Evaporating Polymer Produces Webby Patterns

Controlling the evaporation of a drop of diblock copolymer under just the right conditions leads to self-assembly of weblike macrostructures, report Suck Won Hong, Jun Wang, and Zhiqun Lin of Iowa State University (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200903552). The ordered structures could be useful in preparing multifunctional materials for optoelectronic devices or as platforms to study cell adhesion and motility. The researchers dissolved the asymmetric diblock copolymer polystyrene-b-poly(methylmethacrylate) in toluene and pinned a drop of the solution onto a silicon substrate with a 1-cm-diameter lens. Interactions between polystyrene and the silicon led to the formation of hundreds of ordered, concentric “coffee rings” with a serpentine or wavelike shape at the outer edges of the drop as it evaporated. Hong and colleagues then annealed the rings using acetone vapor and found that the rings became interconnected to form periodic ellipsoidal or circular wells 20 μm across and 80–100 nm deep in a macroscale web of polymer. The approach is “a simple, scalable, low-cost, lithography-free method” for creating ordered nanostructured materials, the researchers write.