Polarity of layer-by-layer deposited polyelectrolyte films as determined by pyrene fluorescence

The polarity of polyelectrolyte (PE) multilayer films is investigated with pyrene as a polarity-sensitive probe. Multilayer films of poly(styrene sulf onate) (PSS) and various polycations were prepared by the layer-by-layer se lf-assembly technique. Pyrene (PY) molecules were inserted into the films b y exposing the multilayers to pyrene solutions. By this method a homogeneou s distribution of pyrene molecules at low concentration within the film was obtained. The ratio of the fluorescence intensities of the first (I) to th e third (III) vibronic band (Py-value) of the pyrene emission spectrum is e mployed here to determine the polarity of the PE films. PSS and poly(allyla mine hydrochloride) (PAH) multilayer films yielded a pyrene value close to the solvent polarity of acetone, while multilayers of PSS and poly(diallyld imethylammonium chloride) (PDADMAC) displayed a value higher than the one c orresponding to water. The pyrene values of the polyelectrolyte films were independent from the solvent employed for probe dissolving. Although no dir ect relationship between solvent polarity and dielectric constant (epsilon) is available, an estimate of the static dielectric constant of the films c an be provided by comparing the Py-values of the films with those of variou s solvents. Changes in the humidity conditions of the film environment in a closed cell did not affect the film polarity. However, a drastic and irrev ersible reduction of polarity could be induced by actively drying the sampl es by a nitrogen flow.