The wide variety of nanostructured morphologies derived from the self-assembly of block copolymers provides a useful route to nanoporous materials. Degrading one of the polymer blocks leaves a porous network of the remaining polymer, which can be used for filtration and selective crystallization. Their main limitation is the small pore sizes currently accessible, but recent research by J. Rzayev and J. Bolton at the University at Buffalo and T.S. Bailey of Colorado State University shows that this range can be significantly extended by the use of so-called “bottlebrush” copolymers, in which polymer side-chains fan out from a central backbone.
The morphology of self-assembled block copolymer materials is normally controlled by altering the length ratio of the two blocks, giving rise to segregated regions in the form of flat planes, cylinders, or spheres. Bottlebrush copolymers, which can have polymer side-chains of different lengths, offer another variable with which to alter the space taken up by each block and thus the morphology they adopt. The research, published in the January 31st online edition of Nano Letters (DOI: 10.1021/nl103747m), uses asymmetrical bottlebrush polymers consisting of a block with long polystyrene side-chains and one with shorter polyactide side-chains. Using a combination of controlled radical and ring-opening polymerizations, the team synthesized a methacrylate-based block copolymer and then grew the side-chains sequentially from this backbone. Melt pressing and annealing the polymer results in randomly oriented polyactide cylinder domains, which were aligned by channel die processing. Degrading the polyactide chains in alkaline conditions leaves a polystyrene material with nanoscale channels (55 ± 16 nm) running the width of the sample. This represents a substantial increase in size over pores derived from ordinary block copolymers (up to 35 nm), and it is likely that a much greater range is accessible by appropriate tailoring of the side-chains.
Asymmetry in the side-chains of bottlebrush block copolymers (bottom) can be used to obtain the same self-assembled morphologies as asymmetric linear copolymers (top). Reproduced with permission from Nano Lett. (2011) DOI: 10.1021/nl103747m. © 2011 American Chemical Society.
A potentially useful side effect of fabricating the material in this way is that the polymethacrylate backbone remains intact and present within the pores. The most striking consequence of this is that despite the material being principally polystyrene, the pores are hydrophilic and can uptake water. These leftover backbones could also provide a useful scaffold for chemically modifying the pores in a host of ways.
