Probing of the mechanical properties and monitoring of the drying process of azopolymer thin films for optical recording

Abstract

Azopolymers prepared in the form of thin films are promising candidates as media for optical recording due to their photo-anisotropic properties. The thermo-physical and mechanical properties of the prepared films, as well as the uniformity of their thickness and smoothness of the formed layers are important characteristics for application of these materials. In the present study, thin films of two amorphous and one liquid crystalline azopolymers were characterized and, in the experiment, a commercially available amorphous azopolymer was included as a base for comparison. Atomic force microscopy was applied for surface topography determination for thin film samples of all four azopolymers and roughness parameters as the average roughness and root mean square values are estimated. These values show that amorphous azopolymers are characterized by a high smoothness and a flatter surface while the thin film from the liquid crystalline polymer shows a structure with pronounced topographic features and thus exhibits much higher surface roughness. The conducted depth-sensing indentation of non-exposed thin films prepared by spin-coating on glass substrates allowed to determine mechanical properties such as universal and indentation hardness, elastic modulus and elastic part of indentation work. The creep behavior of the tested materials was also analyzed and the elastic-plastic behavior for all azopolymers was established. The most pronounced plastic behavior was observed for the liquid crystalline one. We obtained different temporal scales of the drying process of solutions of the tested materials in a polymer casting experiment by a quantitative dynamic speckle analysis. Analysis involved statistical processing of correlated sequences of laser speckle patterns, reflected the properties of the respective materials and was supported by the reported mechanical characteristics.