Seeing the order-order phase transition between double diamond and double gyroid structures in block copolymers

The order-order phase transition between double diamond (DD) and double gyroid (DG) phases occurs in a variety of soft matter systems including water-surfactant liquid crystal phases and in block copolymers. Various geometrical and topological transformational pathways have been proposed based on limited microscopy and scattering data, but the specific structural features of the morphological evolution at the interface between the co-existing phases have been very challenging to elucidate. Here we employ slice and view scanning electron microscopy on polystyrene-polydimethylsiloxane diblock copolymers to examine the phase transition zone between the DD and DG structures. 3D tomograms of the transformation zone show how the mesoatoms and loops of each network are cooperatively modified over a narrow (less than one unit cell wide) zone that shows a facile, low energy transformation pathway between the tubular network phases. We discuss the malleability of the tubular network mesoatoms and the maintenance of key symmetries between the two cubic phases, i.e. the 4sub1/4sub3 screw axes in the DG shift and align with the 2sub1 screw axes in DD, while certain two fold rotational axes remain unchanged across the phase boundary.