Title

Wetting/dewetting behavior of lightly sulfonated polystyrene ionomers on solid surfaces

Date of Completion

January 2010

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

The primary objective of this study was to improve the interfacial wetting properties of the solid surface by developing a uniform, robust film of random sulfonated polystyrene (SPS) ionomer. Meanwhile, to ascertain the mechanism of the suppression of dewetting—i.e., is it thermodynamic or rheological in origin, was our second goal. ^ The wetting/dewetting behavior of thin films of low molecular weight SPS ionomers spin-coated onto silica surfaces were studied using atomic force microscopy (AFM), contact angle measurements, and electron microscopy. The effects of the sulfonation level, the choice of the cation, the solvent used to spin-coat the films, and the molecular weight of the ionomer were investigated. The addition of the sulfonate groups suppressed the dewetting behavior of the PS above its glass transition temperature. Increasing the sulfonation level led to more homogeneous and smoother surfaces. The choice of the cation used affected the wetting properties, but not in a predictable manner. ^ The octadecyltrichlorosilane (OTS)-modified silica surface was employed as a hydrophobic substrate to investigate the mechanism of wetting behavior of SPS. The complex between the ionic groups of ionomers and the –Si-O- groups on the surface of the silicon wafer was very critical for better wetting behaviors of a lower sulfonated Lithium ionomer (2.5LiSPS4), while ionic aggregation between ionomer molecules with a higher sulfonation level (6.5LiSPS4) played an important role for the suppression of dewetting. ^ Rough surfaces composed of discrete but relatively uniform nanoparticles were prepared from a lightly sulfonated polystyrene ionomer by spin coating from tetrahydrofuran (THF) or a THF/methanol mixture onto a silica surface. The particle morphology is consistent with the spinodal decomposition of the film surface occurring during spin coating. The particles are well wetted to the silica, and if heated for a long time above the ionomer's glass-transition temperature, the particles flow and coalesce into a smooth, homogeneous film. The size distribution of particles at the air/solid interface are influenced by the types of solvent, sulfonation level, the counterions and the substrate based on our current research. ^