Poly(butyl acrylate-co-methyl methacrylate)-montmorillonite (MMT) waterborne nanocomposites were successfully synthesized by semibatch emulsion polymerization. The syntheses of the nanocomposites were performed in presence of sodium montmorillonite (Na-MMT) and organically modified montmorillonite (O-MMT). O-MMT was used directly after the modification of Na-MMT with dimethyl dioctadecyl ammonium chloride. Both Na-MMT and O-MMT were sonified to obtain nanocomposites with 47 wt % solids and 3 wt % Na-MMT or O-MMT content. Average particle sizes of Na-MMT nanocomposites were measured as 110-150 nm while O-MMT nanocomposites were measured as 200-350 nm. Both Na-MMT and O-MMT increased thermal, mechanical, and barrier properties (water vapor and oxygen permeability) of the pristine copolymer explicitly. X-ray diffraction and transmission electron microscope studies show that exfoliated morphology was obtained. The gloss values of O-MMT nanocomposites were found to be higher than that of the pristine copolymer. (c) 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42373.