Liquid crystalline assembly of nanocylinders
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Controlled bottom-up assembly of nanocylinders (e.g., nanotubes, nanorods, nanowires) into large area aligned arrays is widely recognized as a key obstacle impeding application development. Processing of lyotropic liquid crystal phases is a promising route for overcoming this obstacle, but nanocylinder liquid crystalline science is a nascent field that tends to be fractionated based on material type. This review explores the common challenges and achievements of nanocylinder liquid crystal research by focusing on three types of systems: (i) carbon nanotubes, (ii) inorganic nanocylinders, and (iii) cellulose nanocrystals. I. INTRODUCTION
Due to their one-dimensional structure and outstanding anisotropic properties, nanocylinders including carbon nanotubes, inorganic nanorods, and cellulose liquid crystals have all garnered significant interest as building blocks for larger structures. However, improved understanding and control over their bottom-up assembly are needed to enable their outstanding nanoscale properties to be manifested in macroscale materials. Processing of nanocylinder dispersions is quickly becoming established as a key research area for the hierarchical bottom-up assembly of anisotropic nanomaterials into functional devices such as transistors,1 macroelectronic devices,2 sensors,3–7 electro-optical devices,3,8–10 and structural materials.11–24 Key advantages to a fluid-phase processing approach are that considerable knowledge and infrastructure already exist as a result of the polymer industry. The US coatings industry alone was worth 24 billion dollars in 2007,25 and solution spinning of liquid crystalline solutions of rodlike polymers is well known for producing high performance materials such as DuPont Kevlar. While the chemical and physical properties of nanocylinders vary broadly, their cylindrical shape results in a common framework for viewing their phase behavior, shear response, and assembly into bulk materials. All rodlike materials have the inherent tendency to form lyotropic liquid crystalline phases at sufficient concentration, and many theories for liquid crystalline phase behavior do not distinguish between cylindrically shaped colloids and rodlike polymers.26 Davidson et al.,27 Solomon and Spicer,28 and others have highlighted that research into the phase behavior and rheological properties of all cylindrical materials can be advanced by viewing them through the framework initially established for rodlike polymers. In a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.10 140
J. Mater. Res., Vol. 26, No. 2, Jan 28, 2011
http://journals.cambridge.org
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spite of this, the majority of research on liquid crystalline assembly has focused on organic entities, particularly polymers. As of the early 1990s, fewer than 20 inorganic liquid crystals composed entirely of inorganic moieties had been discovered,27 and carbon nanotube research was in its infancy. Over the last ten to twenty years, increasing capabilities in nanomater
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