Helical sense of wrapped belts in star terpolymers via slice and view scanning electron microscopy

We study the self-assembly behavior of well-defined 3-miktoarm star terpolymers consisting of polyisoprene (PI), polystyrene (PS), and poly(2-vinylpyridine) (P2VP), (PI)₂-b-PS-b-P2VP. By adjusting the volume fractions of each block (40 vol % for PI, 27 vol % for PS and 33 vol % for P2VP), a unique structure was formed, in which hexagonally packed P2VP cylinders are wrapped by helically twisting PS belts in a matrix of PI. This structure was initially characterized by small angle X-ray scattering of bulk samples and bright field transmission electron microscopy (BF-TEM) of thin microtomed sections. By using I₂ to selectively stain P2VP and OsO₄ for PI, we can distinguish all 3 types of microdomains. In order to study the helical sense and coherent packing between adjacent cylinders, slice and view scanning electron microscopy (SVSEM) tomography is employed. The 3D tomograms of large regions provide information on the detailed nature and packing of the helical PS belts.