Temperature-dependent dielectric relaxation study of binary mixture using the time domain reflectometry method

Dielectric relaxation study of N,N-dimethylformamide (DMF) has been carried out with butylene glycol (BLG, i.e. 1,4-butanediol) at different temperatures. Time domain reflectometry in reflection mode has been used to measure the reflection coefficient in the frequency range from 10?MHz to 20?GHz. The dielectric parameters, static dielectric permittivity (ε ₀) and relaxation time (τ), have been obtained by Fourier transform and least squares fit methods. The experimental results show non-linear variation in dielectric permittivity, and relaxation time with volume fraction of BLG confirms the structural formation due to the intermolecular interaction between DMF and BLG. The variations in excess permittivity (ε^E ), excess inverse relaxation times (1/τ) ^E and Kirkwood correlation factors (g ^eff?;g?^f ) for the binary mixtures have also been reported in this article.     

Dielectric relaxation study of formamide–propylene glycol using time domain reflectometry

Using picoseconds time domain reflectometry, dielectric relaxation studies have been carried out for formamide (FMD)–propylene glycol (PLG) mixtures over the frequency range from 10?MHz to 20?GHz at various temperatures. The dielectric parameters, i.e. static dielectric constant (ε ₀) and relaxation time (τ), have been obtained by Fourier transform and least squares fit methods. The excess dielectric properties and Kirkwood correlation factor of the mixtures have also been determined. The Kirkwood angular correlation factor is greater than one (g ^eff?>?1) in FMD-rich region and less than one (g ^eff?<?1) in PLG-rich region, which indicates that in the mixture the dipole pairs have been formed in such a way that their orientation is parallel in FMD-rich region and antiparallel in the PLG-rich region.     

Dielectric relaxation study of aniline,N-methylaniline and N,N-dimethylaniline and alcohol in 1, 4-dioxane using picosecond time-domain reflectometry

A time-domain reflectometry technique has been used to measure complex dielectric permittivity ε*(ω) = ε?(ω) ? jε″(ω) of 1-propanol–dioxane, 2-propanol–dioxane, aniline–dioxane, N-methylaniline–dioxane and N,N-dimethylaniline–dioxane mixtures in the frequency range of 10 MHz to 30 GHz. The complex permittivity spectrum has been fitted with a single relaxation time with a small amount of Davidson–Cole behaviour. The least squares fit method has been used to obtain the static dielectric constant (ε₀), relaxation time (τ), Bruggeman factor and Kirkwood correlation factor. The Luzar theoretical model is used to compute the binding energies and average number of hydrogen bond between co-solvent–co-solvent and co-solvent–dioxane molecules.     

Dielectric relaxation of Tripropylene glycol–water mixture using time domain reflectometry

The complex permittivity spectra of tripropylene glycol and water solutions have been obtained by time domain reflectometry (TDR) technique in the frequency range from 10 MHz to 30 GHz and the temperature range 20°C–05°C. The dielectric relaxation parameters such as static dielectric constant and relaxation time were obtained by using the non-linear least square fit method. The intermolecular hydrogen bonding of tripropylene glycol–water has been discussed using the Kirkwood correlation factor and thermodynamic parameters. The activation energy decreases with increase in water content in the mixture as expected in the Arrhenius behaviour. The dielectric constant for mixtures has been fitted to the Bruggeman mixture formula in the non-linear case.     

Dielectric Relaxation Studies of Binary Mixture of α‐Picoline and Methanol Using Time Domain Reflectometry at Different Temperatures

Complex permittivity spectra of binary mixtures of varying concentrations of α‐picoline and methanol (MeOH) were obtained using time domain reflectometry (TDR) technique over frequency range 10 MHz to 25 GHz at 283.15 K, 288.15 K, 293.15 K and 298.15 K temperatures. The dielectric relaxation parame‐ ters namely static permittivity (σ₀), high frequency limit permittivity (σ_oo1) and the relaxation time (ρ) were determined by fitting complex permittivity data to the single Debye/Cole‐Davidson model. Complex non linear least square (CNLS) fitting procedure was carried out using LEVMW software. The excess static permittivity (σ₀^E) and the excess inverse relaxation time (1/ρ)^E which contains information regarding mo‐ lecular structure and interaction between polar — polar liquids, were also determined. From the experimental data, effective Kirkwood correlation factor (g^eff) and Bruggeman factor (f_B) were calculated. Excess parameters were fitted to the Redlich‐Kister polynomial equation. The values of static permittivity and relaxation time increase non‐linearly with increase in the mol fraction of MeOH at all temperatures. The values of excess static permittivity (σ₀^E) and the excess inverse relaxation time (1/ρ)^E are negative for the studied α‐picoline — MeOH system at all temperatures.     

Dielectric properties of solutions of tetra-iso-pentylammonium nitrate in dioxane-water mixtures

Dielectric properties of solutions of tetra-iso-pentylammonium nitrate, i-Pen ₄ NNO ₃ . in various dioxane-water mixtures have been studied using dielectric time domain spectroscopy (TDS). The static permittivity of the solutions _s increases for low concentrations of solute but levels off to asymptotic values at higher concentrations. The limiting sloped_s dc, and the asymptotic value depend on the static permittivity of the solvent mixture. The relaxation time due to the solute varies with solute concentration and depends on the solvent mixture. In the solvent mixtures of lowest permittivity the plots of relaxation time vs. concentration go through a maximum, while in the mixtures of highest permittivity the relaxation time initially decreases and then levels off to an asymptotic value. The concentration dependence of the dielectric parameters is discussed in relation to ion association.     

Dielectric relaxation studies of binary mixtures of N-methylacetamide and N-methylformamide in benzene solutions using microwave absorption data

Using standard microwave X-band technique and by following Gopala Krishna's single frequency (9.90?GHz) concentration variational method, the dielectric relaxation times (τ) and the dipole moments (μ) of dilute solution of N-methylacetamide (NMA), N-methylformamide (NMF) and NMA?+?NMF binary mixtures in benzene solutions have been calculated at different temperatures. The energy parameters for the dielectric relaxation process for NMA?+?NMF binary mixture containing 30?mol% NMF have been calculated at 25, 30, 35 and 40°C and compared with the corresponding viscosity parameters. A good agreement between the free energy of activation from these two sets of values shows that the dielectric relaxation process like the viscous flow process can be treated as the rate process. From relaxation time behavior of NMA and NMF binary mixture in benzene solution, solute–solute types of the molecular association has been proposed.     

Microwave dielectric properties of aqueous potassium iodide solutions as a function of temperature

The complex dielectric permittivity of aqueous KI solutions was studied for molalities of 0.50–4.01 m and temperatures of 288–323 K in the region of water dielectric permittivity dispersion. The values of high-frequency of dielectric permittivity (ε) and dielectric losses (ε″) were obtained at seven frequencies ranging between 7.5 and 25 GHz. The low-frequency electrical conductivity of the aforementioned solutions was measured for calculating ionic losses. A single relaxation process is observed in these solutions, fitted by the Debye or Cole-Cole equation with small distribution parameters. The static dielectric constant and dielectric relaxation time were studied as functions of temperature and concentration; the activation enthalpy of dielectric relaxation was calculated. The temperature dependence of the static dielectric constant was found to disappear in highly concentrated solutions. The structure-breaking effect on water caused by K⁺ and I⁻ ions was affirmed, this effect disappearing in going to elevated temperatures.     

Kerr Effect of Xanthan/DTAB Aqueous Solutions

The Kerr effect of the Xanthan (charged polyelectrolyte; 113 ppm)/DTAB (surfactant) mixture in aqueous solution was studied. The static Kerr constant (B) and mean relaxation times (τ) values as a function of surfactant concentration (C_DTAB ) were determined. The observed birefringence (Δn) is not a linear function of the electric applied field (E ²). For small E values Δn grows with E ² and at a certain field value the birefringence tends to saturate. The addition of small quantities of DTAB lowers B values with respect to the Kerr constant of Xanthan (113 ppm) aqueous solution. The relaxation time of the mixtures is linearly dependent on the applied field and it decreases when DTAB concentration increases. The importance of the equivalent point is signalled. It is supposed that the initial rigidity of the polyelectrolite decreases because of the surfactant addition. Conductivity (σ) values as a function of surfactant concentration are presented.     

Temperature dependent dielectric relaxation spectroscopy of amyl acetate-xylene solutions – An approach to molecular and co-operative dynamics

Dielectric relaxation parameters of AMA-Xylene solutions have been investigated thoroughly to discern the associativeness as well as heterogeneous behavior among the hetero molecules in the solutions such that dielectric constant (ε_o) and relaxation time (τ) of solutions has been determined. Complex permittivity spectra (CPS) for Amyl Acetate (AMA)-Xylene solutions were measured in 10 MHz–50 GHz frequency range using Time domain reflectometry (TDR) technique. It is exciting to explore the cooperative nature and association between AMA-Xylene. With co-operative domains (CDs), dynamics in AMA-AMA and AMA-Xylene molecules have been explicated while Kirkwood validates CDs with diverse exchanges through bonding. The experimental values of ε_o are compared with theoretical values obtained from the molecular parameters suggested by the Luzar. Thermodynamic parameters for AMA-Xylene show positive values of Enthalpy and Entropy for all concentrations, indicating that the system is endothermic and less ordered. Gibbs free energy decreases with increase in volume fraction of amyl acetate (V_AMA).     

Stabilization of water structure in ammonium hydrogenphosphate solutions

Radio-frequency permittivity and dielectric loss of (NH₄)₂ HPO₄ solutions at 288–308 K in the water permittivity dispersion region (7.5–25 GHz) are studied. Low-frequency electrical conductivities of these solutions are measured, and ionic losses at high frequencies are calculated. The analysis is carried out in terms of the Cole-Cole relaxation model. Static dielectric constants ?_s and the activation times τ and thermodynamic parameters of activation (ΔH _? ⁺⁺ , ΔG _? ⁺⁺ , and TΔS _? ⁺⁺ ) for the dielectric relaxation of the solutions are calculated. The values of ?_s decrease in response to increasing salt concentration at all temperatures. The increasing τ and ΔH _? ⁺⁺ values indicate the stabilization of water structure in solutions where the anion and cation are hydrophilically hydrated.     

Sub-TG relaxations in LiClO4-poly (propylene glycol)-2000 solutions

The dielectric permittivity and loss of LiClO₄ solutions in poly (propylene glycol (PPG)), molecular weight 2000, have been measured over a concentration range up to a ratio of Li⁺ to oxygen atoms in PPG of 33.3:100, between 77 and 350 K. The data have been analyzed in both the permittivity and electrical modulus formalisms. Addition of LiClO₄ to poly (propylene glycol) first increases the height of the β-relaxation peak, and ultimately a second sub-T_g relaxation peak at a higher temperature emerges. This is in addition to the β-relaxation peak due to the reorientation of PPG dipoles, whose strength decreases from that in pure PPG-2000. For a fixed temperature, the dc conductivity initially decreases with increasing Li⁺ concentration up to 20 Li⁺ per 100 O atoms and thereafter increases. This concentration corresponds to that at which the T_g of the solution reaches its limiting value of ca. 310 K. It is concluded that the formation of ion pairs causes a second and slower sub-T_g relaxation process and that the increase in the efficiency of chain packing reduces the strength of the β-relaxation of the polymer.     

Temperature dependent dielectric relaxation study of ethylene glycol-water mixtures

Using the picosecond time domain reflectometry method, dielectric relaxation measurements for 13 ethylene glycol (EG)-water mixtures have been studied from 0 to 40°C. The dielectric relaxations in the mixtures show a Debye-type behavior, whereas the relaxation in pure EG can be described by the Cole-Cole model. The static dielectric constant ₀, the relaxation time and the dielectric constant at high frequency have also been determined at various temperatures. The dielectric relaxation data suggests that there is no tendency to form hydrogen bonds with the addition of water to EG unlike other alcoholic systems but this tendency becomes increasingly important with decreasing temperature. The activation energy decreases with increased water content in the mixture as expected.     

Mutual viscosity and NMR spin-lattice relaxation time in some benzoic acids

Experimental values of the NMR spin-lattice relaxation time (T ₁) of o-aminobenzoic acid, p-aminobenzoic acid, o-chlorobenzoic acid, p-chlorobenzoic acid and 2,4-dinitrobenzoic acid and mutual viscosity (η₁₂) of o-chlorobenzoic acid, m-chlorobenzoic acid and p-chlorobenzoic acid have been reported. The experimental values of T ₁ have been correlated with the calculated value of T ₁ obtained using different equations of dielectric relaxation time (τ). It is concluded from this comparative study that Murty's equation is a better representation of the dielectric relaxation phenomenon. It is also concluded that the mutual viscosity (η₁₂) is a better substitute for the resistance to the rotation of the individual solute molecule.     

Microwave dielectric properties of potassium hydroxide aqueous solutions

Microwave dielectric properties of potassium hydroxide aqueous solutions were studied at frequencies of 13 to 25 GHz over a wide range of concentrations and temperatures of 288 to 308 K. Experimental electrical conductivity data were used to determine ionic losses at high frequencies. The static dielectric permittivity σ_s, dielectric relaxation time τ, and thermodynamic activation parameters of dielectric relaxation (ΔH _ɛ⁺⁺, ΔG _ɛ⁺⁺, and ΔS _ɛ⁺⁺) were calculated. These quantities decrease in transfer from water to solutions. Thereby we demonstrate that potassium hydroxide has a disturbing effect on the initial hydrogen-bond network of water.     

Temperature changes of dielectric permittivity and relaxation in aqueous lithium iodide solutions

The complex permittivity of aqueous LiI solutions is studied over a wide range of concentrations at temperatures of 288–323 K in the water permittivity dispersion region at seven frequencies in the range of 7.5–25 GHz. One relaxation region describable by the Debye or Cole-Cole equation is observed in these solutions. Dielectric relaxation time τ and static permittivity ?_s are studied as dependent on temperature and concentration. The time and enthalpy of activation of dielectric relaxation decrease in going from water to solutions, which corresponds to the distortion of the initial water structure and the increasing mobility of water molecules in hydration shells of ions. In the initial concentration range, the water activity is a linear function of 1/?_s. The negative temperature dependence of ?_s disappears in going to concentrated solutions. At high concentrations, the static dielectric constant increases in response to increasing temperature. The new trends in ?_s and τ at elevated temperatures of 313–323 K are due to the formation of ion pairs and other ion-water groups having high dipole moments.     

Dielectric spectroscopy and calorimetry during postcuring of a linear chain polymer thermoset formed from a diepoxide and cyclohexylamine,and the nature of the products

The dielectric permittivity and loss spectra of an equimolar liquid mixture of diglycidyl ether of bisphenol-A and cyclohexylamine have been studied during the liquid's isothermal polymerization or curing in separate experiments at different temperatures and thereafter during the postcuring, both on rate-heating and isothermally. The spectra obtained during the growth of the linear chain polymer during the curing and postcuring show the evolution of an intermediate relaxation process whose position in the frequency plane remains relatively insensitive to the decrease in the configurational entropy during the postcuring, but whose strength increases. Postcuring ceases to occur once the calorimetric glass-liquid transition temperature of 345 K, corresponding to the ultimately formed polymeric state, has been reached. The increase in the number of covalent bonds, n, formed during curing and postcuring decreased the equilibrium dielectric permittivity, ε_s, and increased the characteristic relaxation time, τ₀, for all curing and postcuring conditions. For a fixed temperature and n, (dε_s/dT) and (dτ₀/dT), as well as the values ε_s and τ₀ of the ultimately formed state of the polymers differ significantly when the thermal history of polymerization differs. The slow dynamics in the glass-liquid transition region were analyzed in terms of the enthalpy relaxation and fictive temperature concepts. The distribution of relaxation times for these dynamics correspond to the stretched exponential parameter of 0.6, which is significantly greater than 0.39 determined for the dielectric α-relaxation spectra measured at a temperature 30 K higher. The enthalpy relaxation involves a narrower distribution of intermolecular barriers than dielectric relaxation. The results also show that the recently proposed method for determining the gelation time from the plots of the imaginary component of electrical impedance lacks scientific merit. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 303–318, 1998     

Dielectric and visco-elastic properties of laterally fluorinated liquid crystal single compounds and their mixture

The physical properties of three laterally fluorinated liquid crystalline compounds with negative dielectric anisotropy have been studied from static dielectric permittivity, optical birefringence, bend elastic constant, relaxation time and rotational viscosity measurements. Such negative dielectric anisotropy materials find use as components of mixtures for application in vertically aligned mode liquid crystal displays. Moreover, the physical properties of one phenyl cyclohexane compound with positive dielectric anisotropy have also been studied. A five-component mixture comprising these four mesogens and a non-mesogenic component has been formulated and its physical properties have been thoroughly investigated. An attempt has been made to strike a balance between the optical birefringence of the mixture to adjust the cell gap and the dielectric anisotropy and threshold voltage (V_th ) to ensure low driving voltages. The pretilt angle effect on the threshold voltage and the relaxation time has also been studied. At T?=?20°C, the response time decreases to 22% and 41% for the mixture for 2° and 5° pretilt as compared to zero pretilt. On the other hand, at the same temperature the V_th values are decreased by 5% and 9%, respectively.     

Dilectric and thermal properties of PEO doped with cadimium chloride salt

The main aim of this research is to investigate the effect of salt concentration on the dielectric properties(AC (σ_AC),permittivity（ε′）,dielectric loss（ε″）,and dielectric relaxation process) and melting behavior of polyethylene oxide （PEO）/CdCl₂ complexes.The dielectric study was carried out over a frequency range 10-335 kHz and a temperature range 25-45℃.The AC conductivity,permittivity and dielectric loss of the PEO/CdCl₂ complexes increase with increasing salt concentration and temperature.Also,it was found that the addition of CdCl₂ salt to PEO host reduced the melting temperature of PEO host.Dielectric results reveal that the relaxation process of these complexes is due to viscoelastic relaxation or non-Debye relaxation at room temperature.Additionally,it was found that relaxation behavior remained viscoelastic at different temperatures and salt concentrations.     

Dielectric permittivity and relaxation of aqueous cesium iodide solutions

The high-frequency dielectric permittivity and losses of CsI solutions were studied at 288–323 K in the range of water dielectric permittivity dispersion (7–25 GHz). The low-frequency electrical conductivity of these solutions was measured, and ionic losses at high frequencies were calculated. The Debye or Cole-Cole relaxation model was used for describing the spectra. The low-frequency limits of these relaxation region were calculated, which are the static dielectric constants ɛ_S and well as dielectric relaxation times (τ) and activation enthalpies (ΔH _ɛ⁺⁺). The ɛ_S values decrease in going from water to a solution. In concentrated solutions, the slope of the plot of ɛ_S versus temperature become zero. The decrease in τ and gDH _ɛ⁺⁺ is evidence of the structure-breaking effect of ions on water. At elevated temperatures (313 K), the decrease in τ is minimal. At 323 K, τ slightly increases in going from water to a solution.