Surface features in Langmuir-Blodgett monolayers of pre-dominantly hydrophobic poly(styrene)-poly(ethylene oxide) diblock copolymer

Monolayers of a predominantly hydrophobic poly(styrene)-b-poly(ethylene oxide) copolymer (7% PEO by mass) were transferred to a silicon substrate using the Langmuir-Blodgett method. The films were imaged using atomic force microscopy, and three types of features were observed in various proportions: dots (circular aggregates), spaghetti (rodlike aggregates), and continents (planar aggregates). The concentration of the spreading solution on the air-water interface had the most significant effect on the types of features observed. Variations in surface pressure and compression speed had little effect on the distribution and predominance of the different types of aggregates. Single-drop experiments show that feature formation depends on a competition between film spreading and polymer entanglement resulting from solvent evaporation. Aggregates thus formed upon spreading on the air-water interface are kinetically trapped and quite stable upon transfer to the solid substrate.