Molecular pattern dance observed -- Choreography revealed

Envision dancers dressed in either black or white with extremely long, spaghetti-like arms. Each black dancer firmly holds a white dancer by the hand. This is an analogy of the molecules we have studied: block copolymers are long-chained macromolecules consisting of two chemically different parts. They are so long and floppy that they are completely entangled like a heap of cooked spaghetti. The disorder makes it impossible to tell to which dancer a particular arm belongs. Though, when seen from a distance, the individual dancers are indistinguishable and only the resulting average color can be seen. Then a regular pattern is discerned: there are regions on the dance floor where there are more black dancers than white and vice versa. What is fascinating about this phenomenon is that it results form a very simple rule: “like attracts like”. In the case of block copolymers, the regions where like dancers cluster together are called microdomains and they can have different shapes including spheres, rods, lamella, and more complex structures, depending on the type and the architecture of the molecule.

In our recent work published December 2004 in Nature Materials we have used scanning probe microscopy to record images of the pattern formation process in a thin film of block copolymers (see the movie). In our analogy, the thin film would correspond to a situation where the dancers are confined to a narrow, corridor-like dance floor. Here new patterns appear that are very different from those that formed on an indefinitely wide dance floor. We observe how rods reorient and transform into a new pattern known as a perforated lamella. The underlying physical principles of this process correspond to the choreography of the molecular pattern dance. A computer simulation reveals this choreography by reproducing all of the elementary steps of the pattern transformation dance in great detail. Remarkably, the choreography is rather simple and based only on local rules: the amount of preferential attraction of one kind of dancers to the walls and the width of the corridor are the two parameters which select which pattern forms. When one of these external parameters is changed, a pattern transformation occurs where each individual dancer follows a local rule. It tries to move to a region where there are more of its kind while firmly holding its dislike partner.

Our results are expected to have implications in different areas since block copolymers and nanostructured fluids are very common in both nature and technology. They are the physico-chemical basis for morphogenesis in biological cells and are very common in pharmaceutical products, plastic materials, and numerous other applications. The methods and the computer model presented in our work can be used to study and predict pattern formation processes in these materials. We expect a large impact on nanotechnology where block copolymers can be used as self-organised templates for the synthesis of inorganic nanostructured materials. For instance, block copolymer templated patterned magnetic media will be used in the next generation of computer hard disks to increase the storage density and overall capacity.

Reference:

A. Knoll, A. Horvat, K. S. Lyakhova, G. Krausch, G. J. A. Sevink, A. V. Zvelindovsky, and R. Magerle, "Direct imaging and mesoscale modelling of phase transitions in a nanostructured fluid", Nature Materials 3, 886 (2004).

Contact information:

Prof. Dr. Robert Magerle
Chemnitz University of Technology
Faculty for Natural Sciences
Chemical Physics
D-09107 Chemnitz, Germany
Phone: +49-371-531-38033
E-mail: robert.magerle@physik.tu-chemnitz.de

A German version of this text can be found here.

This project is funded by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 481, Teilprojekt A9).