Non-equilibrium adsorbed polymer layers via hydrogen bonding

The layer-by-layer or self-assembly (SA) technique, based on the electrostatic interaction between oppositely charged layers, provides a simple and elegant way of producing multilayer structures from a variety of materials. Multilayers are normally employed because one layer is usually too thin for any device application. We show that considerably thicker layers may be obtained if the adsorption process is non-self-limiting, unlike the case of mechanisms entirely based on electrostatic attraction. This was obtained in spontaneously adsorbed layers of polyaniline (PANi), poly(o-methoxyaniline) (POMA) and tetrahydrotiophenium polyphenylenevinylidene, a PPV precursor. The non-self-limiting adsorption is attributed to H-bonding which is more effective for uncharged polymers. It is also shown that the adsorption process is only non-self-limiting under appropriate experimental conditions, since adsorption must be performed under a non-equilibrium condition. Producing thicker layers via the non-self-limiting adsorption procedure may have important implications in device applications, but for that one needs to overcome the drawback caused by aggregation that affects film quality. For PANi and POMA, we have also demonstrated that multilayers can still be produced from uncharged polymers, simply by protonating them in an acid solution, and then alternating one of them with a polyanion.