability went from 110 to 35 barter (DEGPEA) or to 100 barter (PEGPEA). At the same time, glass transition temperature increased from -37 to -12 degrees C (DEGPEA) or to -28 degrees C (PEGPEA). The observed decrease in chain mobility with phenoxy-terminated co-monomer content indicated by increasing glass-rubber transition temperature apparently had a stronger influence on gas transport properties than the increase in fractional free volume. (C) 2009 Elsevier B.V. All rights reserved.