A poly(styrene-NaSS-UAN) random copolymer (PSSU) consisting of a sulfonated monomer (NaSS) and a non-sulfonated monomer (styrene) was successfully fabricated through a new copolymerization scheme using a urethane acrylate non-ionomer (UAN) as a compatibilizer to reduce solubility differences and enhance the miscibility of each monomer. The TEM image of the PSSU membranes showed that the nano-phase separated structure was comprised of hydrophilic domains dispersed within the hydrophobic polymer matrix along with a peculiar biphasic swelling behavior. UAN also played a role as a dispersant to uniformly distribute the silica nanoparticles of different hydrophilicity and to obtain subsequent sulfonated poly styrene-silica nanocomposite membranes. In the PSSU nanocomposite membranes, the use of hydrophilic silica nanoparticles improved both the hydrophilicity and methanol barrier property of the membranes via a superior dispersion in the hydrophilic domains. Accordingly, it significantly contributed to an increase of the proton conductivity and a reduction of the methanol permeability. On the other hand, hydrophobic silica nanoparticles, which were mainly dispersed in the hydrophobic domains, compensated for excessive water swelling with an increase in the content of ionic groups. The membrane performances in the fully hydrated state could be conveniently controlled through the direct incorporation of nano-sized silica particles using UAN. (c) 2006 Elsevier B.V. All rights reserved.