Mechanism of ion transport in perfluoropolyether electrolytes with a lithium salt.

Perfluoropolyethers (PFPEs) are polymer electrolytes with fluorinated carbon backbones that have high flash points and have been shown to exhibit moderate conductivities and high cation transference numbers when mixed with lithium salts. Ion transport in four PFPE electrolytes with different endgroups was characterized by differential scanning calorimetry (DSC), ac impedance, and pulsed-field gradient NMR (PFG-NMR) as a function of salt concentration and temperature. In spite of the chemical similarity of the electrolytes, salt diffusion coefficients measured by PFG-NMR and the glass transition temperature measured by DSC appear to be uncorrelated to ionic conductivity measured by ac impedance. We calculate a non-dimensional parameter, ß, that depends on the salt diffusion coefficients and ionic conductivity. We also use the Vogel-Tammann-Fulcher relationship to fit the temperature dependence of conductivity. We present a linear relationship between the prefactor in the VTF fit and ß; both parameters vary by four orders of magnitude in our experimental window. Our analysis suggests that transport in electrolytes with low dielectric constants (low ß) is dictated by ion hopping between clusters.

Incipient microphase separation in short chain perfluoropolyether-block-poly(ethylene oxide) copolymers.

Incipient microphase separation is observed by wide angle X-ray scattering (WAXS) in short chain multiblock copolymers consisting of perfluoropolyether (PFPE) and poly(ethylene oxide) (PEO) segments. Two PFPE-PEO block copolymers were studied; one with dihydroxyl end groups and one with dimethyl carbonate end groups. Despite having a low degree of polymerization (N ∼ 10), these materials exhibited significant scattering intensity, due to disordered concentration fluctuations between their PFPE-rich and PEO-rich domains. The disordered scattering intensity was fit to a model based on a multicomponent random phase approximation to determine the value of the interaction parameter, χ, and the radius of gyration, Rg. Over the temperature range 30-90 °C, the values of χ were determined to be very large (∼2-2.5), indicating a high degree of immiscibility between the PFPE and PEO blocks. In PFPE-PEO, due to the large electron density contrast between the fluorinated and non-fluorinated block and the high value of χ, disordered scattering was detected at intermediate scattering angles, (q ∼ 2 nm-1) for relatively small polymer chains. Our ability to detect concentration fluctuations was enabled by both a relatively large value of χ and significant scattering contrast.

Size-dependent dissociation of carbon monoxide on cobalt nanoparticles.

In situ soft X-ray absorption spectroscopy (XAS) was employed to study the adsorption and dissociation of carbon monoxide molecules on cobalt nanoparticles with sizes ranging from 4 to 15 nm. The majority of CO molecules adsorb molecularly on the surface of the nanoparticles, but some undergo dissociative adsorption, leading to oxide species on the surface of the nanoparticles. We found that the tendency of CO to undergo dissociation depends critically on the size of the Co nanoparticles. Indeed, CO molecules dissociate much more efficiently on the larger nanoparticles (15 nm) than on the smaller particles (4 nm). We further observed a strong increase in the dissociation rate of adsorbed CO upon exposure to hydrogen, clearly demonstrating that the CO dissociation on cobalt nanoparticles is assisted by hydrogen. Our results suggest that the ability of cobalt nanoparticles to dissociate hydrogen is the main parameter determining the reactivity of cobalt nanoparticles in Fischer-Tropsch synthesis.

A reaction cell with sample laser heating for in situ soft X-ray absorption spectroscopy studies under environmental conditions.

A miniature (1 ml volume) reaction cell with transparent X-ray windows and laser heating of the sample has been designed to conduct X-ray absorption spectroscopy studies of materials in the presence of gases at atmospheric pressures. Heating by laser solves the problems associated with the presence of reactive gases interacting with hot filaments used in resistive heating methods. It also facilitates collection of a small total electron yield signal by eliminating interference with heating current leakage and ground loops. The excellent operation of the cell is demonstrated with examples of CO and H2 Fischer-Tropsch reactions on Co nanoparticles.

Environmental photochemistry of tylosin: efficient, reversible photoisomerization to a less-active isomer, followed by photolysis.

The environmental photochemical kinetics of tylosin, a common veterinary macrolide antibiotic and growth promoter, were investigated under simulated sunlight. An efficient, reversible photoisomerization was characterized using kinetic, mass spectrometry, and proton nuclear magnetic resonance data. The photoisomerization was confirmed to occur by a rotation about the distal alkene of the ketodiene functionality. Concurrent forward (quantum yield = 0.39 +/- 0.09) and back (quantum yield = 0.32 +/- 0.08) reactions lead to a photochemical equilibrium near a tylosin/photoisomer ratio of 50:50, completed in less than 2 min under a spectrum equivalent to noontime, summer sunlight. The activity of the isomer for the inhibition of Escherichia coli DH5alpha growth was observed to be less than that of tylosin. On a longer time scale than that of isomerization, the isomer mixture undergoes photolysis with a quantum yield of (1.4 +/- 0.3) x 10(-3). The observed quantum yields and UV-vis absorbance data allow for the prediction of the photochemical behavior of tylosin in most environmental systems. Indirect photosensitization was not a significant loss process in solutions of Suwannee River fulvic acid with concentrations from 1 to 20 mg L(-1).