Finer Structures of Polyelectrolyte Multilayers Reflected by Solution ¹H NMR

Water-dispersible polyelectrolyte multilayers (PEM) were obtained by layer-by-layer assembly of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (PSS) on water-soluble poly(ethylene glycol)-graft-multiwalled carbon nanotubes (PEG-g-MWNTs), and their structures were investigated using solution ¹H NMR. It was clearly shown that some segments of the adsorbed topmost layers formed in layer-by-layer assembly processes are still highly mobile in aqueous solution, and these segments are totally compensated by subsequent coatings of the oppositely charged layers to form ionically cross-linked polyelectrolyte complexes (PEC) layers; it was demonstrated that the ionization behaviors of the highly mobile segments of the topmost PAH layers are similar to those in solution rather than those in the bulk multilayers at different pH. Furthermore, the similarity among the PEM on substrates and the PEC colloids prepared under similar conditions in solution was supported by solution ¹H NMR results, and the unequal stiochiometry of the cationic group from PAH and the anionic group from PSS in the cores of the PEC colloids in solution was identified from solution ¹H NMR results using PEG as external references. On the basis of these, it is suggested that the PEC layers in PEM formed from coating of the adsorbed topmost PAH layers with PSS solution contain excess PSS, and reversely those formed from coating of the adsorbed topmost PSS layers with PAH solution contain excess PAH. Hence a new description of PEM is put forward that the polyelectrolyte multilayers are composed of alternate layers excess either in PSS or in PAH.