Polybutadiene emulsion particles observed by scanning tunneling microscopy

Particles of polybutadiene were prepared by an emulsion polymerization process. We have been able to observe their size and shape in a scanning tunneling microscope. Critical to this success was the deposition of gold onto a mica substrate to give a smooth, conductive surface. A drop of methanol was then placed on this surface, and a very small amount of the polybutadiene latex (a stable emulsion of polymerized butadiene particles in water) was injected into the methanol drop. After the methanol evaporated, several areas showed isolated emulsion particles, while aggregates of particles were found in other areas. The surface of the particles was adequately conductive to get stable tunneling currents at a bias voltage of —0.1 V. The conductivity may be related to the layer of surfactant molecules on the surface. Isolated particles were typically oblate ellipsoids: 15 nm long, 10 nm wide, and 3 nm high. Individual particles of about the same size were visible in the aggregates. Because the glass transition temperature of polybutadiene is well below room temperature, the particles would be expected to be soft and deformable. The appearance of the aggregates is that of soft particles which have deformed to achieved a higher packing density.