SELF-ORDER IN FLEXIBLE NONMESOGENIC MACROMOLECULES

Some very flexible linear poly(di-n-alkylsiloxane)s and cyclolinear ol igo- and polyorganosiloxanes, which do not contain classical rigid rod like or disclike mesogens, similar to other semi-organic polymers like polyphosphazenes and polysilanes, can form thermodynamically stable t wo-dimensionally ordered thermotropic mesophases. In many respects the mesophases in polysiloxanes resemble columnar liquid crystals which s eems to be the result of the amphiphilic constitution of the polyorgan osiloxanes. The origin of the mesomorphic state seems to be closely re lated to the competition between the interaction of the polar inorgani c backbones and those of the organic side chains facilitated by the hi gh flexibility of the inorganic main chain. Similarily to discotic liq uid crystals, mesophases in polysiloxanes consist mostly of columns pa cked hexagonally (or closely so) with lateral long-range positional or der, but with only short-range order along the columns. The unique fea ture of the columnar mesophases is that the axes of macromolecules lie along the column axes. The influence of molecular structure and molec ular mass of linear macromolecules is discussed. It has been demonstra ted 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 crystalliz ation of the mesophase are nucleation controlled processes, although t he crystallization of the mesomorphic state is characterized by some s pecific features. The phase behaviour of cyclolinear polyorganosiloxan es is described with particular emphasis on the temperature stability as a function of molecular structure, tacticity, substituents and flex ible spacers.