Revealing nanoscale chemical heterogeneity at the surface of water-based coatings prepared from urethane–acrylic hybrids by photo-induced force microscopy

The surface chemical composition of water-based coatings prepared from urethane–acrylic hybrids or blends is investigated using infrared photo-induced force microscopy. By monitoring the interaction between the optically driven molecular dipole and its mirror image in a metal-coated tip, detailed information of the chemical composition at the coating surface down to 20 nm local resolution in all three dimensions is obtained. The film matrix of the coatings is determined to be the polyurethane while the polyacrylate particles are dispersed in the film. Phase separation of the two materials is found to be at a much smaller length scale in hybrid coatings compared to that in blends. This is because the acrylic monomers are introduced during the dispersion of the polyurethane in the synthesis of the hybrid particles, in contrast to the simple mixing of the two polymer particles in the case of the blends. Photo-induced force microscopy proved to be a powerful tool in the identification and visualization of different chemical species of coating surfaces at the nanoscale.