The study presented here is a fundamental investigation into the molecular origins of the thermal transitions and dynamic mechanical relaxations of Nafion membranes as studied by DSC, DMA, variable temperature small-angle X-ray scattering (SAXS), and solid-state F-19 NMR spectroscopy. While several studies in the literature have attempted to explain the molecular origins of these thermal transitions and mechanical relaxations, the assignments were based primarily on limited DMA results and have at times been contradictory. In DSC traces of Nafion, the low- and high-temperature endotherms are shown to be dependent on thermal history and are now attributed to melting of relatively small and large crystallites, respectively. DSC analysis of Nafion yields information only on the crystalline nature of this ionomer, and neither of the transitions can be assigned to glass transitions. The intensity of the small-angle ionomer peak at ca. q = 2 nm(-1) was monitored as a function of temperature for each alkylammonium neutralized sample. Changes in intensity of the ionomer peak as a function of temperature were shown to correlate well with the a and P relaxations observed in DMA. Variable temperature solid-state F-19 NMR techniques were used to investigate the dynamics of the Nafion chains. Spin-diffusion experiments revealed a profound increase in mobility at the onset of the a relaxation. Sideband analysis indicated that the side chain is more mobile than the main chain and that the mobility is greatly affected by the size of the counterion. Molecular level information from this analysis in correlation with SAXS and DMA data supports the assignment of the beta relaxation to the genuine T-g of Nafion and the alpha relaxation to the onset of long-range mobility of chains/side chains via a thermally activated destabilization of the electrostatic network.

The melt-processible sulfonyl fluoride precursor of a Nafion(R) ionomer was utilized as a sol-gel reaction medium for 3-aminopropyltriethoxysilane (APrTEOS). The diffusion-mediated reaction of APrTEOS with SO2F groups can be controlled with high degree of reaction. Fourier transform infra-red/attenuated total reflection studies show that sulfonamide linkages are formed and condensation reactions of SiOR groups provide covalent crosslinking of chains. Formic acid treatment plus high temperature plus long time resulted in a high degree of polymer crosslinking as seen in Si-29 solid state nuclear magnetic resonance spectra. Mechanical modulus and strength increase, and elongation-to-break decreases with increasing filler. Hybrids with <18% uptake accumulate cracks with crosslinked outer layers, each event signalled by a drop in stress followed by stress recovery. While there are sharp visual material fronts inward from both surfaces, EDAX (energy dispersive analysis of X-rays) showed that there are no sharp Si composition boundaries. Differential scanning calorimetry (d.s.c.) revealed a broad, weak endothermic event peaking at 67 degrees C for the precursor and shifting to higher temperatures while broadening with increasing filler content, indicating progressively-restrictive glass-transition-temperature-related molecular motions within an increasingly nonhomogeneous environment. For the unreacted precursor, this d.s.c. transition occurs at a temperature just above a glass transition detected by dynamic mechanical means. (C) 1997 Elsevier Science Ltd.

The melt-processible sulfonyl fluoride precursor of a Nafion® ionomer was utilized as a sol gel reaction medium for 3-aminopropyltriethoxysilane (APrTEOS). The diffusion-mediated reaction of APrTEOS with SO2F groups can be controlled with high degree of reaction. Fourier transform infra-red/attenuated total reflection studies show that sulfonamide linkages are formed and condensation reactions of SiOR groups provide covalent crosslinking of chains. Formic acid treatment plus high temperature plus long time resulted in a high degree of polymer crosslinking as seen in 29Si solid state nuclear magnetic resonance spectra. Mechanical modulus and strength increase, and elongation-to-break decreases with increasing filler. Hybrids with < 18% uptake accumulate cracks with crosslinked outer layers, each event signalled by a drop in stress followed by stress recovery. While there are sharp visual material fronts inward from both surfaces, EDAX (energy dispersive analysis of X-rays) showed that there are no sharp Si composition boundaries. Differential scanning calorimetry (d.s.c.) revealed a broad, weak endothermic event peaking at 67°C for the precursor and shifting to higher temperatures while broadening with increasing filler content, indicating progressively-restrictive glass-transition-temperature-related molecular motions within an increasingly nonhomogeneous environment. For the unreacted precursor, this d.s.c. transition occurs at a temperature just above a glass transition detected by dynamic mechanical means.

The small angle X-ray scattering technique was used to probe structural heterogeneity on the scale of nanometers within Nafion®/[silicon oxide], Nafion®/[ORMOSIL], and Nafion®/[dimethylsiloxane] hybrid membranes. The results of this study reinforced the working hypothesis of morphological template action for the in situ growth of silicon oxide, or organically modified silicon oxide phases in perfluorosulfonate ionomers via sol-gel reactions for silicon alkoxide or/and silicon alkylalkoxide precursors. Nanophase separation persists when incorporated silicon oxide particles are postreacted with ethoxytrimethylsilane but postreaction with diethoxydimethylsilane generates co-continuous phases that do not generate ionomer SAXS peaks, presumably due to a more homogeneous Si atom distribution within the ionomer. These hybrids are true nanocomposites, as structural heterogeneity exists on the scale of ∼ 5 nm. The variation of the small angle upturn for these hybrids is explained in terms of long-range inhomogeneities in ionomers. © 1996 John Wiley & Sons, Inc.

Attenuated total reflectance FTIR spectroscopy was used to compare the influence of hydrophilic and hydrophobic counterions on the vibrational behavior of the sulfonate and perfluoroether groups in 1100 equivalent weight (EW) Nafion(R) and 808 EW Dow perfluorosulfonate ionomer membranes. For dry, Na+-form membranes, the infrared band attributed to the - SO3- symmetric stretching vibration was found to shift to higher frequencies with an increase in the degree of neutralization. In contrast, neutralization with tetrabutlyammonium ions caused the - SO3- vibrational band to shift to lower frequencies with increasing neutralization. This behavior is attributed to a diminished polarization of the - SO3- groups by the hydrophobic and diffusely charged TBA(+) cations, relative to the b strong polarization observed with Na+ ions. Vibrational bands attributed to perfluoroether groups in close proximity to the sulfonate groups were also found to be influenced by Coulombic interactions within the clusters. The frequency shifts of these bands followed a trend that was virtually identical to that observed for the - SO3- symmetric stretch. This behavior is attributed to a combination of through-space, dipolar field effects, and through-bond inductive effects, from the neighboring ionic groups. In Nafion(R), the ether groups directly attached to the backbone are shielded from the Coulombic interactions within the clusters, while the ether groups within three bonds of the terminal ionic groups are very sensitive to the state of polarization of the sulfonate anion. (C) 1995 John Wiley & Sons, Inc.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTAnisotropic ionic conductivity in uniaxially oriented perfluorosulfonate ionomersKevin M. Cable, Kenneth A. Mauritz, and Robert B. MooreCite this: Chem. Mater. 1995, 7, 9, 1601–1603Publication Date (Print):September 1, 1995Publication History Published online1 May 2002Published inissue 1 September 1995https://pubs.acs.org/doi/10.1021/cm00057a002https://doi.org/10.1021/cm00057a002research-articleACS PublicationsRequest reuse permissionsArticle Views532Altmetric-Citations54LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

A procedure for preparing a melt-processable form of perfluorosulfonate ionomers is presented. The melt-flow Properties of the melt-processable polymer are attributed to the elimination of polymer crystallinity and strong electrostatic crosslinks which act as barriers to flow in semicrystalline ionomers. The ''PTFE-like'' crystallinity is removed by evaporation of the ionomer solutions, and the electrostatic crosslinks are weakened by neutralizing the ionomer with large, hydrophobic tetrabutylammonium counterions. To yield the desirable melt-flow properties, the bulky counterions function as internal plasticizers and form weak ion pair dipoles which diminish the attractive forces responsible for ionic aggregation. Fabrication procedures and the results of preliminary characterization studies of melt-processed films and membranes are discussed here.