The simplest layer-by-layer assembly structure: Best paired polymer electrolytes with one charge per main chain carbon atom for multilayered thin films

Layer-by-layer (LbL) thin film assembly behaviors of the polymethylene-type polyelectrolytes poly(fumaric acid) and poly(methyleneamine) were comprehensively studied as a function of the dipping solution pH values. Since polymethylene-type polyelectrolytes can possess double the density of side-chain charges of conventional vinyl (or polyethylene-type) polyelectrolytes, they exhibited stronger intramolecular side-chain interactions, which were revealed by potentiometrce titrations and their sequential adsorption behaviors. Thicknesses of the LbL thin films obtained, decreased with the degree of ionization of both polyelectrolytes. The pH-dependent variation in film thickness was subtle and continuous when compared with the vinyl polyelectrolytes, probably because of the large variation in pK_a of the polymethylene-type polyelectrolytes in the multilayer thin films. Refractive indices of the thin films were not positively correlated with their thicknesses, but the pH matrix displayed maximum values for the combination of almost fully charged polycation and polyanion. The main chains of highly charged polyelectrolytes are apparently substantially extended, resulting in the optimal packing of polycations and polyanions, in terms of the electrostatic attractions, to form high-density polymer films. AFM measurements revealed that the high-density thin films are composed of densely assembled almost monodispersed polymer nanoparticles with an average diameter of 17 nm. In contrast, flexible polymers with fewer side-chain charges are loosely assembled, giving thicker films of relatively low density. Thus, the pH matrix of the refractive indices is inversely correlated with that of the roughness parameters determined by AFM measurements. Finally, FT-IR-RAS spectra revealed the presence of charged species in all the LbL thin films, suggesting a significantly decreased pK_a for poly(fumaric acid). These results illustrate some of the most fundamental and unexplored aspects of LbL film assemblies based on the simplest of chemical structures.