Evaporation structures of solvent drops evaporating from polymer surfaces: Influence of molar mass

When a solvent drop evaporates from a polymer surface, a characteristic structure remains. To analyze the structure formation during evaporation we deposited microdrops of ethyl acetate on planar PEMA and toluene on PS. The shape of the evaporation structures depends on the molar mass of the polymer, the number of droplets deposited, and the specific polymer/solvent combination. Crater-like structures with a flat bottom were observed for PEMA. With PS, dot-like protrusions were observed for low and intermediate molar masses; for M-w >= 210 kDa, crater-like rims with a depression in the center were formed. These structures are interpreted based on four different processes occurring during evaporation.