Spontaneous homeotropic alignment in films of rigid–flexible polyelectrolyte complexes

Contrary to expectation, the electrostatic complexation of a rigid-rod anionic polyelectrolyte that exhibits nematic phase behaviour in aqueous solution with a flexible cationic polyelectrolyte forms homeotropically aligned films upon casting, i.e. the rigid molecules spontaneously align perpendicular to the substrate. Moreover, such alignment is retained upon the introduction of up to 10 mol% of a flexible cationic polyelectrolyte. The formation of a weak physical gel during film drying or in the precursor solution appears to be critical in facilitating homeotropic alignment in such films. Consequently, it can be envisaged that electrostatic complexation may be fine tuned in order to provide homeotropically aligned films of any rigid charged molecule.