Surface chemical properties of nanoscale domains on UV-treated polystyrene-poly(methyl methacrylate) diblock copolymer films studied using scanning force microscopy.

This paper reports the surface chemical properties of ca. 20 nm wide domains on a UV-treated thin film of a polystyrene-poly(methyl methacrylate) diblock copolymer (PS-b-PMMA; 0.3 as the PMMA volume fraction). UV irradiation and subsequent acetic acid (AcOH) treatment were used for selectively etching horizontally aligned PMMA domains on a thin PS-b-PMMA film to obtain nanoscale trenches and ridges. The surface charge and hydrophilicity of the trenches (etched PMMA domains) and ridges (PS domains) were investigated using three approaches based on scanning force microscopy. Chemical force titration data with a COOH-terminated tip showed a prominent decrease in adhesion force from pH 3 to 4.5 due to electrostatic repulsion between negatively charged functional groups on the tip and film surface but could not clarify the difference in chemical properties between the two nanoscale domains. Friction force images in n-dodecane showed higher friction over etched PMMA and PS domains with an OH-terminated tip and a CH(3)-terminated tip, respectively, exhibiting higher hydrophilicity of the etched PMMA domains. In an atomic force microscopy image of a UV/AcOH-treated PS-b-PMMA film upon immersion in a ferritin solution, approximately 80% of the ferritin deposited on the film was found on the PS domains. The preferential deposition of ferritin on the PS domains was probably due to the electrostatic repulsion between negatively charged ferritin and negatively charged etched PMMA surface in addition to the hydrophobic interaction between ferritin and the PS surface. These results indicated that the etched PMMA domains were more hydrophilic than the PS domains due to the presence of acidic functional groups (e.g., -COOH groups) at a higher density.