Possible presence of hydrophilic SO₃H nanoclusters on the surface of dry ultrathin Nafion® films: a positron annihilation study

Abstract

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.

Probing interfacial interactions of nafion ionomer: Thermal expansion of nafion thin films on substrates of different hydrophilicity/hydrophobicity

“…From Figure , the strong thickness dependency of CTE for thinner Nafion films (<60 nm) can be noted. Similar trends for thickness dependency of other Nafion thin films properties including proton conductivity and water uptake has also been reported where films below 60 nm show stronger changes in properties compared to thicker films . The increase in CTE with decreasing film thickness has been attributed to unfavorable interaction between the substrate and the polymer for PS on Si .…”

“…Similar trends for thickness dependency of other Nafion thin films properties including proton conductivity and water uptake has also been reported where films below 60 nm show stronger changes in properties compared to thicker films. 4,27,[29][30][31][32][33][34][35][36][37][38] The increase in CTE with decreasing film thickness has been attributed to unfavorable interaction between the substrate and the polymer for PS on Si. 39 It has been argued that the unfavorable interactions with the substrates promote polymer chain mobility and thereby an increased expansivity compared to the bulk polymer.…”

Excited‐State Proton Transfer and Conformational Relaxation of 2‐(4′‐Pyridyl)benzimidazole in Nafion Films

“…The structural and transport properties of the commercial Nafion membranes of various equivalent weights and thickness have been studied extensively . For the thin films, the focus has been on the effect of annealing, thickness and on the substrate on which they are cast . To have realistic applications, the films should be mechanically robust and strongly acidic.…”

Enhancing NMR Sensitivity of Natural‐Abundance Low‐γ Nuclei by Ultrafast Magic‐Angle‐Spinning Solid‐State NMR Spectroscopy

“…[16][17][18][19] For the thin films, the focus has been on the effect of annealing, [20][21][22] thickness [23][24][25] and on the substrateo nw hicht hey are cast. [22,[26][27][28] To have realistic applications, the films should be mechanically robust and strongly acidic.H owever,f reshly prepared films are reportedt ob eb rittle and soluble in polar solvents including warm water. [29] Gebel et al [20] have demonstrated that heating impartsc rystallinity to the films, with ac oncomitantd ecrease in solubility.Achange in the structure of the ionomer from micelle to reverse micelle also takes place upon heating.…”