PH-gated porosity and reversible swelling transitions in polyelectrolyte multilayers

Abstract

This thesis explores the design of novel pH-responsive polyelectrolyte multilayer (PEM) films. The discoveries of discontinuous pH-induced swelling transitions and concomitant conformational memory effects as well as pH-gated nano- and microporosity in PEMs, are described. The PEMs in this thesis are created by an aqueous processing technique, which enables the straightforward manipulation of processing variables to create molecular-level designed polymeric coatings. The first part of this thesis examines discontinuous pH-induced swelling transitions and reversible memory effects in certain PEMs of poly(allylamine hydrochloride) and (poly(styrene sulfonic acid) (PAH/SPS). It is demonstrated that the systematic design of molecular organization, hydrophobic character, and a variable electrostatic nature, enables these phenomena in PAH/SPS films. Consequently, co-existing conformational states result, which allow the regulation of the affinity of molecular species to the PEM as well as their sustained release from the PEM. The second part of this thesis describes phase-separation phenomena in PEMs comprised of PAH and poly(acrylic acid) (PAA). It is shown that specifically designed PAH/PAA films exhibit pH-gated nano- and microporosity. Further, the length-scale of pH-gated porosity can be systematically tuned by simple variations in the film's processing conditions. It is shown that nanoporous films can be designed to possess indices of refraction in the range of 1.5 - 1.15 and can be employed as high-performance anti-reflection coatings. Several designs of optical coatings are demonstrated, as is the ability to pattern these coatings by a simple aqueous technique. The ability to reversibly uptake and release molecular species from the pH-gated porous structures is also discussed. Overall, the observations in this thesis underscore the fact that unique pH-responsive behavior can be systematically designed into PEM films by virtue of processing parameters such as solution pH and the choice of polyelectrolytes.