Self-order in flexible non-mesogenic macromolecules

Abstract

Some very flexible linear poly(di-n-alkylsiloxane)s and cyclolinear oligo- and polyorganosiloxanes, which do not contain classical rigid rodlike or disclike mesogens, similar to other semi-organic polymers like polyphosphazenes and polysilanes, can form thermodynamically stable two-dimensionally ordered thermotropic mesophases. In many respects the mesophases in polysiloxanes resemble columnar liquid crystals which seems to be the result of the amphiphilic constitution of the polyorganosiloxanes. The origin of the mesomorphic state seems to be closely related to the competition between the interaction of the polar inorganic backbones and those of the organic side chains facilitated by the high flexibility of the inorganic main chain. Similarily to discotic liquid crystals, mesophases in polysiloxanes consist mostly of columns packed hexagonally (or closely so) with lateral long-range positional order, but with only short-range order along the columns. The unique feature of the columnar mesophases is that the axes of macromolecules lie along the column axes. The influence of molecular structure and molecular mass of linear macromolecules is discussed. It has been demonstrated that the temperature interval of mesophase stability is critically dependent on the length of the alkyl side chains. Both the process of formation of the mesophase from the isotropic melt and the crystallization of the mesophase are nucleation controlled processes, although the crystallization of the mesomorphic state is characterized by some specific features. The phase behaviour of cyclolinear polyorganosiloxanes is described with particular emphasis on the temperature stability as a function of molecular structure, tacticity, substituents and flexible spacers.