MASS-TRANSPORT PHENOMENA IN THIN-FILMS OF POLY(2-VINYLPYRIDINE) STUDIED VIA OPTICAL GUIDED-WAVE TECHNIQUES

The unique ability of optical waveguide techniques to determine simultaneously film thicknesses and mass transport behavior in swollen polymer films is utilized in these experiments to make accurate determinations of diffusion coefficients of fluorescein into H2O-swollen poly(2-vinylpyridine) films from aqueous solution. The diffusion behavior is determined from fitting the fluorescence-time curves to an intensity expression derived from Fick's second law and the appropriate boundary conditions to obtain the diffusion coefficient of the fluorophore in the film. Two techniques for characterizing the fluorescence behavior spatially and temporally are critically compared. Fiber optic-based detection schemes suffer from inaccuracies relative to the use of an imaging camera based on a charge-coupled device (CCD) array. The diffusion coefficients obtained show no significant dependence on the bulk solution concentration in the range 1 nM less than or equal to [fluorescein] less than or equal to 10 mu M of the fluorophore. The value of the diffusion coefficient was found to be in the range 1 X 10-(-12) cm(2)/s less than or equal to D less than or equal to 3 x 10(-11) cm(2)/s and was found to vary widely with small changes in sample preparation conditions.