A complex 3D tubular network structure is investigated using the slice and view electron microscopy technique that directly generates a 3D tomogram of the nanoscale structure. The material studied is a polystyrene (PS) – polydimethylsiloxane (PDMS) diblock copolymer that exhibits the double gyroid network morphology with a lattice parameter of ~ 130nm, with feature sizes on the 20 nm scale, typical of many soft matter assemblies. By alternating between a thin ion beam slice and a secondary electron image using a low voltage incident electron beam, the voxel size is approximately 3 x 3 x 3 nm3 allowing analysis of a comprehensive set of sub-unit cell morphological descriptors and enabling critical comparison to theoretical models of the structure. While the DG phase would nominally be classified as cubic phase based standard morphological characterization techniques (i.e. SAXS and TEM) in accordance with equilibrium theory, a more critical analysis of morphology reveals that sample orders into variable triclinic (vt) variants of the cubic phase. Remarkably, the triclinic symmetry of the network is coherent over the large multi-micron length scales of monodomains, yet the particular triclinic symmetry of the morphology varies substantially from grain to grain. We demonstrate that large, yet coherent, deviations from cubic symmetry are facilitated by non-affine relaxations of sub-unit cell morphology, characterized by variable stretching and relaxation of tubular network struts that accommodate triclinic symmetry while maintaining local angular symmetry of network. This multi-scale analysis of 3D periodic BCP morphology reveals a qualitative and under appreciated distinction between hard and soft crystals with regard to structural relaxations of avoidable residual strains deriving from the inter-grain compatibility constraints arising during crystal formation.

Characterization of sub-unit cell morphology in tubular network block copolymers

2019 Fall National Meeting

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