Photophysical studies of ultrathin films: Characterization of PS and PMMA on fused quartz by fluorescence spectroscopy of pyrene and (4-(1-pyrenyl)butyl)trimethylammomium bromide

Ultrathin films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) coated on fused quartz plates (or slides) by dip coating are examined by using fluorescence spectroscopy. Cations of the pyrene derivative (4-(1-pyrenyl)butyl)trimethylammonium bromide (PBN) are adsorbed onto the quartz slides from aqueous solutions. The rate of dynamic quenching and prompt quenching by O-2 of the PBN fluorescence on the slide in the presence of PMMA and PS films at varying degrees of coverage indicates that, at low coverage, PS films are patchy while PMMA films are continuous. These results are in agreement with published data from AFM studies of similar systems. The fluorescence of pyrene dissolved in ultrathin films on quartz and on hydrophobic (surface derivatized) quartz slides is also examined. As revealed by the trend in pyrene III/I and average fluorescence decay time with coverage, a fraction of the pyrene molecules in PS films diffuse to the polar quartz surface, where they become adsorbed. PMMA, on the other hand, blocks access of pyrene to adsorption sites on the quartz surface presumably by an interaction between carbonyl groups and quartz surface hydroxyls. This fluorescence technique thus provides additional information to AFM in the penetration of reactive species, such as O-2, to the silica-polymer interface and in the polymer-mediated interaction of molecular dopants, such as pyrene, with the silica surface.