Title

Studies of layer-by-layer polyelectrolyte adsorption

Date of Completion

January 2004

Keywords

Chemistry, Polymer|Engineering, Materials Science

Degree

Ph.D.

Abstract

Layer-by-Layer (LbL) assembly of polyelectrolytes is a widely investigated approach for building substrate-supported macromolecular architectures. This dissertation examines several aspects of multilayered structure deposition via polycation/polyanion directed-assembly. Utilizing novel techniques such as polyelectrolyte spin-assembly (PSA), sequential adsorption within microfluidic channels, and hydrogel contact deposition, we have elaborated localized multilayer growth, which can eventually be an invaluable asset to numerous technological fields including microelectronics, biosensors, power generation, and chemical analysis. ^ Employing PSA, sequential adsorption of oppositely charged polyelectrolytes from dilute solutions of different ionic strength undergoing radial flow was achieved. Growth was monitored using UV-Vis spectroscopy and Atomic Force Microscopy (AFM). At low salt concentrations, electrostatic interactions between oppositely charged chains determine chain adsorption, while, at high salt concentrations, chain deformation due to external flow controls the layer structure and, thus, the PSA growth rate. A foray into microfluidics helped further our understanding of localized multilayer deposition under continuous flow. Customized “stamp” defined a microfluidic network that guided sequential adsorption while potentially allowing polymer chain extension. AFM confirmed the preparation of well-defined LbL coatings on a silicon substrate. This first demonstration of locally grown self-assembled films using microfluidics shows similar trends in growth rate with ionic strength to PSA, suggesting similar adsorption mechanisms. ^ Hydrogel contact deposition afforded rapid processing of discrete polymeric multilayer islands by combining aspects of both ESA and contact stamping. Swollen hydrogels consisting of poly(sodium-4-styrene sulfonate) and poly(allylamine hydrochloride) solutions, respectively, were alternatively brought in contact with a substrate in order to build multilayered structures. The growth rate of adsorbing polyelectrolytes was observed to be a function of applied force during contact. ^ Nafion™ is the exchange membrane of choice for direct methanol fuel cells due to its high proton conductivity. Unfortunately, it is a poor methanol barrier. LbL processing was investigated as a means to develop coatings with enhanced barrier properties. Multilayered coatings of sulfonated poly(benzo bisimidazole) complexed with poly(2-vinyl pyridine) were prepared on a Nafion™ membrane serving as LbL substrate. Polyelectrolyte absorption into the Nafion™ membrane was found to precede the expected sequential adsorption. The molecular composite coatings suggested to serve as an additional barrier to methanol, thus improving fuel cell performance without compromising proton conductivity. ^