One Methylene Group in the Side Chain Can Alter by 90 Degrees the Orientation of a Main-Chain Liquid Crystal on a Unidirectional Substrate.

The mechanisms of orientation of columnar liquid crystals (LCs) on a PTFE-rubbed surface are explored on a homologous series of symmetrically substituted poly(di--alkylsiloxanes) (PDAS). It is shown that by increasing the side-chain length in steps of one CH group, the orientation of PDAS switches back and forth from perpendicular to parallel with respect to PTFE chains. These changes are sensitive to the smallest possible variation of the macromolecular structure (i.