Distributed Bragg reflectors: Morphology of cellulose acetate and polystyrene multilayers

The optical quality and photonic properties of all-polymer distributed Bragg reflectors are related to the morphology of the layers and the optical responses of the materials. We introduce the X-ray reflectivity method to determine the thickness, the interface- and surface-roughness of cellulose acetate and polystyrene layers which are two polymers often used in the domain of spin casted multilayer systems. Atomic force microscopy and spectroscopic ellipsom-etry were used as complementary techniques for investigating the surface roughness and the film thickness. The shrinkage and the change of interface roughness of the polymers were investigated up to temperatures of 200 °C Up to 170 °C the interface roughness stays constant at about 1 nm while it increases up to 2nm at 200 °C. The thickness of the polystyrene layer remains constant up to 170 °C, well above its glass transition temperature T_g. For cellulose acetate a monothonic decrease is observed with increasing temperature. It could be shown, that the change in the optical response of a thermally treated distributed Bragg reflector is related to the change of the layer thickness of cellulose acetate. Spectra of (PS CA)₂₀PS distributed Bragg reflectors (DBR) are in a good agreement with calculated spectra with parameters optained from of the X-ray reflectivity measurements.