Aqueous acrylic-polyurethane hybrid emulsions were prepared by the semibatch emulsion copolymerization of methyl methacrylate and butyl acrylate in the presence of eight polyurethane dispersions. The polyurethane dispersions were synthesized with isophorone diisocyanate, 1000 and 2000 molecular weight polyester polyols, 1000 molecular weight polyether polyol, butanediol, and dimethylol propionic acid. Acrylic monomers were added in the monomer emulsion feed. We studied the effect of the use of different polyurethane seed particles and the effect of different weight ratios of methyl methacrylate to butyl acrylate on the emulsion properties, microphase structure, and mechanical properties of hybrid films. The average particle size and distribution were determined by photon correlation spectroscopy. The rheological properties of polyurethane dispersions and hybrid emulsions were tested under destructive conditions by an examination of flow curves and under nondestructive conditions of oscillatory shear in a range of linear viscoelastic responses. Differential scanning calorimetry was performed to characterize the thermal-response properties of polymeric films. The relative average molecular weights were determined by gel permeation chromatography. The interactions between the acrylic and polyurethane components in hybrid particles and particle structure were studied with infrared spectroscopy and nuclear magnetic resonance spectroscopy. Mechanical properties such as the Koenig hardness, tensile strength, elongation at break, and Young's modulus were measured. (c) 2005 Wiley Periodicals, Inc.