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Solutions of Nafion® with an ion exchange capacity (IEC) of 0.91 meq g(-1), which are on the verge of the formation of SO(3)H nanoclusters, were spin coated on silicon (Si), glassy carbon (GC) and platinum/silicon (Pt/Si) substrates to form films of up to 256 nm thickness. Nanostructure of the films was studied using Doppler broadening of annihilation radiation (DBAR), positron annihilation lifetime (PAL), X-ray photoelectron spectroscopy (XPS), an atomic force microscope (AFM) and contact angle measurements. Contact angles as low as 10 degrees indicate that the surface of dry ultrathin Nafion® films on Si is highly hydrophilic. XPS data of 10 nm thick, ultrathin film on Si show that oxygen concentration is enhanced and the SO(3)H group concentration, in other words, IEC on the surface is much higher than other films. The S parameter measured by DBAR of an ultrathin Nafion® film on Si is much higher than that of the films on the other substrates. We consider that a large number of hydrophilic, reversed micelle like SO(3)H groups are on the surface of the ultrathin Nafion® film on Si but not on the surface of other films. Positrons implanted into the film are trapped by the SO(3)H clusters, annihilating with the electrons of oxygen and exhibit the high S parameter. The SO(3)H concentration on the surface of thin Nafion® films on GC and Pt/Si substrates may not be so high as the threshold for the formation of a large number of SO(3)H clusters. Positrons implanted into the films annihilate mostly with fluorine atoms, resulting in a low S parameter. The film-substrate interaction plays an essential role in nanostructuring of Nafion® thin films, which may also be the case for Nafion® on the catalysts of polymer electrolyte fuel cells.

Diffusion coefficients of positronium (Ps) have been determined for polysulfone (PSF), polycarbonate (PC) and polystyrene (PS) by means of positron Iifetime spectroscopy. 2,T-Dinitrobiphenyl (DNB) was added to the polymers as a Ps quencher, and the diffusion coefficients were determined from measured Ps quenching rate constants, assuming that the reaction between Ps and DNB was diffusion controlled. It is shown that the diffusion coefficients of Ps at 25 "C are (2.6-3.2) x lop6 ern' s -I, tw-three orders of magnitude larger than those of oxygen, nitrogen and argon. The effect of DNB on Ps formation is briefly discussed.

Time-resolved momentum distributions of positron-electron annihilation photons were investigated for various polymers by means of positron age-momentum correlation (AMOC) spectroscopy. The momentum distribution exclusively associated with orthopositronium (o-Ps) pick-off annihilation is found to be element-specific and can sensitively probe different light atoms such as C, O, F, and Si that constitute the surface of nanometer-sized free volumes. It is demonstrated that the chemical environment around free volumes in polymers for practical use can be probed by positron annihilation AMOC spectroscopy.

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