Invited Lecture. Defects in small-molecule and polymeric nematics

After a reminder of the essential topological characters of defects in phases with nematic symmetries, a review is given of the observations of textures and defects in polymeric liquid crystals. Emphasis is placed on (a) the diversity of these observations according to the type and character of the polymer, and (b) the differences with small-molecule liquid crystals. However, the topological characters are common. The differences have to be explained in terms of molecular configurations. In this respect, we develop some physical consequences of the scarcity of chain ends, in particular a possible segregation and ordering process that relaxes strong splay deformations and that occurs in the cores of disclinations with a wedge character. After having discussed models of isolated disclinations, we investigate the possibility of the thermodynamical stability of sets of disclinations. We show that this problem has some analogies with the classical Flory-Huggins theory of the stability of polymers in solution, the disclinations playing here the role of the polymeric chains. Finally we indicate the existence of other features specific to polymeric nematics, apart from the question of chain ends, such as the correlations between chains and the related phenomena of frustration.