Preparation of multilayers containing conjugated thiophene-based polyelectrolytes. Layer-by-layer assembly and viscoelastic properties

Abstract

We study the layer-by-layer assembly and properties of polyelectrolyte multilayers containing anionic and cationic poly(alkoxythiophene) derivatives, poly(3-(3'-thienyloxy)propanesulfonate) (P3TOPS) and poly(3-(3'-thienyloxy)propyltriethylammonium) (P3TOPA), together with poly(diallyldimethylammonum chloride) (PDADMA) or poly(styrenesulfonate) (PSS). These polythiophenes are rigid-rod-type polyelectrolytes which tend to aggregate in aqueous medium. Three types of multilayers have been prepared: (P3TOPS/P3TOPA)(n) (all-thiophene films), (P3TOPS/PDADMA)(n), and (PSS/P3TOPA)(n). The layer-by-layer adsorption of polythiophenes was followed by UV-vis spectroscopy and surface plasmon resonance (SPR). Adsorption of P3TOPS and P3TOPA took place within 10 min from dilute aqueous solution (1 mM with respect to monomers). The adsorption of anew polyion layer on top of polythiophene was always accompanied by a partial loss of the polythiophene layer. As the result, the amount of polythiophene in the film oscillated as a function of the number of layers. This behavior was attributed to the adsorption and partial desorption of aggregated polythiophene, supported by atomic force microscope (AFM) images of dry films. Per bilayer, the amount of polythiophene remaining in the film increased linearly and corresponded approximately to the formation of a thiophene monolayer. The viscoelastic properties of the film; also exhibited a marked terminal layer effect in cases where the length and hydrophobicity of the polyions were different. This effect was studied with (PSS/PDADMA) multilayers and was attributed to the formation of loops and tails, All multilayers studied appeared very soft, comparable to protein layers. This softness was attributed mainly to the film/solution interface with a hydrogel-like outer part of the films (zone III).