N.m.r. relaxation in Nafion—the low temperature regime

MacMillan, Bryce; Sharp, A. R.; Armstrong, Robin L.

doi:10.1016/s0032-3861(98)00483-2

Cited by 42 publications

(51 citation statements)

References 21 publications

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“…We believe that this fact can be explained by the following two factors: (i) the irregular nature of the surface of the watercontaining clusters in the Nafion/water system and (ii) the presence of free (unbonded) water molecules and associating polymer groups distributed in the fluorocarbon matrix. As previously hypothesized, [35] the surface area to volume ratio of the aggregates within Nafion is unusually high; from the NMR data, the cluster surface area was found to be proportional to R 2.5 , where R is the effective cluster size. This supports the view of the irregularly shaped cluster surfaces -the "fuzzy" spheres, which have a ratio of surface area to volume considerably higher than for a perfect solid sphere.…”

Section: Local Structure and System Morphologysupporting

confidence: 62%

“…The most important feature of structural organization predicted for the bicomponent system is that, for the wide range of hydration levels (h 7.5 wt.-% H 2 O), the value of r s * is smaller than r w *. Taking into account this observation as well as the results of our previous computer simulation of bicomponent micelles [15] and the experimental NMR data, [14,35] it is believed that the water molecules and polar SO 3 H groups form mixed aggregates with a fairly well-defined three-layer structure consisting of a central water region, an outer layer of side-chain endgroups bonded with the water molecules, and an outer region ("corona") of the "neutral" sections of side-chains, which are immersed into external fluorocarbon matrix. Such a clathrate-like structure has been found for inverted micelles in the presence of low-molecular-weight additives solubilizing the aggregates.…”

Section: Local Structure and System Morphologymentioning

confidence: 78%

“…Our theoretical prediction of well ordered state for incorporated water is in qualitative agreement with existing experimental (NMR) data. [35] It is believed that the water molecules and polar SO 3 H groups form mixed aggregates with a fairly welldefined three-layer structure. Such a clathrate-like structure has been found for inverted micelles in the presence of low-molecular-weight additives solubilizing the aggregates.…”

Section: Discussionmentioning

confidence: 99%

“…[35] In this work, the dynamical properties of water absorbed in Nafion were investigated at temperatures below the dynamical transition temperature (L 253 K). In this temperature regime the characteristics of the physisorbed water were found to be somewhat anomalous.…”

Section: Local Structure and System Morphologymentioning

confidence: 99%

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Structural Organization of Water-Containing Nafion: The Integral Equation Theory

Khalatur¹,

Talitskikh²,

Khokhlov³

2002

Macromol. Theory Simul.

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Using the mutually consistent variant of the integral equation polymer reference interaction site model (RISM) theory and the chemically realistic rotational isomeric state (RIS) model, we perform a molecular level modeling of the specific structural organization of perfluorosulfonic acid ionomer (Nafion) with certain amount of physisorbed water. Our results establish molecular scale information necessary for understanding the equilibrium structure and thermodynamics of water‐containing Nafion as well as water distribution and ionic (molecular) transport phenomena in hydrated Nafion membranes. As a first step in this direction, semi‐empirical quantum mechanical calculations on the molecular structures and energies of the polymeric backbone and pendant chains of Nafion with and without additional water molecules are carried out. These data are used in the single‐polymer RIS Monte Carlo simulation, in which the short‐range intramolecular interactions are taken into account via appropriate matrices of statistical weights. The local structure and morphology of the entire bicomponent water‐containing system is calculated consistently on the basis of the RISM theory at different contents of absorbed water. We find that the addition of even a relatively small amount of water leads to the strong intensification of aggregation processes observed for polar sulfonic acid (SO3H) groups. Water molecules and polar SO3H groups form mixed aggregates with a three‐layer structure. Incorporation of water molecules inside the aggregates results in an increase of their stability and leads to the increase of the number of associating groups in a stable aggregate. With increasing the number of incorporated solvent molecules, the average size of mixed aggregates increases. The results obtained in the present study support the concept of an irregularly shaped cluster surfaces. Such geometries are favorable to the formation of long channels of connected water‐containing aggregates providing the unique permeability characteristics of Nafion membranes. In addition, we investigate the clustering and continuum percolation in the water phase, using the formalism of pair‐connectedness correlation functions. The mean cluster size found theoretically is in reasonable agreement with the corresponding experimental estimates existing in the literature.

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Section: Local Structure and System Morphologysupporting

confidence: 62%

Section: Local Structure and System Morphologymentioning

confidence: 78%

Section: Discussionmentioning

confidence: 99%

Section: Local Structure and System Morphologymentioning

confidence: 99%

See 2 more Smart Citations

Structural Organization of Water-Containing Nafion: The Integral Equation Theory

Khalatur¹,

Talitskikh²,

Khokhlov³

2002

Macromol. Theory Simul.

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“…This prediction can be compared with existing experimental data. Using NMR measurements of the dynamical characteristics of water absorbed in Nafion, MacMillan et al [39] have concluded that the surface area of clusters ("water pools") within hydrated Nafion is proportional toR R 2.5 , whereR R is the effective radius of "water pool". This indicates that the cluster surface is very crumbly.…”

Section: Effect Of Water On Internal Aggregate Structurementioning

confidence: 99%

Structural Organization of Water-Containing Nafion: A Cellular-Automaton-Based Simulation

Mologin

Khalatur²,

Khokhlov

2002

Macromol. Theory Simul.

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Using the cellular‐automaton‐based simulation technique, we study the processes of self‐organization in the systems of comb‐like copolymers with strongly attracting (end‐functionalized) side‐chains in the presence of low‐molecular‐weight water‐like solvent. This molecular level modeling reflects the basic features of the specific structural organization of perfluorosulfonic acid ionomer (Nafion) with certain amount of physisorbed water. We compare the simulation data with the existing phenomenological models used in the literature to describe the structural features of water‐swollen ionomer membranes and the results of our previous calculations based on the mutually consistent integral equation theory and the chemically realistic rotational isomeric state (RIS) model. Our attention is focused on the effect of water on the local aggregate structure, the shape of mixed, water‐containing aggregates, and on system morphology.

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Utilization of prompt‐γ neutron activation analysis in the evaluation of Nafion membranes

Young

Treviño

Tan

et al. 2003

J Polym Sci B Polym Phys

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Researchers have performed evaluations on Nafion membranes for many years. An important aspect of the membranes is their chemical composition and uniformity. One technique that could provide accurate and fast quantitative measurements of the chemical composition and water within the membranes is prompt‐γ neutron activation analysis (PGAA). In this work, PGAA was applied to the chemical characterization of Nafion membranes in various forms. The evaluation confirmed the necessity of pretreatment of the membranes for the elimination of contamination. A series of counterion‐exchanged membranes were examined so that we could determine the degree of conversion achieved under different conditions and residual water levels. The results indicated that the type of counterion was an important factor determining conversion levels, that partial conversion could be achieved, and that counterion displacement could be forced to occur in certain environments. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1485–1492, 2003

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N.m.r. relaxation in Nafion—the low temperature regime

Cited by 42 publications

References 21 publications

Structural Organization of Water-Containing Nafion: The Integral Equation Theory

Structural Organization of Water-Containing Nafion: The Integral Equation Theory

Structural Organization of Water-Containing Nafion: A Cellular-Automaton-Based Simulation

Utilization of prompt‐γ neutron activation analysis in the evaluation of Nafion membranes

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