Introduction: from conventional to unconventional liquid crystals

Liquid crystals have received tremendous attention from both the academic and technological points of view by virtue of their unique molecular structures characterized by the mesogenic cores and fluidity but long-range orientational order and stim- uli-responsive material properties. To date, a variety of liquid crystal materials have been systematically synthesized and extensively employed in a wide spectrum of daily products. Further potential applications have been suggested as well, based on their superior particularities and remarkable advances made in the twenty-first century. Among the thermotropic liquid crystals, rod-like and disk-like molecules are generally considered as conventional liquid crystals, whereas liquid crystals with other molecular shapes, showing unusual material properties distinct from those of conventional ones, are unconventional liquid crystals. In addition, liquid crystal-based mixtures and composites, comprising certain types of substances (e.g. dyes, monomers, and nanomaterials) as additives, can also be regarded as unconventional liquid crystals in that concern of modified and improved properties have led to a surge of research activities to discover and develop new material systems in attempt to tailor the host liquid crystals to extend their applications beyond displays. According to the contents of the book entitled Unconventional Liquid Crystals and Their Applications, this chapter is aimed at providing the reader with a background on the basis of what has been established about conventional rod-like liquid crystals and unconventional liquid crystals in terms of their structures, material properties, and potential applications. We start from an overview on the evolution of liquid crystal research, followed by introducing fundamental concepts, including types, structures, material properties, and well-known applications of various rod-like liquid crystal mesophases. Furthermore, an introduction to recent developments of specific hybrid liquid crystal systems with unusual or modified material characteristics, adoptable for revolutionary liquid crystal technologies, is presented to draw the reader’s attention from conventional to unconventional liquid crystals from an application perspective.