Liquid-crystalline elastomers

Abstract

Introduction

The two parts of the adjective “liquid-crystalline” are not as contradictory as they seem. In qualitative terms, the “crystalline” part refers to the fact that these materials are sufficiently ordered to diffract an X-ray beam in a way analogous to that of normal crystalline materials (with their three-dimensional order). On the other hand, the “liquid” part specifies that there is sufficient disorder for the material to flow like a liquid, at least under some conditions (Collings, 1990). The disorder is typically in one dimension as is the case, for example, with rodlike molecules having their axes all parallel but out of register with regard to their lengths. Because of their intermediate status between crystalline and amorphous materials, they are also called “mesophases” (with “meso” meaning middle).

Both low molecular weight materials (Collings, 1990) and polymers (Ciferri, 1991; Donald and Windle, 1992) can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars© and other aromatic polyamides. It can also occur with flexible chains and it is these flexible chains in the elastomeric state that are the focus of the present discussion. One reason such liquid-crystalline elastomers are of particular interest is the fact that (1) they can be extensively deformed (as described for elastomers throughout this book), (2) the deformation produces alignment of the chains, and (3) alignment of the chains is central to the formation of liquid-crystalline phases.