Formation, Structure, and Properties of Stimuli-Responsive Polyelectrolyte Films

Investigation of the interfacial properties of polyelectrolyte (PE) films is of great importance from both fundamental and application points of view. The present Ph.D. thesis aims to understand the fundamental mechanisms that govern the physiochemical properties of PE monolayer and multilayers in aqueous medium. This thesis in total includes four research studies. One study is devoted to poly(diallyldimethylammonium chloride) (polyDADMAC) monolayer, while the three other works concern polysaccharide multilayer films.
In the first study (Paper I), I investigated the adsorption of polyDADMAC to silica surfaces at different pH values. The charge density of polyDADMAC is independent of the pH of the solution since it is a strong PE. On the other hand, the charge density of the silica surface is sensitive to the pH value. The goal of this study was to understand how changing the pH can affect the adsorption of a strong PE to silica surfaces, as well as the net surface charge. I examined the adsorption of polyDADMAC to the surface at different pH values using quartz crystal microbalance with dissipation monitoring (QCM-D). In addition, I investigated surface forces between two polyDADMAC-coated silica surfaces at different pH values. It was shown that increasing the pH leads to larger adsorption of polyDADMAC. In addition, the surface potential can repeatedly change between positive and negative as continuously injecting the polyDADMAC solution and increasing pH.
In the remaining three studies, I employed chitosan (CHI) and alginate dialdehyde (ADA) to fabricate polysaccharide multilayer films through a layer-by-layer (LbL) technique. The aldehyde groups of ADA can form covalent bonds with amine groups of CHI during LbL deposition, resulting in a self-cross-linked film. Accordingly, I investigated the fundamentals of growth, stability, and stimuli-responsiveness of CHI/ADA multilayer films.
In paper Ⅱ, I employed QCM-D and spectroscopic ellipsometry to examine the growth and stimuli-responsiveness of CHI/ADA multilayer films. It was shown that the film growth demonstrates a linear type behaviour and that the film has a rather compact structure with small water content. The chemical bonds between CHI and ADA was confirmed to enhance the structural stability of the multilayer film. In addition, it was shown that the film can swell at acidic and alkaline conditions and is responsive to the multivalent SO42− and Ca2+ ions.
In paper Ⅲ, I prepared CHI/ADA multilayer films having either CHI or ADA as the outermost layer. I aimed to investigate how changing pH, salt concentration, and the outer layer chemistry affect the properties of the multilayer film. The swelling behaviour, surface forces, and surface charge of CHI/ADA multilayer films were investigated using QCM-D, atomic force microscopy (AFM), and zeta potential measurements. It was affirmed that the outermost layer has a significant effect on the swelling behaviour and stimuli-responsiveness of the multilayer films.
Finally, in paper Ⅳ, I prepared CHI/ADA multilayer films with tunable crosslinking density, swelling capacity, and pH-responsiveness. Here, the swelling capacity and pH-responsiveness of the film are modulated by fabricating the multilayer films at two different pH values and the by systematically changing the oxidation degree of ADA. While non-crosslinked films demonstrated irreversible structural changes in response to pH changes, a systematic increase in the oxidation degree of ADA was shown to enhance the stability and decrease the swelling capacity.