Shulamith Schlick

Visualizing chemical reactions and crossover processes in a fuel cell inserted in the ESR resonator: detection by spin trapping of oxygen radicals, nafion-derived fragments, and hydrogen and deuterium atoms.

We present experiments in an in situ fuel cell (FC) inserted in the resonator of the ESR spectrometer that offered the ability to observe separately processes at anode and cathode sides and to monitor the formation of HO and HOO radicals, H and D atoms, and radical fragments derived from the Nafion membrane. The presence of the radicals was determined by spin-trapping electron spin resonance (ESR) with 5,5-dimethylpyrroline N-oxide (DMPO) as a spin trap. The in situ FC was operated at 300 K with a membrane-electrode assembly (MEA) based on Nafion 117 and Pt as catalyst, at closed and open circuit voltage conditions, CCV and OCV, respectively. Experiments with H(2) or D(2) at the anode and O(2) at the cathode were performed. The DMPO/OH adduct was detected only at the cathode for CCV operation, suggesting generation of hydroxyl radicals from H(2)O(2) formed electrochemically via the two-electron reduction of oxygen. The DMPO/OOH adduct, detected in this study for the first time in a FC, appeared at the cathode and anode for OCV operation, and at the cathode after CCV FC operation of >or=2 h. These results were interpreted in terms of electrochemical generation of HOO at the cathode (HO + H(2)O(2) --> H(2)O + HOO) and its chemical generation at the anode from hydrogen atoms and crossover oxygen (H + O(2) --> HOO). DMPO/H and DMPO/D adducts were detected at the anode and cathode sides, for CCV and OCV operation; H and D are aggressive radicals capable of abstracting fluorine from the tertiary carbon in the polymer membrane chain and of leading to chain fragmentation. Carbon-centered radical (CCR) adducts were detected at the cathode after CCV FC operation; weak CCR signals were also detected at the anode. CCRs can originate only from the Nafion membranes, and their presence indicates membrane fragmentation. Taken together, this study has demonstrated that FC operation involves processes such as gas crossover, reactions at the catalyst surface, and possible attack of the membrane by reactive H or D that do not occur in ex situ experiments in the laboratory, thus implying different mechanistic pathways in the two types of experiments.

Advanced ESR Methods in Polymer Research

Preface. The Editor. Contributors. Dedication. PART I: ESR FUNDAMENTALS. Chapter 1. Continuous-Wave and Pulsed ESR Methods (Gunnar Jeschke and Shulamith Schlick). Chapter 2. Double Resonance ESR Methods (Gunnar Jeschke). Chapter 3. Calculating Slow-Motion ESR Spectra of Spin-Labeled Polymers (Keith A. Earle and David E. Budil). Chapter 4. ESR Imaging (Shulamith Schlick). PART II: ESR APPLICATIONS. Chapter 5. ESR Study of Radicals in Conventional Radical Polymerization Using Radical Precursors Prepared by Atom Transfer Radical Polymerization (Atsushi Kajiwara and Krzysztof Matyjaszewski). Chapter 6. Local Dynamics of Polymers in Solution by Spin-Label ESR (Jan Pila&rcaron ). Chapter 7. Site-Specific Information on Macromolecular Materials by Combining CW and Pulsed ESR on Spin Probes (Gunnar Jeschke). Chapter 8. ESR Methods for Assessing the Stability of Polymer Membranes Used in Fuel Cells (Emil Roduner and Shulamith Schlick). Chapter 9. Spatially Resolved Degradation in Heterophasic Polymers From 1D and 2D Spectral-Spatial ESR Imaging Experiments (Shulamith Schlick and Krzysztof Kruczala). Chapter 10. ESR Studies of Photooxidation and Stabilization of Polymer Coatings (David R. Bauer and John L. Gerlock). Chapter 11. Characterization of Dendrimer Structures by ESR Techniques (M. Francesca Ottaviani and Nicholas J. Turro). Chapter 12. High Field ESR Spectroscopy of Conductive Polymers (Victor I. Krinichnyi). Index.

ESR Spectrum of O3− Trapped in a Single Crystal of Potassium Chlorate

The O3− radical obtained by x irradiation of oxygen‐17 enriched KClO3 was studied by ESR. The molecule is trapped at three distinct sites in the potassium chlorate single crystal, one of which gives rise to a more intense ESR spectrum. For O3− at the dominant trapping site, the principal values of the g tensor are 2.0035, 2.0187, and 2.0123. Two distinct oxygen‐17 hyperfine splittings were observed and their variation as a function of orientation studied. The principal values of the corresponding hyperfine tensors are 82.6, −8, −8 and 43.6, −5, −5 G. The direction of the maximum principal value for both oxygen‐17 hyperfine tensors coincides with the direction of the minimum principal value of the g tensor, as expected for a bent 19‐electron molecule. The higher splitting is assigned to the central atom and the lower splitting to the magnetically equivalent outer atoms. The spin density distribution in O3− deduced from experimental data is compared with theoretical results obtained from a CNDO/II calculati...

Infrared Spectra of the Lithium Halide Monomers and Dimers in Inert Matrices at Low Temperature

The infrared absorption spectra of lithium fluoride, chloride, and bromide vaporization products have been investigated between 2–37 μ in solid argon, krypton, xenon, and nitrogen matrices at liquid‐hydrogen and liquid‐helium temperatures. Both the natural lithium salts containing 93% 7Li and 6Li salts as well as equimolar isotopic mixtures were studied. The solvent shifts of the monomers were found to confirm the isotopic invariance of the ratio Δω/ω as predicted by Buckingham. Serious deviations from the Kirkwood—Bauer—Magat relation were, however, observed. These shifts have also been discussed in terms of specific solute—solvent interactions. Two dimer vibrational frequencies were definitely characterized by means of isotopic substitutions and these frequencies were assigned. They represent in‐plane stretching modes of the planar rhombus, the higher energy vibration being of symmetry b3u (X—X axis polarized) and the lower energy vibration b2u, (Li–Li axis polarized).

ESR line shape studies of trapped electrons in γ‐irradiated 17O enriched 10M NaOH alkaline ice glass: Model for the geometrical structure of the trapped electron

The ESR line shape and width of a trapped electron (et−) has been measured in 34% 17O enriched 10M NaOH alkaline ice glass at 77 K. The second moment of et− due to 17O coupling is 134 G2. Analysis of the second moment contributions from 17O coupling together with the constraint from electron spin echo studies that the et−–O distance is greater than 2 A show that the et−–17O hyperfine coupling is mainly isotropic. Simulation of the et− ESR line under these constraints on 17O coupling and previous ones determined for H or D coupling for various numbers n of equivalent water molecules in the first solvation shell of et− give a best fit to the experimental line shape for n=6. For n?4 definite structure would be expected in the et− line shape which is not seen experimentally. The n=6 result together with previous data allows a rather complete geometrical description of the trapped, hydrated electron to be defined.

Study of phase separation in polyurethanes using paramagnetic labels: effect of soft-segment molecular weight and temperature

Abstract The objective of this study is to assess the effect of temperature and molecular weight of the soft segment on the degree of mixing between the hard and soft segments in segmented polyurethanes. The method of study consists of measuring electron spin resonance (e.s.r.) spectra of a nitroxide spin label in a series of segmented polyether polyurethanes (PU), in the temperature range 100–450 K. The PU are based on 4,4′-diphenylmethane diisocyanate (MDI), poly(tetramethylene oxide) glycol (PTMO) and 1,4-butane diol (BD) as the chain extender. The polymers were prepared from four molecular weights ( MW ) of PTMO: 650, 1000, 2000 and 2900. The hard-segment content is constant in all polymers prepared (38 wt%). The nitroxide probe 4-hydroxy-2,2′,6,6′-piperidine-1-oxyl (TEMPOL) was attached to the polymer chains by reaction with an -NCO group of MDI. The nitroxide label is therefore located at a chain end. Two sites for the label, differing in their dynamical properties, were detected in all polymers around 300 K and most likely represent the hard and soft domains of the PU. Analysis of the e.s.r. spectra indicates that at ambient temperature (300 K) the increase in the MW of PTMO decreases the degree of mixing between the two segments in the PU. At 400 K the trend is reversed, and the PU containing the soft segment with the largest molecular weight has the highest degree of phase mixing. The lineshapes detected at 400 K can be interpreted in terms of a distribution of label sites, characterized by widths δg and δA of the g iso and A iso (from 14 N) values, respectively, and reflecting the polarity profile in the polymers.

Electron spin resonance imaging of polymer degradation and stabilization

Abstract We present the application of one-dimensional (1D) and two-dimensional (2D) electron spin resonance imaging (ESRI) for the study of photo- and thermal degradation of polymers containing hindered amine stabilizers (HAS). The method is based on the formation of stable nitroxide radicals derived from HAS during polymer treatment, and on encoding spatial information in the ESR spectra via magnetic field gradients. The imaging technique allowed nondestructive profiling of the HAS-derived nitroxide radicals. The intensity profile in the sample depth was deduced by 1D ESRI, and the spatial variation of the ESR line shapes (‘spectral profiling’) was determined by 2D spectral-spatial ESRI. Application of this approach to the photodegradation of polypropylene and to the photo- and thermal degradation of poly(acrylonitrile–butadiene–styrene) will be described. The method can be used to detect spatial heterogeneities in the degradation process and to identify morphological domains that are selectively degraded.

Electron spin resonance (ESR) spectra of amphiphilic spin probes in the triblock copolymer EO13PO30EO13 (Pluronic L64): hydration, dynamics and order in the polymer aggregates

Abstract Aqueous solutions of the triblock copolymer poly(ethylene oxide)- b -poly(propylene oxide)- b -poly(ethylene oxide) EO 13 PO 30 EO 13 (Pluronic L64) were investigated over a wide concentration range (20–100%, (w/w) polymer) in the micellar, liquid crystalline and reverse micellar phases, using electron spin resonance (ESR) spectroscopy of spin probes. A series of amphiphilic nitroxide spin probes based on n -doxylstearic acid ( n DSA) with n , the carbon atom to which the doxyl group is attached, equal to 5 and 10 were used to measure the local polarity, dynamics and degree of order in the self-assembled system. The 14 N isotropic hyperfine splitting, a N , was the polarity sensitive parameter. The probe location and the corresponding effective local hydration, Z eff , were deduced by comparing ESR spectra of the probes in L64 solutions with spectra of the probes in aqueous solutions of poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO) and a mixture of PEO and PPO containing the monomer molar ratio 26:30, as in L64. The results indicate that the probes reside in and provide evidence for the presence of hydrophobic and non-hydrated regions consisting of PO blocks and that the order in the aggregates decreases from the PO/EO interface toward the PO domains. Additional support for these conclusions was obtained from ESR spectra of the probes in the lamellar phase as a function of added cholesterol and by simulations of the ESR spectra of the probes in the lamellar phase of L64. This study of the hydrophobic part of the aggregates, together with our previous study based on ESR spectra of cationic probes that reside in the polar and hydrated EO regions, lead to a detailed description of the nature of L64 aggregates in aqueous solutions.

Effect of solvents on phase separation in perfluorinated ionomers, from electron spin resonance of VO2+ in swollen membranes and solutions

Abstract We present a study of perfluorinated ionomers with pendent chains terminated by sulfonic groups (Nafion), partially neutralized by paramagnetic VO2+ cations. Electron spin resonance (e.s.r.) spectra of VO2+ were measured in the temperature range 120–300 K in ionomer solutions and membranes swollen by water, methanol and ethanol, and compared with the spectra of VO2+ in the neat solvents. E.s.r. spectra for all systems in the entire temperature range were simulated, using the Brownian diffusion motional model. The rotational correlation time τc of the cation at 300K in the membranes swollen by water, in ionomer solutions in water and in neat water are all similar, and suggest that the cation is located in large water ‘pools’. This conclusion is in agreement with the expected phase separation of the ionomer into polar and non-polar domains that leads to a reverse micellar structure. For the alcohols, however, the e.s.r. spectra and corresponding τc values of the cation in the swollen membrane and in ionomer solutions are similar, but clearly different from those measured in the neat solvents; these results were explained by assuming that the alcohols penetrate into the perfluorinated regions, and form small solvent pools with a diameter

Using ESR spectroscopy to study radical intermediates in proton-exchange membranes exposed to oxygen radicals

Direct ESR and spin-trapping experiments were used to study the behavior of Nafion, a perfluorinated ionomer membrane used in fuel cells, when exposed in the laboratory to oxygen radicals produced by Fenton and photo-Fenton reactions. DMPO (5,5-dimethyl-1-pyroline) was used as the spin trap. The results suggest that the two ESR methods provide complementary information on Nafion fragmentation. The presence of membrane-derived fragments was suggested indirectly by the presence of a broad signal (line width ≈ 84 G) after prolonged exposure of the membrane to the Fenton reagent based on Ti(III), and by the DMPO adduct of a carbon-centered radical in the spin-trapping experiments. The most convincing proof for the presence of perfluorinated radicals was obtained in Nafion membranes partially neutralized by Cu(II), Fe(II) and Fe(III) upon exposure to UV-irradiation in the presence or absence of H2O2 (photo-Fenton treatment). Identification of the chain-end radical RCF2CF2• with magnetic parameters different to those determined for the chain-end detected in γ-irradiated Teflon, was taken as evidence for the attack of reactive oxygen radicals on the side-chain of the membrane. Additional support for this suggestion was the detection of the “quartet” ESR signal assigned to the CF3C•O radical, and of the “quintet” ESR signal assigned to the radical centered at the intersection of the main and side chains. The limitations and advantages of each approach are discussed.