Hamdy F.M. Mohamed

Effects of ion exchange on the free volume and oxygen permeation in Nafion for fuel cells.

Variations of the free volume, O2 permeability, and structure of the Nafion membrane upon ion exchange of H+ with Na+ and K+ were studied. The free volume was quantified using the positron annihilation lifetime (PAL) technique, whereas the polymer structure was characterized by dynamic mechanical analysis (DMA), nanoindentation, and wide-angle X-ray diffraction (WAXD). It was found that the ion exchange significantly expands the free volume and at the same time decreases the O2 permeability. This is opposed to the simple free volume model in which the structure with less open volume is more amenable to lower permeability. Comparison of experimental data collected by different techniques revealed that not only the free volume but also the polymer stiffness plays an essential role in O2 permeation.

Effect of interface on surface morphology and proton conduction of polymer electrolyte thin films

To understand the relationship between surface morphology and proton conduction of polymer electrolyte thin films, perfluorinated ionomer Nafion® thin films were prepared on different substrates such as glassy carbon (GC), hydrophilic-GC (H-GC), and platinum (Pt) as models for the ionomer film within a catalyst layer. Atomic force microscopy coupled with an electrochemical (e-AFM) technique revealed that proton conduction decreased with film thickness; an abrupt decrease in proton conductance was observed when the film thickness was less than ca. 10 nm on GC substrates in addition to a significant change in surface morphology. Furthermore, thin films prepared on H-GC substrates with UV-ozone treatment exhibited higher proton conduction than those on untreated GC substrates. However, Pt substrates exhibited proton conduction comparable to that of GCs for films thicker than 20 nm; a decrease in proton conduction was observed at ∼5 nm thick film but was still much higher than for carbon substrates. These results indicate that the number of active proton-conductive pathways and/or the connectivity of the proton path network changed with film thickness. The surface morphology of thinner films was significantly affected by the film/substrate interface and was fundamentally different from that of the bulk thick membrane.

Positronium Formation in Aromatic Polymer Electrolytes for Fuel Cells

We measured the yield of positronium (Ps) in sulfonated aromatic proton conducting membranes for the polymer electrolyte fuel cell (PEFC) with and without -SO(2)- in their chemical structures by positron annihilation lifetime spectroscopy (PALS). It was observed that Ps formation is almost totally inhibited in the polymers without -SO(2)- such as sulfonated poly(ether ether ketone) (SPEEK). On the other hand Ps favorably forms in those with -SO(2)- as sulfonated polyether sulfone (SPES), which is due to the anti-inhibition effect of -SO(2)-. The high probability of Ps formation in these polymers enables the study of the free volume and the mechanism of gas permeation by PALS.

Microstructure changes of poly(vinyl chloride) investigated by positron annihilation techniques

Abstract The properties of the free volume hole in pure and doped poly(vinyl chloride) (PVC) have been investigated as a function of cadmium content and temperature using positron annihilation lifetime and Doppler broadening of annihilation radiation techniques. The observed spectra were decomposed into three components, with the longest component (lifetime, τ3 and intensity, I3) being associated with the pick-off annihilation of o-Ps trapped by free volume. The temperature coefficient of τ3 and I3 showed a change at the glass transition temperature (Tg) to lower temperature. Further, the behavior of the annihilation parameters was different during the heating and cooling cycles. These observations are correlated with observations of the electrical conductivity. The free volume hole and the fractional free volume were also derived from the positron annihilation parameters. A correlation between the fractional free volume and conductivity above Tg was experimentally confirmed using measured positron annihilation results. The value of the critical volume for doped PVC is found to be larger than that for pure PVC. This larger difference might be the result of the existence of large ionic charge carriers.

Free Volume and Oxygen Permeability in Polymers Related to Polymer Electrolyte Fuel Cells

The mechanism of gas permeation in sulfonated polymers such as Nafion and sulfonated polyethersulfone (SPES) for polymer electrolyte fuel cells has been investigated from the viewpoint of free volume. Free volume was quantified using the positron annihilation lifetime (PAL) technique and O2 permeability was measured as a function of temperature or ion exchange capacity (IEC). Good linear correlations between the logarithm of the permeabilities and reciprocal free volume indicate that gas permeation in the sulfonated polymers depends strongly on the free volume. Comparison of Nafion with SPES and other relatively flexible chain polymers showed that the stiff chains of the perfluoroethylene backbone in Nafion make the O2 permeability much smaller than the corresponding flexible chain polymer with a similar free volume size. The O2 permeability in the K+-form of Nafion is systematically smaller than the H+-form, showing that the chain stiffness is enhanced by the ion exchange.

Positron annihilation studies of some charge transfer molecular complexes

Abstract Positron annihilation lifetimes were measured for some solid charge transfer (CT) molecular complexes of quinoline compounds (2,6-dimethylquinoline, 6-methoxyquinoline, quinoline, 6-methylquinoline, 3-bromoquinoline and 2-chloro-4-methylquinoline) as electron donor and picric acid as an electron acceptor. The infrared spectra (IR) of the solid complexes clearly indicated the formation of the hydrogen-bonding CT-complexes. The annihilation spectra were analyzed into two lifetime components using PATFIT program. The values of the average and bulk lifetimes divide the complexes into two groups according to the non-bonding ionization potential of the donor (electron donating power) and the molecular weight of the complexes. Also, it is found that the ionization potential of the donors and molecular weight of the complexes have a conspicuous effect on the average and bulk lifetime values. The bulk lifetime values of the complexes are consistent with the formation of stable hydrogen-bonding CT-complexes as inferred from the IR-spectral data.

Studies of the Positron Lifetime and Doppler Broadening Annihilation Radiation of Polyvinyl Chloride Doped with Al2O3

Positron annihilation lifetime and Doppler broadening of annihilation radiation studies of pure and doped polyvinyl chloride (PVC) with Al2O3 reflect the effect of concentration on free volume. Therefore, variations of the relative fractional free volume, which is extracted from positron annihilation parameters, have been correlated with changes in the dielectric properties (dielectric constant and a.c. conductivity) of the pure and doped PVC with Al2O3 from 10 to 50 %. The o-Ps lifetime and its intensity show a linear dependence with a discontinuity at 30 % concentration of Al2O3. A correlation between the relative fractional free volume and the dielectric parameters has been achieved.

Study of the effect of electric field on positron annihilation parameters in polymers

Abstract Positron annihilation lifetime measurements have been performed to study the effect of an external electric field in several polymers. The application of the electric field on polytetrafluoroethylene (PTFE) and ultrahigh-molecular weight polyethylene (UHMWPE) noticeably decreased the ortho-positronium (o-Ps) intensity. The o-Ps intensity increased with increasing electric field strength in the poly(ethylene terephthalate), PET, poly(ethylene naphthalate), PEN, and poly(aryl-ether-ether-ketone), PEEK samples. The data are consistent with a hypothesis that nonpolar polymers (PTFE and UHMWPE) show a decrease in the o-Ps intensity with increasing electric field, while the effect seems to be opposite in polar polymers (PEN, PET and PEEK).

Positron annihilation investigations on poly(methyl methacrylate)

Abstract Positron lifetime and Doppler broadened annihilation radiation were measured for seven different samples of poly(methyl methacrylate) at room temperature in vacuum. The polymerisation of methyl methacrylate was carried out as a bulk polymerisation in the presence of benzoyl peroxide as an initiator. The effect of the amount of the initiator on the viscosity-average molecular weight was studied. It was found that the viscosity-average molecular weight decreased with increasing amount of the initiator. The average lifetime and intensity of ortho -positronium ( o -Ps) increased with increasing viscosity-average molecular weight up to 6.85 × 10 4 and remained constant after that. The S -parameter showed a similar behaviour as that of the o -Ps intensity.

Study of sorption in poly(vinyl alcohol) using the positron annihilation technique

The vacancy states of fresh and annealed poly(vinyl alcohol) polymers sorbing CO2 gas at various pressures have been investigated by the positron annihilation lifetime (PAL) technique. The change of the bulk volume and the amount of sorbed CO2 gas were measured simultaneously to compare the free volume fraction with the size of the ortho-positronium holes. The PAL data indicate that the size of the ortho-positronium hole increases with the progress of the sorption while the sorption and the bulk volume data show that the free volume fraction becomes constant. Also, it supports the existence of local plasticization and the improvement of the dual mechanism model of sorption of gas.