Gyroid Labyrinth of Supertwisted Double Helices in a Liquid Crystal Polymer

TL;DR

A liquid crystal polymer forms a unique four-network gyroid structure with double helices that twist in the opposite direction of their surrounding space.

Contribution

Discovery of a four-network gyroid structure with supertwisted double helices in a liquid crystal polymer.

Findings

The polymer forms a four-network gyroid with double helices in each chiral subspace.

Double helices supertwist with chirality opposite to their surrounding subspaces.

Local double-helical segments persist even in the melt phase above the melting point.

Abstract

A liquid crystal (LC) polymethylsiloxane (PMS) with rod‐like aromatic side‐groups attached via an alkylene spacer and bearing three n‐dodecyl end‐tails is found to form an unusual cubic structure. In a normal LC double gyroid (DG), the two chiral subspaces, one each side of the G‐surface, are occupied by one network each. Here each such network is split into two aromatic strands that wind around the central polysiloxane bundle, forming a double helix, resulting in a four‐network gyroid (4NG). While in previous normal LC DGs the network twist was assumed to follow that of the subspace, in 4NG the twist sense of the double‐helix is opposite to that of the subspace., i.e., while a right‐handed subspace twists by +70.5° between junctions, the double‐helix “supertwists” by −109.5°, and the opposite is true for the left‐handed subspace. Detailed analysis by X‐ray diffraction, DSC, and depolarized fluorescence (DF) shows a gradual but significant reversible change in the degree of mixing between the aromatic side groups and the polysiloxane backbones at 120 °C–130 °C in 4NG. Also, a significant increase in the system mobility starts only at ∼40 °C above the melting point, indicating persistence of local double‐helical segments even in the melt.

A side chain liquid crystal (LC) polymethylsiloxane (PMS) forms an unusual four‐network gyroid (4NG) structure. In each of the two chiral subspaces, separated by the gyroid minimum surface, there are two networks of aromatic moieties instead of one. These wind around the central polysiloxane bundle, forming a double helix. Furthermore, the double helices are found to supertwist, their chiralities being opposite of those of the subspaces they occupy.

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