Shear‐induced lamellar phase of an ionic liquid crystal at room temperature

Abstract

The phase behaviour of a number of N‐alkylimidazolium salts was studied using polarizing optical microscopy, differential scanning calorimetry and X‐ray diffraction. Two of these compounds exhibit lamellar mesophases at temperatures above 50°C. In these systems, the liquid crystalline behaviour may be induced at room temperature by shear. Sheared films of these materials, observed between crossed polarisers, have a morphology that is typical of (wet) liquid foams: they partition into dark domains separated by brighter (birefringent) walls, which are approximately arcs of circle and meet at “Plateau borders” with three or more sides. Where walls meet three at a time, they do so at approximately 120° angles. These patterns coarsen with time and both T1 and T2 processes have been observed, as in foams. The time evolution of domains is also consistent with von Neumann's law. We conjecture that the bright walls are regions of high concentration of defects produced by shear, and that the system is dominated by the interfacial tension between these walls and the uniform domains. The control of self‐organised monodomains, as observed in these systems, is expected to play an important role in potential applications.

Keywords:

• ionic liquid crystals
• shear
• liquid foams
• lamellar phases

Acknowledgments

We would like to thank Fundação para a Ciência e Tecnologia (POCI 2010) and FEDER for the financial support (Ref. POCI/QUI/57735/2004, Ref. POCI/CTM/56382/2004, PTDC/FIS/65037/2006 and Ref. SFRH/BPD/14848/2003).