Introduction

While much of the theoretical and experimental understanding of the influence of external fields has been developed in the context of small molecule liquid crystals, it is nevertheless relevant to the polymeric liquid crystalline state, especially in respect of shear flow. Correspondingly, the first four main sections of this chapter (Sections 8.1 to 8.4) deal with the liquid crystalline state in general. The remaining sections concentrate more on liquid crystalline polymers (LCPs) and the special imprint of basic polymeric behaviour on their properties. The chapter lays the basis for an understanding of flow processing of LCPs as well as their response to electric and magnetic fields.

One of the characteristics of liquid crystals, both small molecule and polymeric, is their low viscosity compared with isotropic fluids. This low value arises from the ready alignment of the molecules under flow, since local alignment already obtains. However, the rheology of liquid crystals is complex; as a consequence of their anisotropy, a single viscosity coefficient is not sufficient for a complete description. Just as PBLG <25> was one of the first LCP systems to be studied morphologically, it was also the first to be studied rheologically. Hermans (1962) measured its viscosity as a function of concentration, molecular weight and shear rate, and discovered that the viscosity in the anisotropic phase is significantly lower than in the disordered isotropic (lower concentration) liquid, in line with earlier studies of small molecule materials such as p-azoxyanisole (PAA) <5>. Figure 8.1 shows that the viscosity versus concentration curve exhibits a predominant maximum.