Nanometer-scale solvent-assisted modification of polymer surfaces using the atomic force microscope

We describe the response of poly(methyl methacrylate) surfaces to localized mechanical stimulation by the tip of an atomic force microscope (AFM) in water, methanol, ethanol, and aqueous alcohol solutions. Simply pressing the AFM tip into the surface with no horizontal motion fails to produce visible features in subsequent low contact force images. A single small-area (40 x 40 nm(2)) high contact force scan has little effect in air but in water or in alcohol-water mixtures produces soft bumps (local volume increase) adjacent to the scanned area. These bumps typically have lateral dimensions of similar to100 nm and rise tens of nanometers above the surrounding surface. Larger, micron-scale scans produce approximately parallel, raised ridges 50-150 nm apart. These structures are stable over time periods of hours or more in air and in solvent. We present evidence that these modifications are due primarily to stress-enhanced solvent uptake in material surrounding the area of tip-polymer contact.