Two-component polymer brushes will allow experimental study of chain fluctuations

Tom Witten of the University of Chicago and I have suggested that chain fluctuations in a polymer brush could be studied experimentally by making a brush composed of two types of chains - call them black and white. Then a diffraction experiment could be done to scatter neutrons from the black-white fluctuations. It turns out that the diffraction patttern would show peaks corresponding to the side-to-side and vertical fluctuation distances.

You can sort of see on the left-hand side of the movie (which is a view down on a polymer brush made of black and white bristles) that there is a characteristic size for black-white side-to-side inhomogeneities. On the right-hand side you can see (side view through the brush) that there the vertical black-white fluctuations occur at the scale of the thickness of the layer. The lifetimes of these fluctuations correspond to the single-chain fluctuation times, and this is also accessible in a (difficult) diffraction experiment.

Here is a nice still picture of fluctuations in the black-white two-component brush: On the left we have the top view, on the right the side view. This is a slightly denser brush than the one that was shown in the movie.

Something else neat might happen in a two-component polymer brush: if black and white polymers have even a small tendency to stay away from one another (oil and water have a strong tendency in this direction - you can't mix them in a glass), then there a peculiar type of demixing is predicted to occur. Remember that the black and white chains are stuck down to a particular place on the surface, so that they can't migrate apart to make big black and white patches. So each chain migrates just a bit to the side to make a `rippled' pattern. Witten and I predicted that it should be energetically favorable to the alternative possibility that a `layering' occurs where all the white chains stay near the bottom of the layer, while the black ones stretch to fill in the top.

The simulated chains in the configuration shown above organize into black-white `ripples' if a repulsion between the black and white monomers is turned on. Note how the fluctuations in the initial state (a) have been amplified to make the domain pattern in (b). Also, you can see in the side view how the chains are being deflected from side to side by the black-white repulsions. This simulation work, which is due to Greg Brown, Amit Chakrabarti, and myself, is currently being extended to block copolymers end attached to surfaces.

Three groups have been working on experiments that could show these effects: Philippe Auroy and Loic Auvray (CNRS Saclay), Miriam Rafailovich and Jonathan Sokolov (SUNY Stony Brook), and Matthew Tirrell (University of Minnesota). However, the experiments are very difficult - imagine gluing down all of those tiny bristles! The fabrication of these brushes is a tour de force of synthetic chemistry.

Last modified: May 23, 1994

John Marko, marko@msc.cornell.edu