Dynamics of rhodamine 6G sorption into a porous tri-block copolymer thin film

The dynamics of sorption of the laser dye rhodamine 6G (R6G+) into a thin ion-exchange polymer film was measured by spectroscopic ellipsometry. The system under study consisted of a nanometer-sized polymer film in direct contact with a liquid phase. A thin film of a partially sulfonated tri-block copolymer preconcentrated the dye from dilute solutions (10(-5) M) driven by ion-exchange to a film concentration of about 0.1 M. Spectroscopic ellipsometry was used to simultaneously measure the dynamics of the film physical (thickness) and optical changes (refractive index n(lambda) and extinction coefficient k(lambda)). The sorption is ultimately accompanied by dye aggregation in the film, which results in shifts and splitting of features in the monomer absorbance spectrum. The formation of nonluminescent aggregates was also confirmed by emission spectroscopy. Desorption of the dye from the film resulted in a reversal of film physical changes. Spectroscopic results were quantitatively accounted for by constructing an optical model that assumed a dye monomer/dimer (R6G'/(R6G+)(2)) equilibrium in the film. The relatively low magnitude of the dimerization constant in the polymer (KD similar to 27 M-1) suggests an approach to minimize molecular aggregation. Concurrent quartz crystal microgravimetry of the sorption process showed generally similar results, confirming the quantitative results from ellipsometry measurements.