Soft matter crystals (e.g. self-assembled block copolymers (BCPs)) display many of the same types of defects as occur in hard matter but with unique features. BCP assemblies have unit cells with length scales of ~ 10 to 100 nm and are comprised of tens of thousands of molecules, each molecule in turn, having ~ tens of thousands of atoms. In the case of network phases such as the double diamond (DD) and double gyroid (DG), the microphase separated domains are continuously connected in 3D. We employ slice and view scanning electron microscopy (SVSEM) to construct high resolution 3D tomograms of various types of grain boundary defects in polystyrene-polydimethyl siloxane (PS-PDMS) diblock copolymers. Depending on the choice of solvent, both the DD and DG phase can form with the two networks comprised of the ~ 40 v/v PDMS component. A common type of low energy, sharp grain boundary observed in hard crystals is a coherent twin boundary, where the structure exhibits mirror symmetry across the boundary plane. Using SVSEM we identified twin boundaries in both the DD and DG phases. In the DD, the twin is a (222). In the DG phase, twins appear on the (422) plane and create an interesting chirality transition of the respective pairs of networks upon crossing the boundary plane. In addition to such sharp, coherent boundaries, we also encounter wide aperiodic boundary regions (~ 100 nm to 10,000 nm wide) comprised of a disordered single network structure with a broad distribution of network strut directions and lengths, a genus over double that of the two interpenetrating ordered networks of the crystal and with a surface to volume ratio very similar to that of the crystal.

Grain boundaries in double network phases: Sharp twins and broad aperiodic

ACS Fall 2021

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