Gyroid Labyrinth of Supertwisted Double Helices in a Liquid Crystal Polymer.

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.