Fundamental studies of polyelectrolyte multilayer films : optical, mechanical, and lithographic property control

Abstract

Polyelectrolyte multilayers (PEMs) are a versatile type of thin film that is created via layer-by-layer assembly of positively and negatively charged polymers from aqueous solutions. Precise control of the PEM thickness, chemical functionality, and molecular architecture is made possible by changing the polyelectrolytes and assembly conditions during film growth, allowing films to be designed with properties suitable for a given application. This thesis elucidates the intra-film structure and interactions of PEMs through the use of optical, mechanical, and chemical techniques. PEM rugate filters, wherein the refractive index varies through the depth of the film in a continuous, periodic fashion, were constructed by confining silver nanoparticle growth to layers of nanometer-scale thickness. The ability to construct such structures is shown to be dependent on the ability to precisely control the concentration of metal-binding carboxylic acid groups throughout the depth of the film. Software to enable the computation design and optical simulation of these and similar structures was developed.

(cont.) A buckling instability technique was used to probe the Young's modulus of PEM assemblies as a function of polyelectrolyte type, assembly pH, and the relative humidity of the ambient environment. In particular, a two-plate methodology was developed to allow testing on a broad array of multilayer films, and an experimental apparatus was constructed to allow in situ modulus measurements of PEM films under controlled humidity conditions. These techniques are used to elucidate the strong effects that polyelectrolyte type, assembly pH, and the ambient humidity can have on the stiffness of PEM films. The controlled removal of material from assembled PEMs was accomplished via etching of films in solutions of increasing ionic strength. The properties of etched films and process dynamics point to evidence of a polydispersity-enabled phenomenon driven by dissolution of polyelectrolyte complexes containing chains of disproportionate molecular weight. Kinetic and equilibrium data are presented that support this hypothesis. Beyond elucidation of the underlying mechanisms governing molecular interactions within PEMs, possible practical applications for the particular PEM assemblies described in this thesis are discussed, including conformable interference filters and buckling-enabled patterning.