Block copolymers, which consist of chains of covalently connected dissimilar polymers, can adopt a wide range of morphologies, such as spheres or cylinders of the minority component within a bulk matrix. The morphologies are influenced by neighboring surfaces, which can disrupt the natural bulk periodicity. These interactions are particularly interesting when the polymers are confined to cylindrical channels that interact preferentially with one of the polymer blocks and where the diameter may or may not be commensurate with the natural dimensions of the polymer.
Wu et al. used block copolymers as surfactants for the formation of silica mesostructures and looked at the influence of the channel diameter. Using transmission electron microscopy (TEM), they imaged the structures that formed after calcination and backfilling of the channels with silver. For larger-diameter channels, they observed a double series of stacked rings that could be separated by sonication. As the diameter of the channel decreased, the rings became helices, which gradually became less densely packed. At a critical diameter, the inner helix transformed into a continuous channel, followed by a similar transition for the outer section as the diameter was decreased further. The ability to form highly tunable, coaxial, high-surface-area mesostructured wires from a number of metals may be of particular use in applications such as sensing and catalysis. — MSL
Nature Mater. 3, 816 (2004).