Dipolar Relaxation Time Research Articles

The interactions of macrocyclic ethers by 13C dipole‐dipole relaxation time measurements

Abstract13C dipole–dipole relaxation time measurement studies of free and cation complexed cyclic polyethers in solution are reviewed briefly. The 13C dipole–dipole relaxation time of the macrocyclic ether backbone has been recognized to be a function of a motion of the molecular segment acting as a macrocyclic ligand site. This has been particularly investigated by NMR under extreme narrowing conditions.

Relaxations in thermosets. X. Analysis of dipolar relaxations in DGEBA‐based thermosets during isothermal cure

AbstractDielectric properties measured during isothermal curing of DGEBA‐based thermosets using a mixture of aromatic amines as curing agent are analyzed. The evolution of the dielectric features of thermosets during curing and after a time when their dc conductivity has reached a negligibly small value are phenomenologically similar to the dielectric features of physically and chemically stable dipolar liquids and solids observed with increasing frequency or decreasing temperature. This equivalence is a consequence of the invariance of the dynamic behavior of dielectric susceptibility with respect to either the frequency of measurement or the relaxation time of the substance and demonstrates that crosslinking of a thermoset causes its relaxation time to increase monotonically.It is shown that the stretched exponential relaxation function formalism satisfactorily describes the dielectric results and that the value of its distribution parameter initially decreases and, after gelation, reaches a constant value, which we denote γ, in the latter part of the cure. The value of the curing parameter, γ, which lies between 0.2 and 0.4, monotonically decreases with increasing curing temperature and tends to a limiting value characteristic of a thermoset at higher temperatures. This is in contrast with the increase found in the corresponding representation in the Kohlrausch‐Williams‐Watts parameter β with increasing temperature. The curing time dependence of the dipolar relaxation time ι has been determined and found to have the shape of an elongated S, with a well‐defined point of inflexion, as ι increases during the cure, from a value characteristic of a liquid to an ultimate value characteristic of a glass.

Dielectric Relaxation Studies of Oligether of Ethylene Glycol at Microwave Frequencies

Abstract Dielectric relaxation studies of dialkyl ethers of ethylene glycol and of diethylene glycol have been carried out at microwave frequencies in benzene solutions. Dipole moment and relaxation times corresponding to overall and group rotations and their weight factors have been determined. The configuration of these molecules and the mechanism of the rotation of end alkoxyl groups have been discussed.

Phenomenological aspects of relaxations in chemically controlled dipolar diffusion in polymers

The phenomenology of chemically controlled dipolar diffusion, or negative feedback process, has been described for a thermoset made from diglycidyl ether of bisphenol-A and diamino diphenylmethane at different temperatures. The dielectric spectroscopy shows an approach of dc conductivity towards a singularity at the gel point according to a power law σ 0 ∝ ( t g − t) x and to a new equation, σ 0 ∝ exp−[ B/( t 0 − t)], where t g, x, B and t 0 are empirical constants that are temperature dependent. The evolution of the dielectric features of thermoset during its curing and after a time when its dc conductivity has reached a negligibly small value is phenomenologically similar to that of the dielectric features of physically and chemically stable dipolar liquids and solids observed with increasing frequency or decreasing temperature. The stretched exponential relaxation function satisfactorily describes the dielectric results. The value of its parameter, which is named γ, lies between 0.26 and 0.31, and monotonically decreases with increasing temperature of the cure. The curing time dependence of the dipolar relaxation time, τ, has been determined and found to have the shape of an elongated S, with a point of inflexion.

Pressure as a probe of the glassy properties of disordered ferroelectrics, antiferroelectrics and dielectrics

Abstract Frustration-induced disorder and impurity-induced disorder can lead to the formation of a glassy state in ferroelectric (FE) and antiferroelectric (AFE) crystals. This paper highlights the important role of hydrostatic pressure as a variable for delicately tuning interatomic interactions and for understanding the short-range interactions and correlations responsible for the glassy state in disordered systems. Specifically, summaries and a comparison of novel glassy properties of mixed hydrogen-bonded FE and AFE crystals [Rb1-x(NH4)xH2PO4] which exhibit frustration-induced disorder and perovskites with dilute substantial impurities (KTa1-xNbxO3 for x≤0.02) are given. The properties of these materials are contrasted with those of more conventional orientational glasses in disordered dielectrics. Features of the results which have broad implications for the glassy state are discussed. These include (1) the pressure-induced crossover from long-range FE order to glassy behavior in soft mode systems, (2) the applicability and generality of the Vogel-Fulcher equation for treating the temperature and pressure dependences of the dipolar relaxation time, and (3) a unique manifestation of the non-equilibrium nature of the glass transition in high pressure measurements.

Electron-self-exchange reactions in solid-state voltammetry: the radical anion of 7,7,8,8-tetracyanoquinodimethane in polymer electrolytes. 1

Voltammetric currents for the oxidation of the lithium salt of the 7,7,8,8-tetracyanoquinodimethane anion radical (LiTCNQ) at Pt microdisk electrodes in network poly(ethylene oxide) (PEO) polymer solvent are substantially larger than currents for its reduction. The difference in currents is interpreted as a combination of electron self-exchange (between TCNQ{sup {sm bullet}{minus}} and TCNQ{sup 0}) and electrostatic migration. The latter is a minor effect in the presence of LiClO{sub 4} electrolyte as shown by a transference number (t{sub TCNQ}) analysis. At (LiClO{sub 4}) {le} 0.4 M, the product k{sub ex,1/0}{delta}{sub 1/0}{sup 2} of electron-self-exchange rate constant and hopping distance (squared) for the TCNQ{sup {minus}/0} couple is 1.0 ({plus minus}0.3) {times} 10{sup {minus}6} cm{sup 2} M{sup {minus}1} s{sup {minus}1}. Assuming {delta}{sub 1/0} = 9.5 {angstrom} gives k{sub ex,1/0} = 1 {times} 10{sup 8} M{sup {minus}1} s{sup {minus}1} in the polymer solvent, which is 47-fold smaller than k{sub ex,1/0} in acetonitrile solvent and 7-fold larger than measurable by methods subject to diffusion-controlled collision rate considerations. Physical diffusion is also slow in the polymer solvent and connections of this to k{sub ex,1/0} are discussed, including encounter rate limitations, longer polymer solvent dipole relaxation times, and longer distance electron transfers.

Molecular Motion and Phase Changes in Long Chain Solid Normal Alkanes as Studied by 1H and 13C NMR

Abstract Proton spin-lattice and dipolar relaxation times and 13C CP/MAS NMR spectra were measured on crystalline samples of n-C50H102 and long chain normal alkanes (n-alkanes) with average molecular weight of 595, 1214, and 2305 in order to characterize the static and dynamic molecular structures in the solid state. The 13C CP/MAS spectra indicate that the molecules take all-trans conformation in the orthorhombic unit cell and do not undergo any discernible change on annealing and/or quenching of specimens. On the other hand, the T1d in each material assumes a deep and clear minimum value in the orthorhombic phase; this relaxation process was unambiguously assigned to the molecular uniaxial reorientation about the long chain axis. There is a linear relation between the activation energy and the reciprocal of the chain length. In the case of n-C50H102 a solid state phase transition was found at 351 K. The T1d in the high temperature phase led to extremely high activation energy, 150 kJ mol−1, which may be attributable to molecular self-diffusion or other motion accompanying drastic change in the molecular structure.

A nuclear magnetic dipolar relaxation study of the internal rotation associated with intramolecular hydrogen bonding

A modified method using the jump model to describe the transition mechanism of internal rotation was developed to analyze the 13C dipolar relaxation times depending on the internal rotation specified by the rotamer dependence of the geometrical factors of functional rotating group. The utility of this analytical method was shown by applying it to analyze the dipolar relaxation time of the 13C nucleus of a model molecule caused by the dipolar interaction with the proton of hydroxyl group undergoing internal rotation pertubed by intramolecular hydrogen bonding.

A free‐volume‐based approach to modeling thermoset cure behavior

AbstractThe free‐volume theory for the temperature dependence of transport properties of glass‐forming polymers is extended to obtain their relationship to the extent of cure. This treatment centers on the unifying role of molecular mobility and yields a model which connects extent of reaction, viscosity, diffusivity, ionic conductivity and dipole relaxation time. The temporal dependence of these properties is expressed by coupling the extended free‐volume model with a relationship for the rate of cure, which included diffusional limitations. Analyses based on this model are applied to the observed behavior of a model epoxy‐amine resin system. The intrinsic kinetics of this model system are shown to be first order. It is shown that diffusional limitations strongly affected the progress of the reaction in the final stages of cure. The diffusion‐modified rate expression predictions agree with extent of reaction versus time data over the range of experimental temperatures. The temporal dependence of viscous behavior of the curing resin is measured. The extended free‐volume model accurately describes the evolution of resin viscosity during cure. The dielectric behavior is similarly characterized and is in close agreement with the predictions of the general free‐volume expression. The results of this study indicate that the free‐volume theory modified to account for molecular weight effects allows prediction of resin properties with a two‐parameter model. The results show that a power‐law relationship exists between viscosity and ionic conductivity. This result suggests that electrical properties may be used for on‐line measurement of resin viscosity during cure.

Pulsed NMR study of molecular dynamics in liquid crystal 40.7

Molecular dynamics governing the nuclear magnetic relaxation times in a liquid crystal, butyloxybenzylidene heptylaniline (40.7), is studied using proton magnetic relaxation spectroscopy. These results are compared with those of similar studies on other compounds belonging to the same homologues series (viz. 40.8, 40.6, and 40.5). Earlier studies indicate that the frequency dispersion of the spin-lattice relaxation time in typical NMR frequency ranges is governed by order director fluctuations (ODF) in 40.8, self-diffusion (SD) in 40.5, whereas by both ODF and SD in 40.6. Molecular reorientations contribute to a frequency-independent component in all these compounds. The present studies show that SD and molecular reorientations effectively mediate the relaxation mechanism in 40.7 in the nematic and the smectic A phases and their relative contributions are quantified. The relaxation data and the line width measurements at magic angle orientation are used to obtain dynamical information on the smectic B phase. These results are supported by dipolar relaxation time measurements. From the temperature-dependence studies the molecular parameters associated with these dynamical processes are obtained.

The dynamics of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers

The techniques of pulsed NMR spectroscopy and inelastic neutron scattering (INS) have been employed to study the coordinated motion of hydrogen atoms in the hydrogen bonds of carboxylic acid dimers. The proton spin-lattice relaxation time T1 has been measured at two frequencies in diglycolic acid and suberic acid and the reorientation rate in diglycolic acid has been measured directly by quasi-elastic neutron scattering. Furthermore T1 data have been collected on benzoic acid, terephthalic acid and malonic acid at different spectrometer frequencies to those in earlier published data. The NMR and INS data have been successfully fitted to a new model proposed by Skinner and Trommsdorff (1988) in which the low temperature dynamics, under the influence of an asymmetric double-minimum potential, are dominated by phonon-assisted tunnelling while that at high temperature follows an Arrhenius rate law. Fitting the T1 data at two different frequencies simultaneously with the INS data restricts the possible values of the parameters in the theory to a narrow range. The values of the parameters, particularly the barrier heights and asymmetry, are discussed in terms of the known molecular structures. Further discussion centres upon some new NMR data of the proton dipolar relaxation time T1D in terephthalic acid and benzoic acid.

An NMR study of 1H and 31P relaxation and molecular dynamics in polycrystalline nicotinamide adenine dinucleotide (NAD +)

Proton spin-lattice relaxation times in the laboratory and rotating frames, dipolar relaxation time, and second moments have been measured as a function of temperature for samples of NAD + · nH 2O, where n varies between 0 and 3.6. The results show clearly that the water molecules execute twofold reorientations with activation energies varying between 11.5 and 22 kJ/mol. It seems that a model in which all crystallographically equivalent water molecules have the same correlation time is adequate in the case of three of the water molecules. The motion of the fourth water molecule is characterized by a distribution of correlation times. The twofold reorientations of the two NH 2 groups have higher activation energies. The temperature dependence of T 1(P), the phosphorus spin-lattice relaxation time in the laboratory frame, supports the interpretation of the proton spin-lattice relaxation results.

Study of thermal stimulated current and effect of annealing on cellulose nitrate

AbstractThe investigated samples were cellulose nitrate CN‐85. The conductivity increases with temperature for unannealed samples and decreases with increasing time of annealing for annealed samples.The activation energy is equal for all samples and amounts to 2.1 eV in a temperature range from 322 to 385 K. The behaviour is metallic (dσ/dT) < 0 for times of annealing larger than 24 h. The TSC current was measured. The position of the maximum slightly shifted towards higher temperatures with increasing rate of heating. The activation energy in the case of the TSC current was 0.21 eV and independent of the rate of heating. The dipolar relaxation time and imaginary dielectric constant were calculated theoretically and studied as a function of temperature.

Hydrostatic pressure dependence of the relaxational mode of KH2PO4studied by light scattering

Abstract The light scattering spectra of the x(yx)y geometry in KH2PO4 were measured by a tandem Fabry-Perot interferometer as functions of both pressure and temperature. From the recorded spectra, the relaxation time of the ferroelectric mode was deduced and was analyzed on the basis of the Curie-Weiss law at several pressures. The relaxation time of individual dipoles (τ0) was found as a function of pressure. The value of τ0 at 1 bar (1.57 × 10–13 s) decreases almost linearly to 0.57 × 10–13 s at 4.17 kbar. A tentative calculation of the protonic relaxation time using a compressible double Morse potential was performed. The result was compared with the relaxation time of the ferroelectric mode.

Ionic Mobilities at Infinite Dilution: Structural Aspects

AbstractEmpirical correlations between ionic mobilities and parameters characteristic of the solvent molecules motions for a variety of single ions and 1‐1 electrolytes at infinite dilution in 19 one‐component solvents and 8 aqueous‐nonaqueous mixed solvents are found, presented and discussed. The parameters are the self‐diffusion coefficient, the dipole relaxation time, the rotational correlation time of various NMR vectors, and the residence time of solvent molecules in the first ion solvation shell. The residual friction coefficients are considered in a more theoretical vein.

N.M.R. study of collective molecular motions in smectogen liquid crystals

Abstract The proton spin relaxation dispersion, T 1(v), was studied by field-cycling techniques over a broad Larmor frequency range in the nematic and smectic phases of several liquid crystals (4,4′-bis-heptyloxyazoxybenzene, 4-cyano-4′-8-alkylbiphenyl, 4-cyano-4′-9-alkylbiphenyl, 4-cyano-4′-11-alkylbiphenyl and 4-cyano-4′-9-alkoxybiphenyl) with different sites of the nitrogen atom. The results can be explained quantitatively in terms of nematic and smectic order fluctuations (T 1 ∝ v 1/2, T 1 ∝ v 1), molecular self-diffusion, molecular rotations and up to six cross-relaxation resonances due to a nitrogen-proton coupling The order fluctuations contribution and the transition from the v 1/2 to the v 1 dispersion profile occurs always at Larmor frequencies in the kilohertz range. Some additional measurements of the frequency dependence of the dipolar relaxation time T 1D(v), are not in accord with existing theories.

Novel effects in transient optical reflectivity from bounded nonlocal media via exciton polaritons

We report results of a theoretical investigation of time-dependent reflectivity from a semi-infinite, spatially dispersive medium subject to an incident pulse of finite duration. A damped oscillatory decay in the tail of transient reflectivity after the pulse cut-off is predicted, sensitive to the material and to the specific additional boundary conditions (ABC's). Physically the origin is in relaxation processes which dephase the emitted light on a time scale larger than the cut-off time of the pulse and shorter than the spontaneous dipole relaxation time. For laser frequency at the exciton resonance the transients are enhanced; near to resonance a double peak occurs due to dispersion of polariton group velocity.

Evaluation of dielectric cure models for an epoxy‐amine resin system

AbstractThe parallel‐plate test fixture on a Rheometrics viscometer was electrically isolated so that the rheological and dielectric properties of a thermoset polymer system could be simultaneously measured. This enabled the relationship between the dielectric properties and the rheological properties to be directly examined. A close relationship was established between the dielectric properties (dipole relaxation time and specific conductivity) and the pre‐gelation bulk viscosity. This relationship suggested that models similar in form to those used to describe the change in viscosity might be used to describe the changing dielectric properties. The limitations and advantages of two such models, which attempt to describe the time‐temperature behavior of the dielectric properties, were then tested for use with a typical aerospace epoxy resin system.

NMR study of the motion of water molecules in the natural zeolite bikitaite

Molecular mobility of water molecules as well as the possibilities for proton transport along the hydrogen bonded chains of water molecules have been investigated in the natural zeolite bikitaite (Li 2Al 2Si 4O 12·2H 2O) by 1H NMR. Spin-lattice relaxation times in the laboratory and rotating frames ( T 1 and T 1 p ) and dipolar relaxation times ( T 1 D ) have been measured as a function of temperature for a polycrystalline sample. One dynamical process has been clearly identified as responsible for the relaxation in the temperature range studied (224–418 K): a 180° flip motion of the H 2O molecule about its quasi-twofold axis. The activation energy and the pre-exponential factor in the Arrhenius relation for the correlation time of this motion are 30 kJ mol −1 and 8 × 10 −15 s, respectively. No evidence of any other dynamical process contributing to the proton relaxation is observed.

Nuclear spin relaxation in some phenyl phosphorus compounds

13C and 31P spin-lattice relaxation times ( T1) and nuclear Overhauser effects were measured in triphenylphosphine, triphenylphosphine oxide, triphenylphosphine sulphide, and triphenylphosphine selenide, over a temperature, range −40°C to +55°C. Isotropic reorientation correlation times were calculated from the dipolar relaxation times for phosphorus and that for the para carbon. The spin-rotation mechanism is the dominant relaxation mechanism for phosphorus in triphenylphosphine, whereas the chemical shift anisotropy is the dominant mechanism in triphenylphosphine oxide at low temperatures. In triphenylphosphine sulphide and triphenylphosphine selenide, the dipolar and the chemical shift anisotropy mechanisms are important in phosphorus relaxation. The motional anisotropy of internal rotation of phenyl rings and overall tumbling is determined from the ratio of T 1's of ortho or meta carbon to para carbon, and is found to increase with the decrease in temperature and with the increase in mol. wt. The angular momentum correlation times were calculated from phosphorus spin-rotation relaxation rates. From viscosity data, reduced correlation times were calculated and compared with that calculated using Stokes-Einstein-Debye, Gierer-Wirtz, and Hu-Zwanzig models.