Fluorescence imaging for studying the water uptake and drying process of polymer films

Coatings and adhesive polymer films, originating from water-borne dispersions, often suffer from water sensi-tivity. Further understanding of the film formation process upon drying and the water transport mechanism upon water uptake for dried films is needed to mitigate this effect. In order to determine the water distribution and migration in such films with high spatial and temporal resolution, a novel and versatile in-situ imaging tool, based on confocal fluorescence microscopy, was developed. Using the water-soluble derivative of tetraphenyl-ethylene, sodium 1,2-bis[4-(3-sulfonatopropoxyl)phenyl]-1,2-diphenylethene (BSPOTPE), the fluorescence emission decreased strongly in contact with liquid and gaseous water because of an increase in the intra-molecular mobility of BSPOTPE. By this method diffusion behaviors of water in dried polymer films are quan-titatively differentiated and for multi-component paint formulations the reversibility of water absorption and desorption on different time scales is analyzed. Furthermore, upon fast film drying the formation of a skin at the air interface and the concomitant decrease of the evaporation rate of water is visualized and analyzed for films. The presented method and its capability to simulate closely real application scenarios under the microscope unveil various application properties of water-borne coatings.