Physical and optical properties of weakly charged polyelectrolyte multilayer films by Ozzy Mermut.

Thin multilayer films were prepared through layer-by-layer adsorption of oppositely charged polyelectrolytes in aqueous media using a recently established electrostatic self-assembly approach. The adsorbing polyelectrolytes can be classified as either weak or strong based on their level of dissociation in solution. The density of charge in weakly charged polyelectrolytes, unlike in strongly charged ones, can be controlled by adjusting the solution pH about the value of its dissociation constant. At the onset of this dissertation, there were some fundamental and unresolved questions regarding assembly mechanisms, kinetics, and internal structures of the layers in polyelectrolyte multilayer films (PEMs). The studies here examine how varying the level of dissociation of polyelectrolytes in solution influences the kinetics of assembly, and adsorption behavior, as well as some structural, mechanical, and optical properties of PEMs. Using mainly ellipsometry, fluorescence, UV-vis spectrophotometry, and atomic force microscopy (used as a nanoindentation tool), the following investigations of weakly charged polyelectrolytes in multilayer assemblies were performed:The affect of assembly pH on the layer thickness of weakly ionized versus strongly charged polycation/polyanion systems, was compared. At adsorption time scales on the order of seconds, weakly charged polyelectrolytes produced layers which were significantly thicker than strongly charged multilayer systems (by as much as 10-fold). Anomalously rapid adsorption of multilayers was found (i.e., within 1 s) when weakly ionized polycation and polyanions were assembled. The strength of counterion binding, and hence the rate of counterion displacement (an integral part of PEM formation), was also shown to significantly affect the time-dependant growth of multilayers. The effect of polyion chain length on the adsorption of a model polyelectrolyte onto an oppositely charged model polyelectrolyte was examined. At long time scales of adsorption (i.e., 24 hr) preferential deposition of long over short chains was observed. These results were rationalized using thermodynamic arguments involving favorable enthalpy (resulting from a reduction in the electrostatic barrier during adsorption), and entropy gain (from having shorter chains free in solution). Upon adsorption, the relative length of loops (formed between "ionic cross-links" in the layers of weakly charged PEMs assembled at varying pH) exhibited variation by a factor of 50, as implied by mechanical measurement of the film's elastic modulus using an atomic force microscope (AFM). Results from the AFM nanoindentation experiments reported here mark first-time measurements of relative loop lengths in PEMs. With an appreciation of the importance of assembly pH in multilayer build-up, the suitability of weakly charged PEMs, (optically functionalized with azobenzene chromophores) as pH sensors in alkaline media, was assessed. The proposed optical application was based on differences observed in the rate of photo-induced isomerization of azobenzenes in varying pH solutions. (Abstract shortened by UMI.