All-Star Polymer Multilayers as pH-Responsive Nanofilms

Abstract

Star polymers with globular architecture and multiple arms are among the simplest forms of polymers with branched topologies. The combination of their unique architecture and high local densities of active functional groups makes star polymers unique candidates for a diverse range of applications. In this article, we describe the synthesis of star polymers with precisely controlled structures via atom transfer radical polymerization (ATRP) using the one-pot arm-first method. Specifically, two types of highly defined, high charge density star polymers with oppositely charged arm structures were prepared: poly[2-(dimefliylamino)ethyl methacrylate] (PDMAEMA) star and poly(acrylic acid) (PAA) star polymers with cross-linked cores. By exploiting the electrostatic interactions between the polyelectrolyte arms, we have integrated the PDMAEMA star and PAA star polymers within alternating multilayer thin films using layer-by-layer (LbL) assembly to generate all-star polyelectrolyte LbL films. The prepared star/star multilayer films illustrate nonuniform and nanoporous structures, which result from the characteristic architecture of star polymers. The thickness, porosity, and refractive index of star/star multilayer films are precisely tunable by assembly pH conditions. Furthermore, as-assembled star/star multilayer films exhibit distinct morphological changes by undergoing extensive structural reorganization upon post-treatment under different pH conditions that do not lead to any changes with their linear compositional counterparts; it is hypothesized that these differences are due to the star polyelectrolyte's compact structure and decreased extent of entanglement and interpenetration, which lead to a low degree of ionic cross-linking compared to their linear counterparts. The pH-responsive structural changes of the films are characterized by AFM, SEM, and FTIR. Finally, we have observed an enhanced ionic (proton) conductivity of star/star multilayers following the pH-induced structural reorganization.