Electrostatic Deposition of Polyionic Monolayers on Charged Surfaces

Abstract

We explored molecular films of polyionic materials built by electrostatic deposition on a charged surface. Formation of self-assembled monolayers is monitored for poly(styrenesulfonate) (PSS) adsorbed on charged surfaces of amine-terminated self-assembled monolayer (SAM) and poly(allylamine) (PAA) on a PSS monolayer. In both cases, polyions are adsorbed on oppositely-charged surfaces. Observations of PSS monolayers at various stages of electrostatic deposition reveal inhomogeneous self-assembly at the earliest stages of deposition. During the first several minutes of deposition, negatively-charged PSS macromolecules tend to adsorb on selected defect sites of positively-charged SAM (scratches, microparticles, and edges) and form islands composed of PSS coils. At this stage, electrostatic adsorption of PSS chains is predominant and equilibration of the surface structure is not achieved by the slow surface diffusion mechanism. Only longer deposition times (>10 min) result in an equilibration of polymer layers and formation of a homogeneous thin PSS layer composed of highly flattened macromolecular chains. The monolayer thickness is between 1.0 and 1.5 nm, with a microroughness of about 0.2 nm. Self-assembly of a second PAA layer on top of a PSS monolayer follows similar tendencies, resulting in the formation of homogeneous PAA/PSS bilayers with an overall thickness of 1.7−2.5 nm.