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Impedance spectroscopy and nuclear magnetic resonance (NMR) were used to investigate the mobility of water molecules located in the interlayer space of H+ - exchanged bentonite clay. The conductivity obtained by ac measurements was 1.25 × 10-4 S/cm at 298 K. Proton (1H) lineshapes and spin-lattice relaxation times were measured as a function of temperature over the temperature range 130-320 K. The NMR experiments exhibit the qualitative features associated with the proton motion, namely the presence of a 1H NMR line narrowing and a well-defined spin-lattice relaxation rate maximum. The temperature dependence of the proton spin-lattice relaxation rates was analyzed with the spectral density function appropriate for proton dynamics in a two-dimensional system. The self-diffusion coefficient estimated from our NMR data, D ∼ 2 × 10-7 cm2/s at 300 K, is consistent with those reported for exchanged montmorillonite clay hydrates studied by NMR and quasi-elastic neutron scattering (QNS). [ABSTRACT FROM AUTHOR]

Measurements of 1 H and 13 C Nuclear Magnetic Resonance (NMR) for the nanocomposite materials formed by the intercalation of hexadecylamine (HAD) in metal oxides (TiO 2 , V 2 O 5 , CrO 3 and MoO 3 ), are reported. The 1 H NMR relaxation results reflects the complexity of the chain segments motions in these systems. The proton (and the carbon) mobility depends on the position in the HDA hydrocarbon chain to the restriction imposed by the anchoring of the head group. The nonexponential behavior of the proton magnetization is clearly a result of the anisotropic motion of the HDA chains. For most of the protons, the rate of motion is not fast enough to fulfill the condition for the nuclear spin-lattice relaxation (NSR) maximum in the laboratory frame but they fulfill the condition for the NSR in the rotating frame. The maximum at around 280 K reflects the relative lower mobility of the protons of the HDA chain inside the low-dimensional space of the metal oxide. The description of the segmental motion of long chains require the use of an asymmetric distribution of correlation times to describe relaxation in flexible polymer chains. [ABSTRACT FROM AUTHOR]

This work describes an Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) study of the vanadium pentoxide xerogel V 2 O 5 :nH 2 O with n ˜ 1.8 Experiments were performed in the temperature range 65 K–310 K. The EPR spectrum at high temperatures exhibits the typical liquid-like eight lines-hyperfine structure. At low temperatures the EPR spectrum change to a V 4+ anisotropic powder spectra. These two different regimes can be delimited by a transition temperature region centered at 280 K. Numerical simulations of the EPR spectra in the two temperature limits are in good agreement with the experimental data. Proton ( 1 H) NMR lineshapes, as functions of temperature were measured in the range 150–323 K and indicate that nuclear motional narrowing is effective at temperatures above 210 K, with an activation energy of 0.14 eV. The NMR spin-lattice relaxation recovery, associated to protons in the water molecules, was found to be non-exponential throughout the temperature range and described by two different relaxation processes. The slow relaxing component is temperature-independent and was attributed to water molecules located far from V 4+ ions. The temperature dependence of the fast relaxing component shows maximum at around 260 K, suggesting a relaxation process sensitive to the temperature induced dynamic structural changes in the vanadium oxide matrix. [ABSTRACT FROM AUTHOR]

Continuous-wave (cw) and pulsed electron paramagnetic resonance (pulsed EPR) techniques were used to investigate spin dynamics in moderately and slightly doped poly(o-methoxyaniline), a polyaniline derivative. EPR line shapes, measured by standard cw EPR, and relaxation rates, measured by electron spin-echo methods, were studied as a function of temperature, in the ranges 6-300 K (cw) and 6-100 K (pulsed). Experimental results were explained by a model which considered two types of spins: isolated polarons in an insulating matrix, and delocalized ones in conductive islands (clusters) formed by dopant aggregates. Experimental data agreed with the assumption that EPR spectra are generated by isolated polarons coupled to the clusters by dipole-dipole interactions. Furthermore, some experimental evidence suggests that spin dynamic within the clusters, which play an essential role in the interpretation of the EPR results, is determined by exchange interactions. Consequences of the proposed model are discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]