In this study, effect of duration of ultrasonication process on structural characteristics and barrier properties of solvent-free castor oil-based polyurethane (PU)/organically modified montmorillonite (OMMT) nanocomposites was investigated. A series of PU/OMMT composites were synthesized by in situ polymerization technique through an ultrasonication-assisted process at various processing durations. Effect of ultrasonication duration on de-agglomeration of clay stacks in castor oil dispersions was evaluated by optical microscopy, sedimentation test, and viscosity measurement. Wide angle X-ray diffraction and Fourier-transform infrared spectroscopy were employed to investigate the effect of processing time on degree of delamination of clay platelets and interfacial strength between clay layers and PU matrix. Also, surface morphology of the nanocomposites was analyzed by atomic force microscopy. The results showed that by increasing the ultrasonication time up to 60 min, the size of clay agglomerates decreased and the interlayer spacing of clay platelets increased. To evaluate the effect of ultrasonication duration on transport properties of the PU/OMMT composites, diffusion coefficient and permeability were determined through water uptake test. Electrochemical impedance spectroscopy was carried out to analyze the barrier properties and to evaluate the corrosion performance of these composite coatings on carbon steel panels. It was found that by increasing sonication time, the barrier property of nanocomposites against diffusion of water molecules improved, which is due to further separation of clay platelets, enhancement of the traveling pathways for water molecules and improvement of interactions between the two components. (c) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012