Mark E. Smith

Complete resolution of SiOSi and SiOAl fragments in an aluminosilicate glass by 17O multiple quantum magic angle spinning NMR spectroscopy

Abstract Complete NMR resolution of Siue5f8Oue5f8Si and Siue5f8Oue5f8Al species in an aluminosilicate glass (NaAlSi3O8) is shown using 17O multiple quantum magic angle spinning (MAS) NMR and indicates that no Alue5f8Oue5f8Al is present at the detection limit of 0.5%.

Composition effects in polyetherurethane-based solid polymer electrolytes

Abstract Nuclear magnetic resonance spectroscopy (n.m.r.), dynamic mechanical thermal analysis (d.m.t.a.) and AC impedance techniques have been used in combination to probe the effect of electrolyte composition in an archetypal solid polymer electrolyte (SPE). A series of solid polymer electrolytes (SPEs) based on a urethane-crosslinked trifunctional poly(ethylene glycol) polymer host containing dissolved ionic species (LiClO4 and LiCF3SO3) have been studied. D.m.t.a. has established that increasing LiClO4 concentration causes a decrease in the polymer segmental mobility, owing to the formation of transient crosslinks via cation–polymer interaction. Investigation of the distribution of mechanical/structural relaxation times for the LiClO4/polymer complex with d.m.t.a. reveals that increasing LiClO4 concentration causes a slight broadening of the distribution, indicating a more heterogeneous environment. Results of n.m.r. 7Li T1 and T2 relaxation experiments support the idea that higher salt concentrations encourage ionic aggregation. This is of critical importance in determining the conductivity of the material since it affects the number of charge carriers available. Introduction of the plasticiser tetraglyme into the LiClO4-based SPEs suppresses the glass transition temperature of the SPE, and causes a significant broadening of the relaxation time distribution (as measured by d.m.t.a.).

NMR determination of ionic structure in plasticized polyether-urethane polymer electrolytes

Abstract Solid polymer electrolytes based on amorphous polyether-urethane networks combined with lithium or sodium salts and a low molecular weight cosolvent (plasticizer) have been investigated in our laboratories for several years. Conductivity enhancements of up to two orders of magnitude can be obtained whilst still retaining solid elastomeric properties. In order to understand the effects of the plasticizers and their mechanism of conductivity enhancement, multinuclear NMR has been employed to investigate ionic structure in polymer electrolyte systems containing NaCF 3 SO 3 , LiCF 3 SO 3 and LiClO 3 salts. With increasing dimethyl formamide (DMF) and propylene carbonate (PC) concentration the increasing cation chemical shift with fixed salt concentration indicates a decreasing anion-cation association consistent with an increased number of charge carriers. 13 C chemical shift data for the same systems suggests that whilst DMF also decreases cation-polymer interactions, PC does the opposite, presumably by shielding cation-anion interactions. Temperature dependent 7 Li spin-lattice relaxation times indicate the expected increase in ionic mobility upon plasticization with a shift of the T 1 minimum to lower temperatures. The magnitude of T 1 at the minimum increases upon addition of DMF whereas there is a slight decrease when PC is added. This also supports the suggestion that the DMF preferentially solvates the cation whereas the action of PC is limited to coulomb screening, hence freeing the anion.

Examination of the mixed-alkali effect in (Li,Na) disilicate glasses by nuclear magnetic resonance and conductivity measurements.

Results from 29Si, 23Na and 7Li magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy, 7Li NMR relaxation and electrical conductivity in a series of [Li(1-x).Nax]2O.2SiO2 (disilicate) glasses are used to investigate the mixed-alkali effect. From the 29Si NMR spectra there is relatively little change of the network with alkali composition. 23Na and 7Li NMR linewidths and shifts change continuously as a function of composition, indicating that the alkali ions are intimately and uniformly mixed rather than separated into lithium and sodium-rich domains. The activation energy from electrical conductivity shows a distinct maximum at the central composition (x = 0.5), whereas the local activation energy for lithium motion determined from NMR shows only a smaller but monotonic increase as the lithium-content decreases.

A study of Li2TiOSiO4 and Li2TiOGeO4 by X-ray powder and electron single-crystal diffraction, 17O MAS NMR and O K-edge and Ti L2,3-edge EELS

X-ray powder and electron single-crystal diffraction of crystals of Li(2)TiOSiO(4) and Li(2)TiOGeO(4) showed them to be tetragonal, space group P4/nmm unit-cell parameters a = 6.4379(2), c = 4.40032(2) Å for Li(2)TiOSiO(4) and a = 6.6110(8), c = 4.4372(6) Å for Li(2)TiGeO(4). The compounds are isostructural with their sodium analogues but are considerably compressed along the c axis owing to the smaller size of lithium compared with sodium atoms. Square-pyramidal TiO(5) groups are joined in these compounds by tetrahedral SiO(4) and GeO(4) groups, respectively. (17)O nuclear magnetic resonance spectra of the two compounds, isotopically enriched with (17)O, showed peaks due to the apical titanyl, Ti-O, and basal, bridging, Ti-O-Si or Ti-O-Ge, oxygen atoms of the title compounds. By comparison with reference spectra, oxygen K edges and titanium L(2,3) edges of electron energy-loss spectra were tentatively assigned.

Orientation of the quadrupole and dipole tensors of hydroxyl groups by 17O quadrupole separated local field NMR

The static 17O NMR spectra of Mg(OH)2 and amorphous Mg(OH)x(OCH3)2−x were measured. Simulation of these spectra gave e2qQ/h=6.8u2009MHz, η=0, and δiso=20u2009ppm, and e2qQ/h=7.25u2009MHz, η=0, and δiso=−25u2009ppm for the hydroxyl oxygen in, respectively, Mg(OH)2 and Mg(OH)x(OCH3)2−x. An OH distance in Mg(OH)2 of 1.001 A was obtained using Lee–Goldberg decoupling to obtain the OH dipolar coupling constant. Dipolar oscillations in the 1H–17O cross-polarization curve provided an OH distance of 0.995 A in Mg(OH)x(OCH3)2−x; these oscillations were not observed in Mg(OH)2. Based on differences in the OH distance and in the 17O quadrupole coupling constant it was concluded that the OH bond in Mg(OH)x(OCH3)2−x was more covalent. 17O 2D quadrupole separated local field experiments were performed on both samples in order to obtain the relative orientation of the 17O quadrupole and OH dipole tensors. In both cases the interaction tensors were found to be collinear. Lee–Goldberg decoupling during the dipolar evolution time t1 impro...

Solid state NMR characterisation of the thermal transformation of Fuller's Earth

Fullers Earth, a dioctahedral calcium montmorillonite clay mineral of the smectite group, undergoes thermal transformation via a series of complex intermediate states that are highly structurally disordered. Fullers Earth is a commercially significant material with considerable levels of paramagnetic iron (Fe3+) substitution into octahedrally coordinated metal sites. Despite the high iron-content (approximately 10% of the occupied octahedral metal sites) in these samples 29Si and 27Al magic angle spinning (MAS) NMR spectra of sufficient quality are obtained to reveal structural changes in samples that have been heated from room temperature to 1400 degrees C. Two major structural changes are clearly observed, initial dehydroxylation and then collapse of the layer structure into more highly connected silica-rich domains and an alumina-rich phase.

An nmr investigation of ionic structure and mobility in plasticized solid polymer electrolytes

23Na and 19F nuclear magnetic resonance spectroscopy is used to investigate the effect of plasticizer addition on ionic structure and mobility in a urethane crosslinked polyether solid polymer electrolyte. The incorporation of dimethyl formamide and propylene carbonate plasticizers in a sodium triflate/polyether system results in an upfield chemical shift for the 23Na resonance consistent with decreased anion-cation association and increased cation-plasticizer interactions. The 19F resonances appears less susceptible to changes in chemical environment with only minor chemical shift changes recorded. Spin lattice relaxation measurements for the 19F nucleus are also reported. Two minima are observed in the relaxation measurements consistent with both an inter and intramolecular relaxation mechanism.

Structural characterization of conducting polypyrrole using 13C cross-polarization/magic-angle spinning solid-state nuclear magnetic resonance spectroscopy

13C nuclear magnetic resonance (n.m.r.) has been used to study polypyrrole and N-substituted polypyrrole in the solid state. The extent of oxidation appears to be counterion-dependent; in particular, the quinoid structure appears favoured in the films prepared with dodecyl sulfate. Resonances associated with the quinoid unit are lost upon reduction of the polypyrrole film, which supports the idea that the quinoid structure is associated with the oxidized form of polypyrrole. N-substituted polypyrroles have a more distinct resonance at 110 ppm, which is linked to lower degrees of oxidation or charge delocalization in these systems. The decrease in conductivity of polypyrrole upon thermal ageing in air is associated with both the loss of counterion (‘thermal dedoping’) and the decomposition of the quinoid structure in the polymer backbone. There is no indication of carbonyl formation in the solid-state n.m.r. spectra obtained in the present study.

Conductivity and NMR properties of plasticized polyethers complexed with lithium salts

NMR provides a tool whereby the dynamic properties of specific nuclei can be investigated. In the present study, a poly(ethylene oxide-co-propylene oxide) network has been used as the polymer host to prepare solid polymer electrolytes (SPE) containing either LiClO4 or LiCF3SO3. In addition, a low molecular weight plasticizer [propylene carbonate (PC), dimethyl formamide (DMF) or tetraglyme] has been added to several of the samples to enhance the mobility of the polymer and, thus, of the ionic species. The effects of plasticizer and salt concentration on the ionic structure and mobility in these SPEs, as measured by NMR relaxation times, and correlation to the conductivity behaviour in these systems are discussed. Temperature dependent triflate diffusion coefficients, as measured by Pulsed Field Gradient 19F-NMR, in plasticized SPEs are also reported.