Ultrathin Binary Grafted Polymer Layers with Switchable Morphology

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Abstract
Polymer surface layers comprised of mixed chains grafted to a functionalized silicon surface with a total layer thickness of only 1−3 nm are shown to exhibit reversible switching of their structure. Carboxylic acid-terminated polystyrene (PS) and poly (butyl acrylate) (PBA) were chemically attached to a silicon surface that was modified with an epoxysilane self-assembled monolayer by a “grafting to” routine. While one-step grafting resulted in large, submicron microstructures, a refined, two-step sequential grafting procedure allowed for extremely small spatial dimensions of PS and PBA domains. By adjusting the grafting parameters, such as concentration of each phase and molecular weight, very finely structured surfaces resulted with roughly 10-nm phase domains and less than 0.5-nm roughness. Combining the glassy PS and the rubbery PBA, we implemented a design approach to fabricate a mixed brush from two immiscible polymers so that switching of the surface nanomechanical properties is possible. Post-grafting hydrolysis converted PBA to poly(acrylic acid) to amplify this switching in surface wettability. Preliminary tribological studies showed a difference in wear behavior of glassy and rubbery surface layers. Such switchable coatings have practical applications as surface modifications of complex nanoscale electronic devices and sensors, which is why we restricted total thickness for potential nanoscale gaps.