Polymer patterning: Electrostatics makes it easy

(Nanowerk News) The ability to inscribe ever-smaller patterns in thin polymer films is key to the development of light-emitting diode-based displays and plastic electronics. Methods to achieve such patterning can be broadly divided into top-down methods like inkjet printing, which aims to increase the resolution at which large-scale patterning techniques can be performed, and bottom-up methods, which build up small patterns using intermolecular interactions.
One such bottom-up technique involves the use of an externally applied electric field to cause an initially uniform thin layer of molten polymer to evolve into a pattern. To do this, the substrate on which the pattern is to be formed is constructed as a capacitor with two conducting layers and one insulating layer. Tingbing Cao and co-workers from the Renmin University of China in Beijing and the Colorado School of Mines in the US have now used a static electric field to pattern thin polymer films ("Self-Organization of Thin Polymer Films Guided by Electrostatic Charges on the Substrate").
Kelvin-probe microscopy images of geometric patterns formed by electret-based patterning of thin poly(methylmethacrylate) films
Kelvin-probe microscopy images of geometric patterns formed by electret-based patterning of thin poly(methylmethacrylate) films. (© 2011 Wiley-VCH)
"Materials called electrets can permanently store electrostatic charge," explains Cao. "This phenomenon is used in laser printing, where tiny charged toner particles are attracted to a pattern with opposite charge to create an image. In our experiments, we created a static charge pattern, then applied a thin uniform coating of polymer." On heating, the molten polymer self-organizes into a pattern resembling the original charge pattern, and since it is no longer necessary to apply a constant electric field, the patterns can be formed on a simple, single-layer substrate rather than the complex multilayer capacitor. "These features makes the technique more flexible and less expensive," says Cao.
So far, Cao and his co-workers have shown that their technique can be used to form geometric patterns (see image) using one of a variety of different polymers — poly(methylmethacrylate), poly(styrene) and poly (4-bromostyrene). Future work by the team will examine the possibility of producing other types of patterns using this approach. "The formation of non-geometric charge patterns is possible," explains Cao, "so the technique should not be limited to such patterns. We are also working on methods to apply multiple types of polymers by matching the type of electrostatic charge with polymers possessing corresponding characteristics."
Source: Tokyo Institute of Technology