According to a group of researchers from MIT, block copolymers, self-assembling materials that scientists know to form a variety of predictable, regular patterns, can now be made into much more intricate models to open up new areas of materials design. The team, comprising of Yi Ding, a post-doctoral student, Alfredo Alexander-Katz and Caroline Ross, both professor of materials science and engineering, published their new findings in the journal Nature Communications.
Alexander-Katz explained that this is a discovery that was in some sense, fortuitous. Everyone believed this was not possible. He described the discovery of the team of a phenomenon that allows the polymers to self-assemble in patterns that deviate from regular symmetrical arrays.
Self-assembling block copolymers are materials whose chain-like molecules, which are initially disordered, will spontaneously arrange themselves into periodic structures. Scientists have discovered that if there was a repeating pattern of lines or pillars created on a substrate, and then a thin film of the block copolymer was formed on that surface, the designs from the substrate would be duplicated in the self-assembled material. But this method could only produce simple designs such as grids of dots or lines.
The new technique had two different, inconsistent patterns. One is from a set of posts or lines etched on a substrate material, and the other is an inherent pattern that is created by the self-assembling copolymer. For instance, there may be a rectangular pattern on the substrate and a hexagonal grid that the copolymer forms by itself. One would expect the resulting block copolymer arrangement to be poorly ordered, but that's not what the researchers discovered. Instead, Ross noted that it was forming something much more unexpected and complicated.
Working on the project as an undergraduate, Katherine Mizrahi Rodriguez explained that the team prepared many of these block copolymer samples and studied them under a scanning electron microscope. Yi Ding, who worked on this project for his doctoral thesis, said that he started looking over and over to see if any exciting patterns came up. That was when all of these new findings began to evolve.
As Ding noted, they understand the system fully in terms of the thermodynamics, and the self-assembling process allows them to create fine patterns and to access some new symmetries that are otherwise hard to fabricate. Ding explained that this process removes some existing limitations in the design of optical and plasmonic materials, and thus creates a new path for materials design.
Ultimately, Ross noted that the team's work had been confined to two-dimensional surfaces, but in ongoing work, they are hoping to extend the process into the third-dimension. She explained further that three dimensional fabrication would be a game changer.
