How does your gyroid grow?: A mesoatomic perspective on supramolecular, soft matter network crystals

TL;DR

This paper introduces a mesoatomic framework to understand the formation and structure of triply periodic network crystals in soft matter, focusing on diblock copolymer melts and proposing a kinetic model for their assembly.

Contribution

It develops a novel mesoatomic decomposition method for complex network crystals and links mesoatomic shapes to the kinetics of crystal assembly.

Findings

Mesoatoms are non-convex, smoothly curved building blocks.

A minimal energetic model predicts assembly pathways for double-network crystals.

Mesoatoms may serve as intermediate structures in soft matter crystallization.

Abstract

We propose a framework to understand supramolecular network crystals formed in soft matter in terms of mesoatomic building blocks, collective groupings of amphiphilic molecules that play a role analogous to atomic or molecular subunits of hard matter crystals. While the concept of mesoatoms is intuitive and widely invoked in crystalline arrangements of sphere- or cylinder-like (micelle-like) domains, analogous notions of physically meaningful building blocks of triply periodic network crystals, like the double-gyroid or double-diamond structures are obscured by the complex, bicontinuous domain shapes and intercatenated topologies of the double networks. Focusing on the example of diblock copolymer melts, we propose generic rules for decomposing triply-periodic network crystals into a unique set of mesoatomic building blocks, leading to mesoatomic volumes that are non-convex and bound by smoothly curved faces, unlike the more familiar polyhedral shapes associated with sphere- and cylinder-like mesoatoms. We analyze the shapes of these mesoatoms, their internal structure and importantly their local packing with neighbor mesoatomic units. Hypothesizing that mesoatoms are kinetically favored intermediate structures whose local shapes and packing template network crystal assembly on long time scales, we propose and study a minimal energetic model of mesoatom assembly for three different cubic double-network crystals, which predicts a detailed picture for kinetics of intercatenation and surface growth. We discuss potential extensions and elaborations of the mesoatomic description of supramolecular soft matter network crystals, experimental observations of malleable mesoatomic units in the precursor sponge phase as well as in ordered cubic networks, as well as possibilities for observing mesoatoms in primordial, pre-crystalline states.