Chemical and morphological characterization of sulfonated polystyrene brushes in different environments

Surface tethered polyelectrolyte polymer brushes composed of polystyrene (PS) were grafted into a silicon wafer substrate. PS chains were polymerized by grafting from method employing different reaction times in order to obtain polymer brushes with different molecular weights. Then, the grafted polystyrene brushes were chemically modified with a soft sulfonation reaction by employing different times of sulfonation. The sulfonation degrees and chemical structure were analyzed by proton nuclear magnetic resonance (1H NMR) and Fourier transformed infrared spectroscopy (FT-IR). The grafted brushes were fully characterized by contact angle measurements and atomic force microscopy (AFM) after exposed to solutions with different pH values. The morphological analysis revealed different behavior for the brushes depending on the pH of the environments. By immersing polyelectrolytes brushes in water or in basic solutions, polymer brushes that are negatively charged due to sulfonic acid group deprotonation are formed, causing repulsion between the negative charges. This repulsion stretched away the chains to the surface adopting an extended configuration. Thus, brushes generated a continuous layer in the outermost surface of the brushes. Therefore, in water or basic media, the configuration adopted by the brushes can be considered as a uniform charged layer on top of the substrate. However, when the samples were treated by an acid solution, brushes adopted a random distribution. Contact angle (CA) measurements showed differences in surface arrangements keeping in all cases the hydrophilic character of the surface.