Introduction

The term mesomorphism (exhibiting an intermediate form) is generally reserved for spontaneously ordered fluids – liquid crystals. Liquid crystals were discovered in 1888 and studied extensively in the early 1900s, but essentially remained a laboratory curiosity until the 1960s when electro-optic applications for these unusual fluids were initiated and prototypes of the now-commonplace liquid-crystal display (LCD) were first demonstrated. During this period of renewal of interest in liquid crystals, polymer scientists discovered that the unusually good mechanical properties of ultra-high-strength synthetic poly(arylamide) fibers were in part due to the fact that such fibers were spun from liquid-crystalline polymer solutions, e.g. DuPont's Kevlar® and Akzo's Twaron®. Consequently, macromolecular-design strategies for synthesizing new high-performance polymers and modeling polymer processing now routinely consider the potential role of the mesomorphic state. In this chapter we will try to develop an understanding of the liquid-crystalline state in materials of low molecular weight, since the author is convinced that this is a prerequisite for appreciating how mesomorphism affects high molecular weight polymers.

General concepts

The mesomorphic state may be realized in two ways, namely, the two ways in which ordinary fluid phases are formed from solids: dissolution and fusion. These two categories of mesomorphism are called lyotropism (liquid-crystalline solutions) and thermotropism (liquid-crystalline melts), respectively. The latter category consists of single-component substances and encompasses the large variety of low molecular weight mesogens used in LCDs. More recently, thermotropic “specialty” polymers have been commercialized.