Photoacoustic monitoring of thermal wave interference effects during the formation of polymeric thin films from solutions

In this work, the potential of photoacoustic (PA) technique in the analysis of the formation of polymer films by evaporation from aqueous solutions is explored. Experiments are performed using different aqueous volume fractions of two different polymers, PEDT:PSS and a white liquid glue. The liquid sample is deposited on a metallic substrate attached to a modified PA cell that allows the monitoring of the evaporation process and development of the polymeric film. It is shown that when the thickness of the sample is of the order of the thermal diffusion length, maxima and minima of the PA signal, due to thermal wave interference effects, are observed. In the last stage of the process, the PA signal reaches a stable value which depends on the thickness of the formed film. The kinetics of the polymeric film formation, the dependence of the evaporation rate of the solvent as well as the final thickness of the polymeric film is discussed. These results can be useful in establishing a methodology for the photothermal monitoring and thermal characterization of the development of polymeric films from solutions.