Molecular modeling of self-organized polyelectrolyte end-grafted layers

MARIO TAGLIAZUCCHI; MÓNICA OLVERA DE LA CRUZ; IGAL SZLEIFER

Beer Sheva & Ein-Gedi

Workshop; The 9 th Stadler Minerva student workshop; 2011

Reimund Stadler Minerva Center for Mesoscale Macromolecular Engineering

Responsive materials embrace several kinds of bulk and surface systems that can alter their structure, behavior or physical properties as a response to a change in their chemical or physical environment. We use a molecular theory to study polyelectrolyte and redox polyelectrolyte end grafted layers, explicitly including the competition between different physical and chemical interactions. This competition drives the self‑organization of the film into aggregates of different morphologies (some of them previously observed in experiments) such as micelles, stripes, holes and micelles coexisting with non-aggregated chains. These morphologies arise because grafted hydrophobic polymers tend to form domains since their chemical attachment to the surface does not enable macrophase separation. In polyelectrolytes and redox polyelectrolytes the coupling between chemical equilibrium (i.e. redox and/or acid base equilibrium), van der Waals interactions and electrostatic forces affects the morphology of the aggregates and allows control over surface patterns by bulk pH, electrode potential and salt concentration. We present here a systematic investigation on the effect of bulk pH, bulk salt concentration, grafting density and electrode potential on the morphological behavior. We also show that the local environment within the grafted film can drastically differ from that in the bulk solution, for example the local pH inside micellar aggregates may differ by more than 1 unit with that of the bulk solution. For redox polymers, these local properties have an impact on the (experimentally accessible) current-potential curves. The picture that emerges from this study is that the final state of the system cannot be explained or analyzed in terms of each individual molecular interaction, but it emerges from their global and highly coupled optimization.