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Jorge Luis Menchaca Arredondo

Laboratoire de Physique des Solides
Université Paris-Sud, Orsay

Liquid-Cell Atomic Force Microscopy (LC-AFM) is used to image in situ self-assembled polyelectrolyte films (SAPFs). We show that the technique is appropriate for studying the surface structure of these systems. We report images of layer-by-layer deposition for negative poly(sodium 4-styrenesulfonate) (PSS) and positive poly(allylamine hydrochloride) (PAH). Height AFM images were obtained using the contact mode for different pH values of the initial polyelectrolyte solutions. The technique was then used for imaging other systems. Weak/weak and strong/weak polyelectrolyte films are formed by the alternated deposition of poly(acrylic acid) (PAA)/PAH, and PSS/PAH, respectively, forming a granular surface structure. Grain number and area (GN, GA) are used to characterize the surface of these films during their build up process. We show that hydrophilic PAA increases GA and decreases GN, while these parameters follow an opposite behaviour with PSS. In both cases, GA and GN always have a simple inverse relationship, while the grain surface coverage (GS=GN × GA) is nearly constant and independent of polyelectrolyte nature and substrates. We also studied self-assembled polyelectrolyte nanorings formed by poly(ethylenimine) (PEI) and PSS during the two first steps of SAPF formation. Nanoring formation strongly depends on the preparation method and parameters such as polyelectrolyte filtration, air and CO2 presence during SAPFs formation and buffer solution. The outer diameter of nanorings can be varied from hundreds of nanometres to microns by changing these parameters. Nanorings are stable in the liquid cell for hours but they disappear on contact with air.