Block copolymers form a wide range of morphologies because of the interplay between the tendency of the two polymer phases to separate and the mixing promoted by entropy. The two blocks are of course connected, leading to ordered structures (ranging from a few to a few hundred nanometres in size) that are widely used in nanotechnology. Fully elucidating these structures, however, is not easy, because existing techniques are limited by low resolution, their destructive nature or the sample size required.
Now, Stefan Hell and colleagues from the Max Planck Institute for Biophysical Chemistry in Göttingen have combined stimulated emission depletion microscopy (STED) and 4Pi microscopy to reveal the nanoscale structure of a block copolymer (Nano Lett. 9, 2497–2500; 2009). One block is doped with a fluorescent dye to improve the contrast between the phases. In STED, part of the beam 'turns off' fluorescence in the sample apart from a tiny portion at the centre of the beam, thus improving resolution to below the diffraction limit.
Tuning the shape of the point spread function (PSF) for different morphologies further improved the results — an oblate PSF for a lamellar structure, for example. The obtained images showed the structure of the polystyrene–poly(2-vinylpyridine) copolymers in exceptional detail. The kinetically trapped structure (pictured) shows a bicontinuous arrangement with both lamellar and worm-like features, and was acquired using a spherical PSF with a diameter smaller than 50 nm.