We have synthesized several H+ conducting siloxane copolymers with a methyl polyethylene glycol siloxane (MePEG(n)SiO(1.5); n = 3, 7, 12, 16) and diphenyl siloxane, isobutyl siloxane, and 3,3,3-trifluoropropyl siloxane copolymers. These copolymers where prepared to test if copolymerization with bulky groups can add free volume to our MePEG(n) polymer, and thus affect ion transport properties. We also found that the fractional free volume (FFV) could be increased in short MePEG(n) copolymers with the diphenyl and isobutyl copolymers, while with longer MePEG(n) groups (n = 12, 16), the pure (MePEG(n)-SiO3)(m) polymer already has a large free volume and that the added copolymers only dilute that already maximized free volume. The increase in FFV did not correspond to an increase in ionic conductivity. Ionic conductivity appears to be more a function of the volume fraction of PEG (V-f,V-PEG) than FFV. (C) 2013 Elsevier B.V. All rights reserved.