Control of coating properties by tailored particle interactions: relation between suspension rheology and film structure

Abstract

The microstructure of dip-coated particulate thin films is studied by varying the stability of an aqueous silica sol used as coating bath. Different stability conditions are adjusted by changing electrolyte concentration and pH-value. The sol is characterized by acoustic and electroacoustic spectroscopy to obtain particle size distribution (PSD) and ζ-potentials. Furthermore, photometric measurements supplement the studies of stability behavior giving clear evidence of a non-DLVO contribution that is attributed to hydration forces. The structure of coatings, defined as the order of particles within the layer, was found to vary with stability parameters. It is observed that the surface roughness scanned by atomic force microscope (AFM) increased with increasing electrolyte concentration. Moreover, the structural evolution of the particulate network as a function of solid concentration and stability parameters is studied. For these purposes the sol is accumulated by dialysis against polyethylenglycol (PEG) and evaluated in terms of the viscosity as a function of shear rate. Finally, the influence of coating structures on the UV-light transmission properties is determined. Especially in case of LaCl3 a strong dependency between sol stability and the optical transmission property of the thin film is observed. The results explicitly confirm a correlation between microscopic structure and macroscopic property.