Pattern-directed to isotropic dewetting transition in polymer films on micropatterned surfaces with differential surface energy contrast.

Surface chemical patterns can both cause and direct dewetting in overlying thin polymer films. In this paper we focus on a key factor in this phenomenon, the magnitude of the surface energy difference between surface pattern domains (Δ). To probe the influence of Δ on film dewetting, we utilize novel combinatorial test patterns exhibiting a gradient in Δ. Specifically, our test patterns consist of a series of micron-scale striped regions that continuously change in their surface energy () relative to background striped regions having a fixed and calibrated . Using polystyrene (PS) films as a demonstration case, we employ these test patterns to quantify the morphology and kinetics of dewetting as Δ diminishes. Our study indicates a transition from pattern-directed to isotropic PS dewetting at critical Δ values. For Δ > 14 mJ m, ordered droplet arrays are formed, while for Δ < 7 mJ m, the dewetting is isotropic. A competition between these limiting behaviors is found for a "crossover regime", 7 mJ m < Δ < 14 mJ m. These combinatorial test patterns provide a powerful approach for investigating the large number of parameters that govern the stability of ultrathin polymer films, and the physical factors that influence the dewetted film morphology.

Gradient chemical micro patterns: a reference substrate for surface nanometrology.

We present fabrication routes for a new type of surface specimen that exhibits a micro pattern with a gradient in chemical contrast between the pattern domains. Design elements in the specimen allow chemical contrast in the micro pattern to be related to well-established surface characterization data, such as contact angle measurements. These gradient specimens represent a reference tool for calibrating image contrast in chemically sensitive scanning probe microscopy techniques and a platform for the high-throughput analysis of polymer thin film behavior.

Y-shaped amphiphilic brushes with switchable micellar surface structures.

We observed novel nanoscale surface structures of segregated pinned micelles and craterlike micelles formed by grafted Y-shaped molecules and their reversible reorganization in selective solvents. The Y-shaped molecules have two incompatible polymer chains (polystyrene and poly(tert-butyl acrylate)) attached to a functional stemlike segment capable of covalent grafting to a functionalized silicon surface. Postgrafting hydrolysis of poly(tert-butyl acrylate) arms imparts amphiphilicity to the brush. We demonstrated that spatial constraints induced by a chemical junction of two relatively short (6-10 nm) dissimilar arms in such Y-shaped molecules lead to the formation of segregated micellar surface nanostructures in the grafted layer. We proposed a model of these segregated pinned micelles and the corresponding reverse micelles (craterlike structures) featuring different segregation states of hydrophobic polystyrene and hydrophilic poly(acrylic acid) arms. The arms undergo conformational rearrangements in selective solvents in a controlled and reversible fashion. These nanoscale structural reorganizations define adaptive macroscopic wetting surface properties of the amphiphilic Y-shaped brushes. This surface structure and switchable behavior can be considered as a promising way toward the patterning of solid substrates with adaptive nanowells, which could be used for trapping of adsorbing nanoscale objects.

Ultrathin binary grafted polymer layers with switchable morphology.

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.