Depolarization Technique Research Articles

Rotational dynamics of pyrrolopyrrole derivatives in glycerol: A comparative study with alcohols

The rotational dynamics of two structurally similar nonpolar molecules, 2,5-dimethyl-1,4dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c] pyrrole (DPP) has been studied in glycerol in the temperature range of 300–380 K using both time-resolved and steady-state fluorescence depolarization techniques. While the reorientation times of both the probes are varying linearly as a function of viscosity over temperature, the rotational dynamics of DMDPP is described by the Stokes–Einstein–Debye hydrodynamic theory with slip boundary condition, whereas the reorientation times of DPP are in between slip and stick limits and are about a factor of 1.5 longer than that of DMDPP. This is due to the hydrogen bonding between the two NH groups of the probe molecule and the oxygen atoms of the hydroxyl groups in glycerol. It has also been observed that the rotational dynamics of a nonpolar and noninteracting molecule like DMDPP is essentially the same, both in glycerol and in n-alcohols.

Evaluation of the LEP centre-of-mass energy above the W-pair production threshold

Knowledge of the beam energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55 GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. Several further corrections are applied to derive the the centre-of-mass energies at each interaction point. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of 2.7x 10^{-4}. Prospects for improvements are outlined.

Asphaltene Molecular Size and Structure

The rotational correlation times of individual asphaltene molecules have been determined using fluorescence depolarization techniques, addressing an active, long-standing controversy. Using simple ...

α, β and γ relaxations of functionalized HD polyethylene: a TSDC and a mechanical study

The relaxation behavior of high density polyethylene functionalized with diethylmaleate has been determined by dynamic mechanical method and thermally stimulated depolarization technique. The transitions observed have been related to the structural parameters as well as the α-olefin comonomer content and degree of grafting, and compared with previous TSDC results on LLPE functionalized with the same polar group. The lowest relaxation is composed of a sub- γ and a γ mode assigned to localized motions of the polar groups and to the glass transition (at 137 K), respectively. The β-relaxation intensity grows with the grafting degree in agreement with its assignment to motions taking place in the interfacial regions. The behavior of the α-mode with different content of diethylmaleate has confirmed its dependence with the crystallite size.

Conformational mobility in a polymer with mesogenic side groups: Dielectric and DSC studies

The relaxation times of the conformational rearrangements of the main-chain segments of a liquid crystalline side-chain polymer was determined from differential scanning calorimetry experiments in the temperature interval around and below its glass transition. Phenomenological models with fitting parameters were used to evaluate the temperature dependence of the relaxation times and the form parameter of the relaxation times distribution. These parameters were compared with its counterparts in the dielectric α relaxation process which appear in the temperature interval immediately above the calorimetric glass transition. For the temperature interval below the calorimetric glass transition the differential scanning calorimetry (DSC) results were compared with the dielectric results obtained by the thermally stimulated depolarisation technique.

Rotational dynamics of pyrrolopyrrole derivatives in alcohols: Does solute–solvent hydrogen bonding really hinder molecular rotation?

Rotational reorientation times of two structurally similar nonpolar molecules, 2,5dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP) have been measured in n-alcohols using steady-state fluorescence depolarization technique. While both DMDPP and DPP contain two C=O groups, the latter also has two NH groups. As these groups are known to form hydrogen bonds with alcohol solvents, the present work is aimed at finding out whether or not such solute–solvent hydrogen bonding is effecting the rotation of the probe molecules. The rotational dynamics of DMDPP is explained reasonably well by the Stokes–Einstein–Debye (SED) hydrodynamic theory with slip boundary condition. It is also found that the hydrogen bonding between the two C=O groups of the probe and the solvent molecules is not influencing the rotation of DMDPP. However, the reorientation times of DPP are found to be longer by a factor of 2.2 to 3.3 compared to that of DMDPP, and followed a super-stick behavior which has been observed for the first time for a nonpolar solute molecule. This is due to the strong hydrogen bonding between the two NH groups of the probe, and the alcoholic solvent molecules.

Study on thermal transitions of toluene diisocyanate-based polyurethane elastomers with poly (tetramethylene oxide) as the soft segment by TSC/RMA, DSC and DMA thermal analyzers

Thermal transitions of TDI-based polyurethane elastomers with PTMO as the soft segment were characterized by the depolarization technique in TSC and by using with the thermal windowing technique on selected specimens in the RMA measurements. Results indicate that the broadened thermal transition in the glass transition region as observed in the DSC thermogram is related to the combined Tg transition and the Tglobal transition in the TSC spectrum. This Tglobal transition is associated with the macromolecular property as detected by tan δ in DMA measurement. The increase in the Tg with a high NCO content may be explained by the structural modification found on the urethanic chain with the additional linkage of the hard segment that affects the cooperative motion of the molecular chain. Data measured from DSC, TSC/RMA and DMA with simulated DEA and wide angle X-ray data are presented for the characterization of the polyurethanes. The RMA measurement leads to a compensation search on Tg transition and provides pertinent thermokinetic data that correlates the NCO content with changes in enthalpy and entropy on the relaxation behaviors in the Tg transition of polyurethane elastomers.

Evaluation of the LEP centre-of-mass energy above the W-pair production threshold

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55 GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the the centre-of-mass energies at each interaction point. In addition the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of 2.7x10^{-4}. This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined.

Synthesis of a well-defined anthracene-labelled polystyrene by atomtransfer radical polymerization

The atom-transfer radical polymerization of styrene was carried out in bulk at 110°C with a newly synthesized bromine-containing anthracene derivative, 9,10-bis(1-bromoethylcarbalkoxymethyl)anthracene (ANTDBr), as a bifunctional initiator and a cuprous bromide complex Cu(I)Br/2dHbipy (dHbipy = 4,4′-di- n-heptyl-2,2′-bipyridine). g.p.c. and n.m.r. studies revealed that the initiation efficiency of ANTDBr is 100% and the polymerization proceeds in a controlled fashion, providing low-polydispersity polystyrenes with one anthracene chromophore in the middle of the main chain. This success opens up a new route to well-defined anthracene-labelled polymers including random and/or block copolymers, which will be useful as probe polymers to study, e.g., local chain dynamics by the fluorescence depolarization technique.

Electrical characterization of Ca1−xErxF2+x luminescent thin films

Ca 1−x Er x F 2+x thin films, epitaxially grown on silicon substrates, present a 1.53 μm infrared luminescence line, which gives them an evident interest for optical communications. At room temperature, using an argon laser as the excitation source, the maximum of this emission is obtained in thin films for x=0.16, when it is observed for very low erbium contents in bulk CaF2:Er3+ single crystals. Thus, we can think that the Er3+ ion environment, which governs the self-quenching phenomena, differs in thin films from that in bulk single crystals. In this paper, the nature of the erbium doping centers and their spatial distribution into the host material are studied versus x by using complex admittance and thermally stimulated depolarization techniques. In thin films, it is shown that for x⩽0.01, the luminescent centers correspond to isolated Er3+–Fi− ion pairs in nn sites, which behave like dipoles, D1. Their activation energy is 0.68 eV. From x=0.01 to x=0.05 clustering begins to be observed and leads when x>0.05, to the formation of extended clusters, usually called D2a in the literature. So, the clustering process in thin films appears only for concentrations two orders of magnitude higher than in single crystals. Moreover, it is shown that, whatever x varying from 0.001 to 0.20, charges can be transferred between pairs or clusters. Then, the activation energy depends on the mean distance between these defects, and thus, on the erbium content: the dielectric response corresponds to a homogeneous distribution of the pairs or clusters into the bulk of the host matrix.

Magnetic texture determination by means of the monochromatic circularly polarized Mössbauer spectroscopy

An expression for the line intensities in the case of a monochromatic, circularly polarized Mössbauer radiation is derived and a method of correcting for the incomplete degree of polarization is worked out. Experiments with circularly polarized radiation allows one to determine the average direction of magnetic moment of iron. The advantages of the polarized source are demonstrated by showing differences in the magnetization distribution in a recording material magnetized along different directions. Links between Mössbauer and neutron depolarization technique are discussed.

Solvent Dependence of the Hydrodynamical Volume of Dendrimers with a Rubicene Core

Four generations of a dendrimer with a fluorescent core consisting of a rubicene moiety are synthesized. The biexponential nature of fluorescence decay in toluene indicates the presence of two emitting conformations. Molecular modeling suggests that a conformation where the dendrons interact with the core is not improbable. The relative weight of the two decay times indicates that the contribution of that conformation in toluene increases as the dendrimer generation increases. In acetone and acetonitrile however the fluorescence decay is, except for the fourth generation, monoexponential. The hydrodynamical volume of the dendrimer is determined in toluene, a good solvent, acetone, a medium quality solvent, and acetonitrile, a poor solvent, with the time-resolved fluorescence depolarization technique. No change of the hydrodynamical volume is found in toluene in a temperature range between 20 and 94 °C. This suggests that the dendrimers of all the generations are fully expanded in this solvent. In acetonitrile however the hydrodynamical volume of the dendrimers is substantially smaller than in toluene and acetone. This effect is most clear for the fourth-generation dendrimer. For this compound the hydrodynamical volume is only a few percentages larger than the excluded volume.

Ferrite nucleation and growth in a medium-carbon steel studied by neutron depolarization

The neutron depolarization technique is used for a simultaneous observation of the grain size and the volume fraction of ferrite and pearlite forming from austenite in a medium-carbon steel at different temperatures. The effect of the transformation temperature is mainly visible in the particle density, a property that is governed by the nucleation behaviour. Pearlite formation leads to very distinct effects in neutron depolarization, the interpretation of which, however, is not yet fully developed.

A proposal to visualize magnetic domains within bulk materials

The recently proposed neutron magnetic tomography, an extension of the three-dimensional neutron depolarization technique, is considered. The new method allows visualization of magnetic domains within bulk materials. In concept it is similar to X-ray tomography but the analysis is much more involved because of the tensorial character of neutron spin rotations. An improved iterative numerical procedure is presented which fully accounts for the non-Abelian properties of the spin rotation. The method is applied successfully to the reconstruction of domain structures from simulated data of schematic examples. The experimental feasibility is discussed.

Wavelength dependent neutron depolarization in some metallic glasses

Ferromagnetic metallic glass ribbons Fe 34.6Ni 36.5Mo 6B 22.9, (Fe 50Ni 50) 94.9B 5.1, Fe 78Co 8.1B 13.9 and Fe 56.2Co 7Ni 15.6 Si 4B 17.2 were studied by magneto-optical methods (Kerr effect, magnetic colloid fluid technique) sensitive to the surface in combination with bulk domain structure determination by means of the neutron depolarization technique. The dependence of polarization of a polychromatic beam of polarized neutrons transmitted through the samples on the neutron wavelength was analysed by fitting to the data domain structure models consistent with the surface domain patterns; it enabled to estimate the mean domain size as well as the mean magnetic induction, its fluctuations, and orientation with respect to applied magnetic field.

New possibilities of polarised neutron reflectometry in the study of domain structure of thin magnetic films

It has been shown that the use of neutron reflection from magnetically non-collinear media allows to determine the local (averaged over the region coherently illuminated by the neutron) and the mean magnetic induction, the mean square direction cosines of the local magnetic induction of thin (∼ 10–500 nm) films. Thus, polarised neutron reflectometry (PNR) can be used to study magnetic properties of such films, when the use of the transmission depolarisation technique is problematic. The first two Fourier transform coefficients of the angular distribution function of the domain magnetisation can be found in zero field. Small-angle scattering at domains under reflection has been used to evaluate the mean domain size. The study of magnetisation processes in a Fe 36Co 64 (170 ± 3 nm) film has been undertaken to demonstrate the new possibilities of PNR. Rotation of the polarisation vector under neutron reflection has been observed.

Neutron depolarization study of internal stresses in amorphous Fe40Ni40B20

The magnetic domain structure of amorphous ferromagnets with nonzero magnetostriction is mainly determined by the internal stress state because of the magneto-elastic coupling. The stress and field dependence of the domain structure contains important information on the internal stresses in the material. The three-dimensional neutron depolarization technique has been used to study the stress- and field-dependence of the bulk domain structures in both as-quenched and annealed ribbons of the metallic glass Fe40Ni40B20. A three-layer domain structure model corresponding to compressive and tensile internal stresses is presented to explain the measured data. The influence of surface roughness on the interpretation of neutron depolarization measurements in amorphous ribbons is discussed. Finally, the internal stress relaxation due to the annealing is explained in terms of the viscous behaviour of the glass.

Beam energy measurement at LEP2 using Møller scattering

A detailed study has been made of the feasibility of Møller scattering as a method for continuous monitoring of the LEP beam energy at high energies where the currently used resonant depolarisation technique may be unusable. Two methods, based on Møller scattering, to measure the energy with different dominant systematic errors are discussed. Monte Carlo studies of the effect on the energy measurement of radiative corrections, target binding effects, beam parameters and calorimetric energy resolution have been made. It is shown how the beam parameters necessary to control the systematic error may be deduced from the Møller scattering events, and the crucial target-detector distance monitored using ep scattering events. The beam energy precision is expected to be a few MeV at 90 GeV. The reduction of the systematic error on the W-mass measurement can be appreciable, and for a fixed total error on the W-mass may provide a considerable economy in running time at LEP2. The method is also applicable to precise energy measurement at high-energy electron linear colliders.

Time evolution study of the domain structure in amorphous Fe64Co21B15 and Co77B23 using dynamic 3D-neutron depolarization

The most recent results on the relaxation of magnetic permeability in amorphous ferromagnets have been obtained using the ac technique. The magnetic relaxation is attributed to the existence of structural defects, which are typical for the amorphous metallic state. The short-time magnetic relaxation in amorphous Fe64Co21B15 and Co77B23 alloys was measured by the 3D-neutron depolarization technique after impulsive sample saturation. The behaviour of the magnetic decay shows that this effect has a predominant diffusive character, ascribed to the activated process of directional ordering of defects.

Electrokinetic Remediation. I: Pilot-Scale Tests with Lead-Spiked Kaolinite

The feasibility and efficiency of transporting lead under electric fields are investigated at pilot scale in three 1 t Georgia kaolinite specimens spiked with lead nitrate solution and at an electrode spacing of 72 cm. Enhancement methods such as cathode depolarization and/or catholyte neutralization techniques are not used in processing. A constant direct current density of 133 μmA/cm 2 is applied. Two of the tests are conducted on specimens spiked with lead at concentrations of 856 mg/kg and 1,533 mg/kg. The third test is conducted on a 1:1 mixture of compacted kaolinite/sand spiked with lead at a concentration of 5,322 mg/kg. Lead was transported toward the cathode and precipitated at its hydroxide solubility value within the basic zone in direct contact with the cathode compartment. Subsequent to 2,950 h of processing and an energy expenditure of 700 kWh/m 3, 55% of the lead removed across the soil was found precipitated within the last 2 cm close to the cathode, 15% was left in the soil before reaching this zone, 20% was found precipitated on the fabric separating the soil from the cathode compartment, and 10% was unaccounted. Heavy metals and species that are solubilized in the anodic acid front can be efficiently transported by electromigration under an electrical field applied across electrodes placed in soils.