Nanoscale craters in poly(methyl methacrylate) formed by exposure to condensing solvent vapor

Abstract

We report atomic force microscope observations of small pits formed on thin poly(methyl methacrylate) films after exposure to formic acid vapor, which condenses to form small drops on the surface and then evaporates. This procedure produces large numbers of small pits, 50–5000 nm in diameter, with aspect ratios (depth-to-diameter) as high as 0.5. About 25% of the volume removed from high-aspect-ratio pits has been transported to form a raised ring around the rim of the pit. We attribute the remaining 75% of the volume loss to densification of the surrounding polymer. Nanoindentation measurements show that material inside the pits is harder and stiffer than material outside the pits, consistent with densification. The effects of solvent concentration, exposure time, and exposure to ammonia vapor are described. Similar treatments with volatile solvents have potential applications in large-scale surface patterning, submicron hole formation, and controlled alteration of surface properties.