Dielectric Relaxation Data Research Articles

Temperature dependent magnetic and electrical transport properties of lanthanum and samarium substituted nanocrystalline nickel ferrite and their hyperthermia applications

Structural, optical, temperature dependent magnetic and electrical properties have been investigated for the nanocrystalline NiFe1.97RE0.03O4 (RE = La3+ and Sm3+) compound. Without any signs of a secondary REFeO3 phase, the Rietveld refinement reveals a single-phase spinel structure possessing cubic symmetry for current samples. The decrease in lattice constant has been observed as a result of RE ions substitution. The TEM micrographs reveal nanosized particles with an average size of 35 ± 3 nm and 32 ± 3 nm for La3+ and Sm3+ substituted samples. EDX spectra of both samples show good compositional homogeneity. HRTEM micrographs of both samples show well resolved lattice fringes and their inter-planar spacing matches with that obtained from the Rietveld analysis of the XRD patterns. The concentration of Fe2+ and Fe3+ ions has been estimated from the XPS spectra analysis and it is found to be ∼ 22% and 78% for NiFe1.97La0.03O4 and, 20% and 80% for NiFe1.97Sm0.03O4 compound. The optical energy band gap for rare earth substituted samples is found to be more than that of pure nickel ferrite. The value of saturation magnetization (Ms) and magneto-crystalline anisotropy constant (K1) estimated from the “Law of Approach to Saturation” equation for rare earth substituted samples are found to be less than that of pure nickel ferrite. However, an enhanced value of coercivity has been observed with RE substitution. ZFC (zero field cooling) and FC (field cooling) DC magnetization curves measured at 100 Oe in the temperature range of 60–400 K reveal a combination of weak and intermediate forms of magnetic interaction between the particles for both samples. Temperature dependent analysis of saturation magnetization and coercivity employing modified Bloch’s law and Kneller’s relation supports the nanomagnetic behavior of both samples. Under an AC magnetic field of 14.92 kA/m and frequency 337 kHz, the SAR and ILP values have been found to be ∼ 331 W/g and 4.22 nHm2/kg for NiFe1.97La0.03O4 and, ∼ 241 W/g and 3.21 nHm2/kg for NiFe1.97Sm0.03O4. The dielectric relaxation data have been analyzed at various temperatures ranging from 40 to 300 0C over the frequency range 100 Hz-1 MHz using electrical impedance spectroscopy. The dielectric constant for rare earth substituted samples is found to be more and their AC conductivity is observed to be less than that of pure nickel ferrite. The analysis of temperature dependent AC conductivity employing Jonscher’s power law suggests that the charge carrier’s conduction mechanism of both samples follows the small polaron hopping mechanism below 200 0C, thereafter; it follows the correlated barrier hopping mechanism. The activation energy estimated by imaginary impedance spectra is found to be 0.411 eV and 0.398 eV for NiFe1.97La0.03O4 and NiFe1.97Sm0.03O4 which is more than that of pure nickel ferrite. The Cole-Cole plots at different temperatures suggest the presence of non-Debye-type relaxation. The modeling of Cole-Cole plots with equivalent circuits for both samples confirms the contribution of both grain and grain boundary in electrical conduction. The estimated stretching exponential factor by fitting the modified KWW (Kohlrausch-Williams-Watts) equation to the imaginary modulus curve reveals relatively more dipole-dipole interaction in NiFe1.97Sm0.03O4 than that of the La3+ substituted sample.

Terminal stage of highly viscous flow.

The shear misfit model for highly viscous flow is based upon a theoretical prediction for its terminal stage in terms of irreversible Eshelby relaxations in five-dimensional shear space. The model is shown to predict a small δ-function (Debye peak) in the dielectric spectrum, in agreement with experimental evidence. It is extended to density fluctuations, and a relation between adiabatic and isothermal compressibility jumps at the glass transition is derived. The model is applied to high-precision measurements of the shear, dielectric, and bulk relaxation data in two vacuum pump oils and in squalane, a short chain polymer with a strong secondary relaxation peak. The terminal stage of aging data in squalane demonstrates that the adiabatic density fluctuations contribute a fast component to the thermal expansion, explaining why the thermal expansion seems to equilibrate a bit faster than the dynamic heat capacity.

Arriving at the most plausible interpretation of the dielectric spectra of glycerol with help from quasielastic γ -ray scattering time-domain interferometry

Glycerol is one of the glass-forming liquids selected by Robert H. Cole in 1950 to start his study of molecular dynamics by dielectric spectroscopy. Seventy-one years have gone by and remarkably no consensus has been reached on the nature and identity of the relaxation processes observed in the dielectric spectra. The macroscopic dielectric relaxation data allow different interpretations to yield contrasting results, and it is not possible to determine which one is most plausible. Coming to the rescue is the application of the nuclear γ-resonance time-domain interferometry (TDI) to glycerol by Saito et al. [Phys. Rev. E 105, L012605 (2022)10.1103/PhysRevE.105.L012605]. Their microscopic TDI data potentially can decide which interpretation of the dielectric spectra of glycerol is most plausible. The attempt was made by Saito et al., but there is a problem in their analysis of the dielectric data of glycerol and hence their conclusion is untenable. In this paper, we critically compare four major interpretations with the TDI data in an effort to identify the most plausible interpretation of the relaxation processes constituting the dielectric spectra of glycerol.

Influence of UV light intensity on dielectric behaviours of pure and dye-doped cholesteric liquid crystals

This study aims to explore the ultraviolet (UV) light effects on dielectric properties of azo dye methyl red (MR)-doped cholesteric liquid crystal (CLC) composite. The CLC composite formed in the study contains 5CB nematic liquid crystal (LC) and S811 chiral additive. To obtain CLC/MR composite, CLC was dispersed with 2% wt/wt MR azo dye. Dielectric measurements of the CLC and CLC/MR samples were performed using dielectric spectroscopy technique in wide frequency range in the absence of UV light and at various UV light intensities (30, 60 and 90 mW/cm2) for 0 and 40 V. It was observed that the dielectric constant increased with UV light at low-frequency values. Dielectric loss data were utilized to extract relaxation frequency which was observed to increase with UV light exposure. Dielectric anisotropy of CLC sample exhibited typical transition from p-type to n-type, but this behaviour disappears for CLC/MR composite above 60 mW/cm2 UV light intensity. Cole–Cole plots were used for extraction of dielectric relaxation data and obtained results showed that CLC sample shows Debye type relaxation only at 0 V whereas CLC/MR composite exhibits non-Debye type relaxation both at 0 V and 40 V. Moreover, conductivity properties of the samples were also investigated, and dc conductivity values were extracted from experimental ac conductivity values. It was found that MR incorporation increased dc conductivity, also it was significantly increased by UV light exposure and thus similar effect was also observed in current–voltage characteristics of the samples. The results show that azo dye MR molecules are suitable for CLCs since they lead to some enhancements in dielectric and electrical properties.

Linear viscoelastic relaxation in the α and α+ regions of linear polymers, crosslinked polymers and small molecules

The dynamic linear viscoelastic behavior in the region above Tg was studied for a series of diglycidyl ether of bisphenol-A (DGEBA) based epoxy resins cured with 4,4′-methylenedianiline (MDA) and 4,4′-diaminodiphenyl sulfone (DDS) as well as a linear DGEBA-based polymer – phenoxy. Master curves were constructed from the frequency dependent storage and loss isotherms. The relaxation spectra were determined for the MDA and DDS cured systems and also, using the reported dielectric relaxation data (Casalini et al. Phys. Rev. B, 1997), for the DGEBA monomer. For the tighter DGEBA networks, a single broad relaxation peak was observed vs looser DGEBA networks and a phenoxy thermoplastic where two relaxation processes were observed: the main α-process and an α+-process that occurs at longer relaxation times and higher temperatures. The spectrum for DGEBA monomer exhibited only an α-process with a very abrupt end. The temperature dependence of logaT follows one of the two distinct branches when adjusted for Tg: the tight DGEBA networks all exhibit a steep response; in contrast, the DGEBA monomer, phenoxy and looser DGEBA networks exhibit a less steep response. Relaxation spectrum for the loosely crosslinked Epon1009-MDA system consists of α and α+ components; intriguingly, logaT for the α-process follows the steep branch vs the α+-process following the less steep branch.

Molecular Insights into Dipole Relaxation Processes in Water-Lysine Mixtures.

Dielectric spectroscopy is a robust method to investigate relaxations of molecular dipoles. It is particularly useful for studies of biological solutions because of the potential of this method to cover a broad range of dynamical time scales typical for such systems. However, this technique does not provide any information about the nature of the molecular motions, which leads to a certain underemployment of dielectric spectroscopy for gaining microscopic understanding of material properties. For such detailed understanding, computer simulations are valuable tools because they can provide information about the nature of molecular motions observed by, for example, dielectric spectroscopy and to further complement them with structural information. In this work, we acquire information about the nature of dipole relaxation, in n-lysine solutions by means of molecular dynamics simulations. Our results indicate that the experimentally observed main relaxation process of n-lysine has different origins for the single monomer and the polypeptide chains. The relaxation of 1-lysine is due to the motions of whole molecules, whereas the experimentally observed relaxation of 3-lysine and 4-lysine is due to the motions of the residues, which, in turn, are promoted by water relaxation. Furthermore, we propose a new structural model of the lysine amino acids, which can quantitatively account for the experimental dielectric relaxation data. Hydrogen bonding and the structure of water are also discussed in terms of their influence on relaxation processes.

Atomic-scale evidence for displacive disorder in bismuth zinc niobate pyrochlore

Pyrochlores characterized by the chemical formula A2B2O7 form an extended class of materials with interesting physical and chemical properties. The compound Bi1.5ZnNb1.5O7 is prototypical. Its excellent dielectric properties make it attractive, e.g. for capacitors, tunable microwave devices and electric-energy storage equipment. Bi1.5ZnNb1.5O7 shows an intriguing frequency-dispersive dielectric relaxation at 50 K ≤ T ≤ 250 K, which has been studied intensively but is still not fully understood. In this first study on a pyrochlore by atomic-resolution transmission electron microscopy we observe the Bi atoms on A sites since, due to their low nuclear charge, the contribution of Zn atoms to the contrast of these sites is negligible. We find in our [1¯00]and [112] oriented images that the position of the atomic intensity maxima do not coincide with the projected Wyckoff positions of the basic pyrochlore lattice. This supplies atomic-scale evidence for displacive disorder on split A-type sites. The Bi atoms are sessile, only occasionally we observe in time sequences of images jumps between individual split-site positions. The apertaining jump rate of the order of 0.1–1 Hz is by ten orders of magnitude lower than the values derived in the literature from Arrhenius plots of the low-temperature dielectric relaxation data. It is argued that these jumps are radiation induced. Therefore our observations are ruling out a contribution of Bi-atom jumps to low-temperature dielectric A sites-related relaxation. It is suggested that this relaxation is mediated by jumps of Zn atoms.

Possible relations between supercooled and glassy confined water and amorphous bulk ice.

In this paper we discuss apparent contradictions in the literature between dynamical results on supercooled confined water obtained by different experimental methods. The reason for the lack of a clear glass transition of confined water is also discussed. Dielectric relaxation data and results from differential scanning calorimetry measurements provide a consistent picture, but it is still unclear why the glass transition related structural (α) relaxation disappears before the normal time-scale of a calorimetric glass transition (i.e. about 100 s) is reached. From recent results on amorphous bulk ice we propose that this anomalous phenomenon may not be an effect of confinement, but an intrinsic property of water when it transforms to a crystal-like glassy state, probably around 225 K. Thus, the results from the studies of confined water in the so-called no man's land (the temperature range 150-235 K) where bulk water rapidly crystallizes may be of more relevance for supercooled and glassy bulk water than previously thought. Furthermore, the structural difference between glassy water (or amorphous ice) and crystalline ice is likely to be rather small, due to the large degree of disorder in crystalline ice.

Electrode reactions in slowly relaxing media.

Standard models of reaction kinetics in condensed materials rely on the Boltzmann-Gibbs distribution for the population of reactants at the top of the free energy barrier separating them from the products. While energy dissipation and quantum effects at the barrier top can potentially affect the transmission coefficient entering the rate pre-exponential factor, much stronger dynamical effects on the reaction barrier are caused by the breakdown of ergodicity for populating the reaction barrier (violation of the Boltzmann-Gibbs statistics). When the spectrum of medium modes coupled to the reaction coordinate includes fluctuations slower than the reaction rate, such nuclear motions dynamically freeze on the reaction time scale and do not contribute to the activation barrier. Here we consider the consequences of this scenario for electrode reactions in slowly relaxing media. Changing the electrode overpotential speeds the electrode electron transfer up, potentially cutting through the spectrum of nuclear modes coupled to the reaction coordinate. The reorganization energy of electrochemical electron transfer becomes a function of the electrode overpotential, switching between the thermodynamic value at low rates to the nonergodic limit at higher rates. The sharpness of this transition depends on the relaxation spectrum of the medium. The reorganization energy experiences a sudden drop with increasing overpotential for a medium with a Debye relaxation but becomes a much shallower function of the overpotential for media with stretched exponential dynamics. The latter scenario characterizes the electron transfer in ionic liquids. The analysis of electrode reactions in room-temperature ionic liquids shows that the magnitude of the free energy of nuclear solvation is significantly below its thermodynamic limit. This result applies to reaction times faster than microseconds and is currently limited by the available dielectric relaxation data.

Toward a better understanding of dielectric responses of van der Waals liquids: The role of chemical structures

Exhaustive analysis of dielectric relaxation data of van der Waals glass-forming liquids revealed a strong correlation between the width of the frequency dispersion of the α-relaxation and the dielectric strength Δε, originating from the dipole-dipole interaction contribution to the intermolecular potential [M. Paluch et al., Phys. Rev. Lett. 116, 025702 (2016)]. The two van der Waals liquids, 4-vinyl-1,3-dioxolan-2-one (VPC) and 4-ethyl-1,3-dioxolan-2-one (EPC), have chemical structures modified from that of propylene carbonate. All three glass-formers have very similar values of dipole moments, exactly the same dielectric strength, and hence identical frequency dispersion of the α-relaxation in all three glass-formers is expected if the correlation holds. Based on this expectation, we performed dielectric relaxation measurements of the VPC and EPC at ambient and elevated pressures. The results obtained show not only identical α-relaxation frequency dispersion for the three glass-formers but also the excess wing which is the unresolved Johari-Goldstein β-relaxation. On the other hand, the other thermodynamics related parameters of the α-relaxation dynamics, including the glass transition temperature Tg, the fragility index mP, and activation volume ΔVact, are not uniformly the same for all three glass-formers.

A dielectric study of water uptake in epoxy resin systems

ABSTRACTThe influence of epoxy resin formulation on the nature and extent of moisture is studied using dielectric and gravimetric analysis for samples cured at 40, 50, 60, and 70 °C and aged at 50 °C and 70 °C. The equilibrium moisture uptake depends on the difference between the glass transition and aging temperature. Dielectric relaxation data measured from 1 to 3 × 1010 Hz indicates the presence of water in at least two different environments. The high frequency relaxation ca. 1 × 1010 Hz is associated with water clustered in nano‐voids, whereas the relaxation at 105 Hz arises from a combination of OH pendant group reorientation motion coupled with that of molecularly dispersed water molecules. A correlation exist between the dielectric permittivity and the amount of moisture absorbed. Water initially resident in voids is re‐dispersed with aging into the resin matrix aiding plasticization and allowing densification of the matrix. The extent to which changes occur depends on the chemical functions forming the matrix and densification leads to a drop in the amount of water absorbed. In the complex resin systems, water interacts with both OH groups and polyether of the amine curing agent which is not possible with the aliphatic diamine in the simple system.] © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44717.

Evidence of Coupling between the Motions of Water and Peptides.

Studies of protein dynamics at low temperatures are generally performed on hydrated powders and not in biologically realistic solutions of water because of water crystallization. However, here we avoid the problem of crystallization by reducing the size of the biomolecules. We have studied oligomers of the amino acid l-lysine, fully dissolved in water, and our dielectric relaxation data show that the glass transition-related dynamics of the oligomers is determined by the water dynamics, in a way similar to that previously observed for solvated proteins. This implies that the crucial role of water for protein dynamics can be extended to other types of macromolecular systems, where water is also able to determine their conformational fluctuations. Using the energy landscape picture of macromolecules, the thermodynamic criterion for such solvent-slaved macromolecular motions may be that the macromolecules need the entropy contribution from the solvent to overcome the enthalpy barriers between different conformational substates.

Contrasting dynamics of fragile and non-fragile polyalcohols through the glass, and dynamical, transitions: A comparison of neutron scattering and dielectric relaxation data for sorbitol and glycerol

BackgroundGlycerol and sorbitol are glass-forming hydrogen-bonded systems characterized by intriguing properties which make these systems very interesting also from the applications point of view. The goal of this work is to relate the hydrogen-bonded features, relaxation dynamics, glass transition properties and fragility of these systems, in particular to seek insight into their very different liquid fragilities. MethodsThe comparison between glycerol and sorbitol is carried out by collecting the elastic incoherent neutron scattering (EINS) intensity as a function of temperature and of the instrumental energy resolution. ResultsIntensity data vs temperature and resolution are analyzed in terms of thermal restraint and Resolution Elastic Neutron Scattering (RENS) approaches. ConclusionsThe number of OH groups, which are related to the connecting sites, is a significant parameter both in the glass transition and in the dynamical transition. On the other hand, the disordered nature of sorbitol is confirmed by the existence of different relaxation processes. General significanceFrom the applications point of view, glycerol and sorbitol have remarkable bioprotectant properties which make these systems useful in different technological and industrial fields. Furthermore, polyols are rich in glassforming liquid phenomenology and highly deserving of study in their own right. The comparison of EINS and calorimetric data on glycerol and sorbitol helps provide a connection between structural relaxation, dynamical transition, glass transition, and fragility. The evaluation of the inflection point in the elastic intensity behavior as a function of temperature and instrumental energy resolution provides a confirmation of the validity of the RENS approach.This article is part of a Special Issue entitled “Science for Life” Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo

Does the Johari–Goldstein β-Relaxation Exist in Polypropylene Glycols?

Secondary relaxations with properties closely related to the α-relaxation have fundamental importance in glass-forming substances including polymers. To distinguish these secondary relaxations from those involving intramolecular degrees of freedom, they are called the Johari–Goldstein (JG) β-relaxations. Acting as the precursor of the α-relaxation, the JG β-relaxation is supposedly ubiquitous in all glass-formers, a thesis supported by experiments on a variety of glass-formers. Notwithstanding, the JG β-relaxation has not been identified definitively in the hydroxyl-terminated polypropylene glycols (PPGs) with various molecular weights, despite these polymers have been intensively studied experimentally in the last several decades. The difficulty of finding the JG β-relaxation is due to the presence of a faster intramolecular γ-relaxation and a slower relaxation originating from residual water. This is demonstrated in two recent papers by Gainaru et al. Macromolecules 2010, 43, 1907, and Kaminski et al. Macromolecules 2013, 46, 1973. In this paper, we show the presence of the JG β-relaxation in the PPGs from the dielectric relaxation data by using the time honored criterion derived from the coupling model in conjunction with the observed anomalous temperature dependence of the γ-relaxation caused by merging with the JG β-relaxation, and new experimental data obtained by applying pressure on PPG4000 before and after drying the sample to remove the residual water. From the results, we conclude that the behavior of the PPGs is no different from the other glass-formers as far as the omnipresence of the JG β-relaxation is concerned.

An alternative explanation of the change in T-dependence of the effective Debye-Waller factor at T(c) or T(B).

The cusp-like temperature dependence of the Debye-Waller factor or non-ergodicity parameter f(Q)(T) at some temperature T(c) above T(g) found by experiments in several fragile glassformers has been considered as critical evidence for validity of the ideal Mode Coupling Theory (MCT). A comprehensive review of experimental data of f(Q)(T) and beyond brings out various problems of the MCT predictions. For example, the molten salt, 0.4Ca(NO3)2-0.6KNO3 (CKN), was the first glassformer measured by neutron scattering to verify the cusp-like behavior of f(Q)(T) at T(c) predicted by ideal MCT. While the fits of the other scaling laws of MCT to viscosity, light scattering, and dielectric relaxation data all give T(c) in the range from 368 to 375 K, there is no evidence of cusp-like behavior of f(Q)(T) at T(c) from more accurate neutron scattering data obtained later on by Mezei and Russina [J. Phys.: Condens. Matter 11, A341 (1999)] at temperatures below 400 K. In several molecular glass-formers, experiments have found at temperatures below T(c) that [1-f(Q)(T)] is manifested as nearly constant loss (NCL) in the frequency dependent susceptibility. The NCL persists down to below T(g) and is not predicted by the ideal MCT. No clear evidence of the change of T-dependence of f(Q)(T) at any T(c) was found in intermediate and strong glassformers, although ideal MCT does not distinguish fragile and strong glassformers in predicting the critical behavior of f(Q)(T) a priori. Experiments found f(Q)(T) changes T-dependence not only at T(c) but also at the glass transition temperature T(g). The changes of T-dependence of f(Q)(T) at T(c) and T(g) are accompanied by corresponding changes of dynamic variables and thermodynamic quantities at T(B) ≈ T(c) and at T(g). The dynamic variables include the relaxation time τ(α)(T), the non-exponentiality parameter n(T), and the generalized fragility m(T) of the structural α-relaxation. The thermodynamic quantities are the free volume deduced from positron annihilation spectroscopy, and the configurational entropy obtained from adiabatic calorimetry measurements. These changes of dynamic variables and thermodynamic quantities in temperature dependence at T(B) ≈ T(c) occur concurrently with the change of f(Q)(T) and suggest the effects are related, and have to be explained altogether. Since this task cannot be carried out by the ideal MCT, we have provided a different interpretation of f(Q)(T) and an alternative explanation of the change in its T-dependence of f(Q)(T) at T(B) ≈ T(c) as well as the other dynamic variables. We show f(Q)(T) originates from the dissipation of the molecules while caged by the anharmonic intermolecular potential, and manifested as the NCL at lower temperatures. The cusp-like change of T-dependence of f(Q)(T) at T(c) originates from the corresponding change of free volume and configurational entropy at T(B) ≈ T(c), which also explains the simultaneous changes of the T-dependencies of the other dynamic variables. The alternative explanation is able to resolve the conundrum in CKN because T(B) is ≥400 K, and hence the change of T-dependence of f(Q)(T) at T(c) ≈ T(B) was not observed in data taken at temperatures lower than 400 K by Mezei and Russina. The alternative explanation also can rationalize the difference between fragile and non-fragile glassformers in the strength of the observed changes of f(Q)(T) at T(c) and T(g) as well as the other dynamic quantities at T(B) ≈ T(c) and T(g).

Polymorphism and disorder in caffeine: Dielectric investigation of molecular mobilities

Using dielectric relaxation data we have characterized the molecular mobilities of caffeine both in phase I (stable and metastable) and in phase II. In phase I effects of sublimation and phase transformation kinetics were carefully considered. In plane rotational motions were followed on a wide temperature range. A noticeable antiferroelectric short range order developing at the approach of the glass-like transition is characterized. Condition for occurrence of a critical-like behaviour is discussed. At high temperature the emergence of an additional ultra slow relaxation process is highlighted. Possible molecular mechanisms are proposed for both processes. In phase II the existence of a less intense relaxation process is confirmed. Close similarity with the main process developing in phase I hints at a common origin of the dipolar motions. Careful consideration of recent structure determinations leads to suggest that this process is associated to similar molecular in plane rotations but developing at the surface of crystalline samples. Lower cooperativity at the surface is reflected in the smaller activation entropy of the relaxation.

Comparative Study on the Properties of Hydration Water of Na- and K-Halide Ions by Raman OH/OD-stretching Spectroscopy and Dielectric Relaxation Data

Properties of hypermobile water (HMW) were studied by Raman OH-stretching spectroscopy. Hydration water properties measured by Raman OH-stretching spectra of NaX/KX (X: Cl, Br, I) solutions (0.05-0.2 M) were comparatively analyzed with the data by dielectric relaxation spectroscopy (DRS), NMR, and statistical mechanical studies. The Raman OH-stretching spectra were well-fitted with linear combinations of the spectra of pure water both at the same and the higher temperatures. The fitting analysis determined the "structure temperature" Tstr and mole fraction of the high Tstr water region, giving the hydration number Nhyd, for each electrolyte solution. The determined Tstr was much higher than the solution temperature of 293 K for each tested salt and was higher for larger halide ions, consistent with commonly known "structure-breaking" order Cl < Br < I. No significant differences in Nhyd were observed between NaX and KX and among even halide ion species within the experimental errors. Measured Nhyd values of 25-27 were much greater than the reported numbers by NMR chemical shift and (17)O NMR relaxation studies and comparable to the numbers of hypermobile water reported in the previous DRS studies. The results indicated that the hydration region around NaX or KX measured by the present Raman study was nearly overlapped with the region of HMW by DRS. It was also suggested that differences in the ion size effects on Tstr and the DR frequency resulted from the sensitivity difference to long-range many-body interactions among water molecules. High structure-temperature regions were also detected by the analysis of OH-stretching and OD-stretching bands for 0.2 M NaI in H2O/D2O mixed solvent of 50 mol %, and we found that both OH-stretching and OD-stretching bands have almost equivalent Tstr ≈ 330 K and mole fractions with each other.

Comment on “Temperature divergence of the dynamics of a poly(vinyl acetate) glass: Dielectric vs. mechanical behaviors” [J. Chem. Phys. 136, 154901 (2012)

The temperature dependence of polymer dynamics below the standard glass transition is of considerable interest. Glassy relaxation dictates the process of physical aging, and understanding these slow dynamics helps in assessing a possible divergence at a finite temperature. In a recent paper, Zhao and McKenna have measured the equilibrium dynamics of poly(vinyl acetate) in the range from Tg − 15 K to Tg + 30 K using Struiks protocol [J. Zhao and G. B. McKenna, J. Chem. Phys. 136, 154901 (2012)]. In a figure that is meant to compare their results with dielectric relaxation data from Wagner and Richert [Polymer 38, 255 (1997)], dielectric retardation data from that work is shown instead. The corrected figure shows a considerable difference in the two measurements below Tg − 10 K, Arrhenius versus Vogel-Fulcher-Tammann behavior, and a possible source of the difference is discussed.

Plasticization of Fibrous Cellulose Acetate: Part II – Physical Property Measurement

Birefringence, broadband dielectric and dynamic mechanical relaxation measurements are reported on fibrous cellulose acetate (CTA) plasticized with dibutylphthalate (DBP). Marked differences in physical properties are attributed to retention of nano-structure. Dielectric and mechanical relaxation data show two relaxation processes. The lowest temperature process is attributed to segmental type motion of the polymer chain and the higher temperature process to side-chain/plasticizer relaxation. The plasticization behavior is very different from that observed with other materials. The high-temperature relaxations are consistent with relative sliding of the polymer chain or nano crystallites past one another as in a liquid crystalline material.

Dielectric behavior and impedance analysis of lead-free CuO doped (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 ceramics

Pure (Na0.50K0.50)0.95(Li0.05Sb0.05Nb0.95)O3 (NKNLS) and CuO doped NKNLS perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. x wt% of CuO (x = 0.2–0.8 wt%) was added in the NKNLS ceramics. X-ray diffraction patterns indicate that single phase was formed for pure NKNLS while a small amount of second phase (K6Li4Nb10O30 ∼ 3%) was present in Cu2+ doped NKNLS ceramics. Dielectric anomalies around the temperatures of 120 °C and 350 °C have been identified as the ferroelectric–paraelectric transition (orthorhombic to tetragonal and tetragonal to cubic) temperatures for pure NKNLS compound. The electrical behavior of the ceramics was studied by impedance study in the high temperature range. Impedance analysis has shown the grain and grain boundary contribution using an equivalent circuit model. The impedance response in pure and Cu2+ doped NKNLS ceramics could be resolved into two contributions, associated with the bulk (∼grains) and the grain boundaries. From the conductivity studies, it is found that activation energies are strongly frequency dependent. The activation energy obtained from dielectric relaxation data may be attributed to oxygen ion vacancies.