Poly( n-butyl methacrylate)/clay and poly(n-butyl methacrylate-co-methyl methacrylate)/clay nanocomposites were synthesized via emulsion polymerization to act as barrier membranes. Cloisite (R) Na(+) was modified via ion exchange to tether acryloxy reactive groups on the surface of the silicate platelet in order to promote polymerization and exfoliation of the stacked platelets. The addition of organically modified clays in the matrix improved the thermal and barrier properties of the nanocomposite. Results indicate that the polymer particle size in the emulsion dictated the dispersion of the montmorillonite clay platelets in the polymer matrix and the subsequent barrier properties of the membranes. Membranes fabricated utilizing emulsions with polymer particle sizes down to 120 nm gave rise to the highest barrier properties. The presence of 5 wt% clays decreased gas permeability by up to 85% in certain copolymer systems compared to the pristine polymer. (C) 2010 Elsevier B.V. All rights reserved.