Visualizing the double-gyroid twin

Abstract

Significance The characterization of periodic morphologies in soft-matter supramolecular crystals has generally relied on X-ray scattering and transmission electron microscopy techniques. However, understanding the detailed nature of geometrical and topological defects in complex supramolecular assemblies requires high-resolution three-dimensional visualization over large sample volumes. Taking advantage of the recent development of slice-and-view scanning electron microscopy tomography for block copolymers, we clearly identify a sharp, coherent grain boundary in a double-gyroid structured polystyrene-b-polydimethylsiloxane diblock as a (422) twin boundary, likely formed during self-assembly. Knowledge of the geometric and topological nature of defects is important for further improving the performance of supramolecular soft crystals. Periodic gyroid network materials have many interesting properties (band gaps, topologically protected modes, superior charge and mass transport, and outstanding mechanical properties) due to the space-group symmetries and their multichannel triply continuous morphology. The three-dimensional structure of a twin boundary in a self-assembled polystyrene-b-polydimethylsiloxane (PS-PDMS) double-gyroid (DG) forming diblock copolymer is directly visualized using dual-beam scanning microscopy. The reconstruction clearly shows that the intermaterial dividing surface (IMDS) is smooth and continuous across the boundary plane as the pairs of chiral PDMS networks suddenly change their handedness. The boundary plane therefore acts as a topological mirror. The morphology of the normally chiral nodes and strut loops within the networks is altered in the twin-boundary plane with the formation of three new types of achiral nodes and the appearance of two new classes of achiral loops. The boundary region shares a very similar surface/volume ratio and distribution of the mean and Gaussian curvatures of the IMDS as the adjacent ordered DG grain regions, suggesting the twin is a low-energy boundary.