Effect of the charge on the morphology of sodium salt form of the randomly sulfonated polystyrene ionomer cast onto silicon wafers.

The morphology of the sodium salt form of randomly copolymerized polystyrene sulfonate (Na-PSS) in water/THF(99/1 v/v) cast onto silicon wafers, was studied by using scanning electron microscope (SEM). The contents of the sulfonate repeat units in Na-PSS were 1.1, 2.4, 4.6, 10.8, and 15.6 mol%. Based on the observed SEM images, the morphology of the Na-PSS changed with increasing ionic group content. For 1.1 and 2.4 mol%, sphere-shaped aggregates were formed with average sizes of 90 nm and of 77 nm, respectively. For 4.6 mol% and 10.8 mol%, 20-30 nm-sized aggregates were close-packed and fused together, resulting a surface with large roughness and ca. 10 nm-sized pores were formed. As the mol% increased to 15.6, the surface became smoother and flat films were formed.