Defects in Liquid-Crystalline Polymers
- PDF / 1,768,123 Bytes
- 6 Pages / 576 x 777.6 pts Page_size
- 3 Downloads / 247 Views
S BULLETIN/SEPTEMBER 1995
There are two main molecular architectures, either mainchain polymers (Figure la) or sidechain polymers (Figure lb). Biological polymers, which are rather rigid, also show mesomorphic order in solution when the concentration is high enough, by virtue of steric effects.2 Such phases are known because of the pioneering work of Bernal et al.3 Mesomorphic phases (also known as liquid crystals) are conveniently classified4 into nematics N (the molecules are oriented parallel to each other as in Figure lc), cholesterics N* (chiral molecules that display helical orientational order about a constant direction \), smectics Sm (in which the molecules gather into rather flexible layers), columnar phases, and blue phases BP (chiral molecules arranged locally in a stringlike manner). The following will make no mention of polymeric smectic phases—whose viscosity is often so high that it is not possible to heal the samples of their defects and get them suited to optical microscopy (note, however, a few electron-
Figure 1. Polymeric architectures: (a) Monomer for a mainchain polymer. (b) Monomer for a sidechain polymer. (c) Mainchain nematic phase. The "boxes" represent the rigid, mesogenic parts, and the "springs" represent the flexible parts.
microscopy works5)—or of polymeric columnar phases, which, to the best of my knowledge, are not yet documented. Nematic and cholesteric phases have been studied more extensively. We will discuss mainchain polymers (synthetic or biological) only. These phases have been studied intensely because their defects differ in many respects from the defects of low-molecularweight liquid crystals (LMWLCs) more than those of sidechain polymers do, at least with regard to static properties. The viscoelastic properties of sidechain polymers are ruled by the conformational properties of the polymeric backbone, which are still quite obscure, as well as by the nature of the coupling between the backbone and the side chains. Again, their high viscosity is an obstacle to direct observation of defects. The topological classification of defects in nematics and their application to the case of LMWLCs are well-understood.6-7 Nematic liquid-crystal polymers (LCPs) have been extensively studied on the basis of our knowledge of LMWLCs. The classification of defects is the same since it depends only on symmetries. However, our understanding of their physical properties is still limited. Because of the characteristic features of the polymeric chains,8 the large viscoelastic anisotropy to which they give rise, the critical competition between the relaxation times of the chains and their possible entanglements—which are finite, and the relaxation time of the nematic textures, one needs to use new elasticity and hydrodynamic approaches, as well as new models for cores in their static and dynamic states, etc. Most of the observations are based on various thermotropic melts, such as homo- or copolyesters; semiflexible polymers in solution, for example, poly(p-phenylene terephthalamide) in sulfuri
Data Loading...
