Development of Glassy Bicontinuous Cubic Liquid Crystals for Solid Proton-Conductive Materials

Liquid-crystalline materials in bicontinuous cubic phases are of utmost interest due to their potential in developing a 3D continuous proton conduction pathway along an infinite periodic minimal surface. Previous studies showed it can be achieved by assembling amphiphilic zwitterions in the presence of an acid, bis(trifluoromethane)sulfonimide (HTf₂N) to form a bicontinuous cubic structure. Ion change reaction between the two components to form ionic liquid-like ion pair is a key for the co-assembly.

In a recent article published in the journal, Advanced Materials, a new glassy liquid-crystalline material was produced, having ability to stabilize a bicontinuous cubic structure.

The authors used pyridinium-based amphiphilic zwitterions (PyZI_n) and imide-type acids HA-R in equimolar reactions to form adducts exhibiting bicontinuous cubic liquid-crystalline phases. The resultant liquid crystals show glassification behavior upon cooling with keeping their 3D structures, which was characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction measurements.

They found that a higher glass transition temperature was observed for the mixture of pyridinium-based zwitterion and HA-R acid when compared with HTf₂N acid and as a result supports the fact that the freezing of the ionic domain comes before alkyl chains ordering when determining the phase behavior.

The glassy bicontinuous cubic structures were able to absorb water for a longer period without observing any change in their structures or morphology. This shows that other additives can be added to the glassy liquid crystals without change in their nanostructures.

With the use of HTf₂N acid and amphiphilic zwitterion of PyZI₁₆, crystallization that occurs during the continuous addition of water led to a change in morphology of the resultant liquid-crystalline nanostructures due to an outflow of the HTf₂N acid to water phase. This result confirms that the glassification of the liquid crystals in reaction of the amphiphilic zwitterions and imide-type acid is far more preferable in keeping both the 3D molecular arrangements and assemblies of the nanostructures. Moreover, they confirmed the rapid increase in ionic conductivity of the glassy bicontinuous cubic structures due to arrangement of water molecules on the infinite periodic minimal surface which leads to the induction of proton hopping conduction.

The development of the glassy bicontinuous cubic liquid achieved in this study opens an avenue for induction of additives into the nanostructured liquid-crystalline materials without changing the 3D nanostructures.