Temperature dependence of the free volume in polytetrafluoroethylene studied by positron annihilation spectroscopy

Mohamed, Hashim; Abdel-Hady, E.E.; Mohamed, Salwa S.

doi:10.1016/j.radphyschem.2006.03.026

Cited by 23 publications

(13 citation statements)

References 10 publications

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“…Following a number of successes in the past, the positron annihilation lifetime (PAL) technique has been used for sensitively probing the nanostructure of various polymers. [6][7][8][9][10] Positrons emitted from a radioactive atom (most commonly 22 Na) slow down quickly in a polymer and survive until annihilation either in the form of free positrons or as hydrogen-like positronium atoms (Ps). Depending on the spins of the electron and the positron, Ps can form as para-Ps ( p-Ps, singlet spin of the positron and the electron) or ortho-Ps (o-Ps, spin triplet).…”

Section: H Pendant Side Chainsmentioning

confidence: 99%

Impact of Heating on the Structure of Perfluorinated Polymer Electrolyte Membranes: A Positron Annihilation Study

Hamed

Kobayashi

Kuroda

et al. 2011

Macro Chemistry & Physics

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The ortho‐positronium (o‐Ps) lifetime was measured as a function of temperature for the perfluorinated PEMs Nafion NR212 and Aquivion E8705. The o‐Ps lifetime in Aquivion E8705 was found to be shorter than Nafion NR212 in the entire temperature range studied. Abrupt changes in the temperature coefficient of the o‐Ps lifetime were noticed and the α‐relaxation, β‐relaxation, and stress relief temperatures were identified. It turned out that Aquivion E8705 has higher α‐relaxation and stress relief temperatures than Nafion NR212. Dissociation of the ionic clusters and large change in the nanostructure after heating to 160 °C were observed for Nafion NR212 but not for Aquivion E8705. It is demonstrated that Aquivion E8705 has a higher thermal stability than Nafion NR212. magnified image

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Section: H Pendant Side Chainsmentioning

confidence: 99%

Impact of Heating on the Structure of Perfluorinated Polymer Electrolyte Membranes: A Positron Annihilation Study

Hamed

Kobayashi

Kuroda

et al. 2011

Macro Chemistry & Physics

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“…In polymers, there exist three states of positrons characterized by lifetimes τ 1 , τ 2 , and τ 3 and the corresponding relative intensities I 1 , I 2 , and I 3 , respectively; these are the spin-antiparallel para-positronium (p-Ps) with τ 1 ∼ 125 ps, a free positron with τ 2 ∼ 450 ps, and spin-parallel ortho-positronium (o-Ps) with τ 3 ∼ 1-10 ns. [12][13][14][15] The longestlived component is of particular importance for the polymer studies because the lifetime τ 3 is related to the average free volume hole size. Meanwhile, the relative intensity I 3 represents the probability of o-Ps formation, 16 which is influenced by various reactions of an energetic positron leading to the formation and inhibition of Ps as well as polymer morphology and crystallinity.…”

Section: Introductionmentioning

confidence: 99%

Effects of Ion Exchange on the Free Volume and Oxygen Permeation in Nafion for Fuel Cells

et al. 2009

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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.

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“…PTFE is a porous semicrystalline material (Xiuli et al, 2003;Yi-Yan et al, 1980;Mohamed and Abdel-Hady, 2007), i.e. it is formed of amorphous and crystalline regions.…”

Section: Permeation Devicesmentioning

confidence: 99%

An unheated permeation device for calibrating atmospheric VOC measurements

Brito

Zahn

2011

Atmos. Meas. Tech.

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Abstract. The development of an unpowered permeation device for continuous calibration of in-situ instruments measuring atmospheric volatile organic compounds (VOCs) is described. Being lightweight and compact, and containing only negligible amounts of chemicals, the device is especially suited for field use such as on board aircraft. Its speciality is to maintain the permeation process in thermal equilibrium, so that the instantaneous permeation rate can be ascribed to a simple temperature measurement. This equilibrium state is maintained by a combination of three features: (i) a thin PTFE membrane as permeation medium which guarantees short stabilization times, (ii) a water bath as heat buffer, and (iii) a vacuum-panel based insulation, in which features (ii) and (iii) minimize temperature drifts to ∼30 mK h −1 per Kelvin temperature difference to the environment. The respective uncertainty of the permeation rate due to thermal non-equilibrium is kept below 1 %. An extensive theory part details the major permeation processes of gases through porous polymers, being Fick's diffusion, Knudsen flow, and viscous flow. Both the measured stabilization time and the measured temperature dependence of the permeation rate independently indicate that the permeation can be described by a viscous flow model, where diffusion of the gas molecules in large pores (having a diameter of >0.05 µm) dominates.

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Temperature dependence of the free volume in polytetrafluoroethylene studied by positron annihilation spectroscopy

Cited by 23 publications

References 10 publications

Impact of Heating on the Structure of Perfluorinated Polymer Electrolyte Membranes: A Positron Annihilation Study

Impact of Heating on the Structure of Perfluorinated Polymer Electrolyte Membranes: A Positron Annihilation Study

Effects of Ion Exchange on the Free Volume and Oxygen Permeation in Nafion for Fuel Cells

An unheated permeation device for calibrating atmospheric VOC measurements

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