pH-Gated Porosity Transitions of Polyelectrolyte Multilayers in Confined Geometries and Their Application as Tunable Bragg Reflectors

Abstract

Polyelectrolyte multilayers with alternating porous and fully dense regions have been assembled from poly(acrylic acid) (PAA), poly(allylamine hydrochloride) (PAH), and poly(sodium 4-styrenesulfonate) (SPS) in appropriate combinations. The porous zones were developed postassembly via immersion of the heterostructure film into an aqueous acidic medium followed by rinsing in deionized water. Properly assembled PAH/PAA regions exhibit a reversible, pH-gated nanoporosity transition while PAH/SPS regions remain unchanged upon treatment at low pH. The application of these structures as one-dimensional dielectric mirrors and vapor sensors has been investigated. Organic liquid crystal molecules were also successfully loaded selectively into the nanoporous regions via a capillary force driven wicking action. Potential applications for drug delivery and as Bragg stacks with tunable refractive index layers are also discussed.