A poly(styrene-co-acrylic acid) copolymer was synthesized by surfactant-free polymerization with the assistance of power ultrasound in water. Fourier transform infrared, NMR, and differential scanning calorimetry measurements revealed that the copolymer was random. Atomic force microscopy and laser light scattering were used to investigate the self-assembly of the copolymer, and it was found that the copolymer chains formed micelles or other self-assemble structures in solution. Atomic force microscopy also indicated that the self-assembled structures developed into nanospheres with a poly(acrylic acid)-rich or polystyrene-rich Surface in a film, depending on the solvent used for the preparation of the film. In particular, a wheel-like structure could resulted in a film when the copolymer film was prepared in a moist environment; it resulted from heterogeneous aggregates of poly(acrylic acid) at the rim of water bubbles. (c) 2006 Wiley Periodicals, Inc.