Voltage-assisted asperity formation in styrene butadiene at room temperature: Cross-linking at the nanoscale

Nanoscale surface modification is reported for styrene butadiene rubber using an electrically biased conducting atomic force microscope tip. Under appropriate bias conditions, the local electric field magnitude is of the order of 10 8 ‐10 9 Vm 1 , which is sufficiently large to initiate cross-linking in the rubber. Peaklike surface features, surrounded by a circular trough and a raised ring, are created by careful and controlled retraction of the biased tip. The features’ aspect ratios can be controlled by modifying the tip retraction protocol, tip geometry, and bias voltage. Typical feature dimensions reported here vary from approximately 0.5‐10-nm high and up to several hundreds of nanometer in diameter. Although the temperature of the rubber is above the glass transition and the rubber is in a viscous state, the features are stable over a period of several days once created—which is believed to be due to cross-linking of the rubber during feature formation. Modeling of the electric field distribution in the vicinity of the tip is presented, which strongly supports the assertion that the resulting nonuniform electric field induces nanostructure formation and initiates cross-linking. A mechanism is proposed whereby source material is redistributed in the proximity of the tip/surface region to form the observed features.