Dielectric spectroscopy and viscosity studies of aqueous poly(ethylene oxide) and poly(vinyl pyrrolidone) blend

The complex dielectric function, electric modulus, impedance and ac electrical conductivity behaviour of aqueous solutions of 5 wt% poly(ethylene oxide) (PEO) and poly(vinyl pyrrolidone) (PVP) and their different volume percent blends were investigated in the frequency range 20 Hz to 1 MHz at 15, 30 and 45 °C. It is found that the real part of dielectric function of these blends at 1 MHz decreases with the increase of PEO concentration and their dc electrical conductivity has strong correlation with the electrode polarization relaxation time. The static permittivity, ionic conductivity, electrode polarization relaxation time and apparent viscosity have linear behaviour with temperature variation at fixed volume concentration of the aqueous polymers blend. The viscosity of these aqueous polymeric blends increases with the increase of PEO concentration. The behaviour of hydrogen bond interactions between the polar segments of PEO and PVP were explored from the comparative change in dielectric parameters and viscosity of the two phase aqueous polymeric systems.     

Dielectric Dispersion Study of Poly(vinyl Pyrrolidone)–Polar Solvent Solutions in the Frequency Range 20 Hz–1 MHz

Influence of polar solvents environment and polymer concentrations on the electrical properties (complex dielectric constant, ac electrical conductivity, complex electric modulus and complex impedance) of the solutions of poly(vinyl pyrrolidone) (PVP) in polar solvents, namely water, ethyl alcohol, ethylene glycol, diethylene glycol, poly(ethylene glycol), glycerol, dimethyl sulfoxide and dimethyl formamide, have been investigated in the frequency range 20 Hz–1 MHz at 25°C. Comparative analysis of the dielectric dispersion curves confirms that the solvent molecular size and number of its hydroxyl groups, and the solutions viscosity, are the major factors which governs the PVP chain segmental motion. The ionic conduction and electrode polarization phenomena has a dominant influence on the large increase of complex dielectric constant values of the solutions of PVP‐polar solvent in the lower frequency region. The values of relaxation times corresponding to these phenomena are also reported.     

Dielectric Relaxations and Structures of Nanoclay in Solution Cast Poly(Ethylene Oxide)– Montmorillonite Clay Nanocomposites

The relative complex dielectric function, electric modulus and alternating current electrical conductivity spectra and complex impedance plane plots of aqueous solution cast poly(ethylene oxide)–montmorillonite clay (PEO–MMT) nanocomposite films were investigated over the frequency range 20 Hz to 1 MHz at ambient temperature. The intercalated and exfoliated structures of nanoclay dispersed in PEO matrix were recognized by the significant change in real part of dielectric function with clay concentration in the range 0%–20 wt%. The relaxation times corresponding to PEO chain segmental motion and ionic conduction relaxation processes were used to explore the interactions compatibility between PEO molecules and the dispersed MMT clay nanoplatelets and their effect on PEO chain dynamics. Real part of conductivity spectra of these nanocomposites over five decades of frequency has nonlinear behavior, which is influenced by the MMT clay concentration. The complex impedance plane plots confirm the bulk properties of these nanocomposites over the experimental frequency range.     

醋酸N-正辛基吡啶离子液体在不同溶剂中的荧光光谱分析

离子液体具有熔点低、可忽略的蒸气压、电化学窗口宽、热稳定性高和良好的导电性等独特性能,引起了化学工业和相关领域的广泛关注。离子液体具有低蒸气压,不会造成空气污染,但这并不意味着它们对环境完全无害。大多数离子液体易溶于水,可能会因为意外泄漏或通过污水进入水生环境。常用离子液体[BMIM][PF₆]和[BMIM][BF₄]的水溶液中,很容易形成氢氟酸,磷酸,具有一定的腐蚀性。将离子液体列为绿色溶剂,也需要提供其关于代谢和降解的毒性、生态毒性研究数据,或者其对环境影响的数据,离子液体在不同溶剂中的检测方法是非常重要的。离子液体的光谱分析法用量较少、方法简单、结果准确。离子液体和许多有机溶剂互溶,可形成均一、稳定的溶液。荧光检测法具有灵敏度高,选择性好,线性范围宽和受外界干扰少等优点。本工作研究了醋酸N-正辛基吡啶（OP-OAc）离子液体在水、乙醇、乙腈、乙酸等4种溶剂中的荧光光谱。研究结果表明,OP-OAc离子液体在不同溶剂中的荧光强度：I_乙酸＞I_乙腈＞I_乙醇＞I_水;最大发射波长的大小顺序：λ_{em, 水}＞λ_{em, 乙醇}＞λ_{em, 乙腈}＞λ_{em, 乙酸};它们的最大发射波长相对于激发波长发生红移;水中OP-OAc的荧光强度与浓度存在较高的相关性;当加入的甲醇、乙醇、乙腈溶剂不断增加时,OP-OAc离子液体的荧光强度增加,溶剂与水的比例为8∶2时,OP-OAc离子液体的荧光强度最强,溶剂的比例超过80%时,荧光强度突然降低;水中OP-OAc离子液体在pH 10时,荧光强度最高,在pH 14时,荧光强度最低。     

A comparative study of non-polar solvents effect on dielectric relaxation and dipole moment of binary mixtures of mono alkyl ethers of ethylene glycol and of diethylene glycol with ethyl alcohol

Dielectric relaxation and dipole moment of binary mixtures of homologous series of mono alkyl ethers of ethylene glycol and of diethylene glycol, i.e., mono methyl, mono ethyl and mono butyl ethers of ethylene glycol (ROCH₂CH₂OH) and mono methyl, mono ethyl and mono butyl ethers of diethylene glycol (ROCH₂CH₂OCH₂CH₂OH) with ethyl alcohol (C₂H₅OH) of different concentrations were studied in dilute solutions of benzene, dioxane and carbon tetrachloride at 35 °C. Permittivity (ε′) and loss (ε″) at 10.1 GHz, static dielectric constant ε_o at 1 MHz and high frequency limiting dielectric constant ε_∞ = n_D² at optical frequency of these molecules and their binary mixtures at different concentration were measured in dilute solutions of non-polar solvents. The average relaxation time τ_o, relaxation times corresponding to overall molecular reorientation τ₁ and group rotations τ₂ were determined using Higasi's single frequency measurement equations for dilute solutions. The evaluated values of relaxation times and free energy of activation ΔF were used to explore the solvent effect on molecular dynamics of these polar binary systems in non-polar solvents. The excess inverse relaxation time and excess free energy of activation were determined to confirm the existence of hydrogen-bonded heterogeneous cooperative domains of the ethers and alcohol molecules at different concentration their binary mixtures in non-polar solvents. The dipole moment of the binary mixtures was evaluated using Higasi's and Guggenheim's equation for dilute solutions. The evaluated values of dipole moments and computed dipole moment values using a simple mixing equation of the polar molecules binary mixture were used to explore the effect of non-polar solvent environment on heterogeneous molecular interactions between ethers and alcohol molecules. The effect of number of carbon atoms in the molecular structure of these homologous series molecules was also considered for the interpretation of various evaluated dielectric parameters.     

Spectroscopic Study of Solvent Polarity on the Optical and Photo-Physical Properties of Novel 9,10-bis(coumarinyl)anthracene

Novel 7,7′-((anthracene-9,10-diylbis(methylene))bis(oxy))bis(4-methyl-2H-chromen-2-one) (BisCA) was prepared as fluorescent probe. The chemical structure of the novel BisCA was confirmed by spectroscopic data as well as elemental analyses. The solvatochromic characteristics of the new proble and its precursors were investigated in different solvents including, ethanol, DMF and toluene as protic polar, aprotic polar and non-polar solvents, respectively. Photo-physical parameters of probes, such as fluorescence quantum yields, fluorescence lifetime of excited state, radiative and non-radiative decay, were assessed in different media. The intermolecular H-bond effect on absorption and excitation spectra of the novel probe was reported in different solvents. Also, Onsager cavity radius and dipole moment of ground state and excited state of the probe were calculated as described by Bakhshiev and Reichardt methods.     

Dielectric alignment of a non-polar molecule substituting a polar molecule in a lattice of molecular dipoles

The dielectric alignment of a dilute non-polar component in a polar solvent may be determined by NMR spectroscopy. In this paper a series expansion of the alignment up to second order terms in the dipolar interaction is presented for a non-polar molecule replacing a single polar molecule in a rigid lattice of molecular dipoles. Assuming isotropic polarizabilities for all molecules Van Vleck earlier applied the lattice model in a theory of the dielectric constant of a dilute solution.In the present calculation the polarizabilities of the non-polar molecule and the dipoles are assumed to be anisotropic and isotropic, respectively. The anisotropy of the non-polar molecule is relevant, since the value of the alignment is zero for the isotropic case. Different results are obtained for cubic lattices and for a lattice points are uniformly distributed.In absence of a rigid dipole moment of the solvent molecules the latter formula may be compared with that implicity deduced by Buckingham for the Kerr effect. In the limit of strong dipoles the orientation imposed by the rigid dipole moments appears to be the major contribution to the alignment of the non-polar molecule.     

Dielectric dispersion studies of aromatic ketones in non-polar solvents

Measurements of dielectric constant (ε′) and loss (ε″) have been made at five different microwave frequencies from 1000 MHz to 67.7 GHz for acetophenone, benzophenone and propiophenone in four different non-polar solvents namely benzene, cyclohexane, 1-4-dioxane and n-heptane. Results are reported at five different temperatures from 25°C to 60°C. The dielectric data of these ketones in different solvents are analysed in terms of the Cole-Cole arc plots and superposition of two Debye-type absorptions. Values of mean relaxation times (τ_o), dipolement (μ), overall relaxation time (τ₁) and group relaxation time (τ₂) have been obtained and presented here. The values of relaxation time and dipolemoment are in reasonable good agreement, at the temperatures, at which there are available known data.     

Dielectric behaviour and AC conductivity of LiCF3SO3 doped H-chitosan polymer films

H-chitosan that exhibited solubility in THF was prepared by acyl modification of chitosan. Films of H-chitosan containing LiCF₃SO₃ were prepared by the solution cast technique. The effect of salt concentration on the frequency-dependent dielectric properties of H-chitosan: LiCF₃SO₃ complexes were investigated by impedance spectroscopy, in the temperature range from 243 to 373 K. The dielectric properties and ac conductivity of the samples prepared have been analyzed. The dielectric constant increases sharply with temperature in the low frequency region. At higher frequencies, the effect of temperature on the dielectric constant is negligible. The values of dielectric constant were also found to increase with increasing conductivity of the samples. The imaginary part, M_i of electrical modulus shows the formation of dispersion peak. Relaxation times for the ionic charge carriers were extracted from the loss tangent maximum peak at various temperatures. The plot of relaxation times as a function of temperature shows Arrhenius type behaviour. The ac conductivity was found to obey the universal power law and as the temperature increases, the feature of σ(θ) α θⁿ is more predominant. The temperature dependence of the power law exponent n is reasonably interpreted by the overlapping large polaron tunneling (OLPT) model.     

Construction and performance of an NMR tube with a sample cavity formed within magnetic susceptibility-matched glass

We describe the construction and performance of an NMR tube with a magnetic susceptibility matched sample cavity that confines the solution within the detection zone in the axial direction and in a quasi-rectangular region in the radial direction. The slot-like sample cavity provides both good sample volume efficiency and tolerance to sensitivity loss in the sample space. The signal-to-noise ratio per unit volume of the constructed tube was 2.2 times higher than that of a cylindrical tube of 5mm outer diameter with a sample containing 300 mM NaCl at a static magnetic field of 14.1T. Even the overall signal-to-noise ratio of the slot tube was 35% higher than that of the conventional 5mm tube for a sample containing 300 mM NaCl. Similar improvements over existing sample tube geometries were obtained at 950 MHz. Moreover the temperature rise resulting from RF heating was found to be significantly lower for the slot tube even when compared to 3 and 4mm outer diameter cylindrical tubes as measured in a 5mm cryoprobe. A further advantage of this type of tube is that a sample cavity of any desired size and shape can be formed within a cylindrical tube for use in a single cryogenic probe.     

On the theory of the alignment of a polar molecule in a non-polar solvent

The alignment average of a dilute polar substance dissolved in a non-polar solvent may be derived from the NMR spectrum of the polar component. Likewise, the Kerr effect is related to this quantity.In this paper a series expansion of the alignment of a polar molecule in a non-polar solvent is presented for a rigid-lattice model, first introduced by Van Vleck for the dielectric constant of a very dilute solution. Different results up to second order in the dipolar interaction are obtained for a simple, a body centred, or a face centred cubic lattice and a lattice whose lattice points are uniformly distributed in a continuum. The latter formula is compared with that calculated on the basis of the Lorentz local field model and with that of Buckingham in which the interaction between the dipoles themselves is neglected.     

A high-resolution NMR probe in which the coil and preamplifier are cooled with liquid helium. 1984

In a well designed NMR spectrometer, the noise originates predominantly from the resistance of the receiver coil. Significant improvements in sensitivity can be achieved by cooling the coil to cryogenic temperatures, provided that a preamplifier can be designed to match the coil’s performance. A probe is described in which the coil and preamplifier are cooled with liquid helium, but the sample is maintained at room temperature. Carbon-l3 spectra at 45 MHz demonstrate improved sensitivity over conventional probes at the same field.     

Electret-thermal and dielectric analyses of electrically active colloids

The nature and mechanism of interfacial electrical interaction in organic colloids containing an active solid phase and a polar liquid phase are studied. Such systems are shown to offer specific properties. Specifically, high electrical forces producing gradients of the potential of the self-electric internal field arise at the liquid-solid interface. In the presence of free charges and ionic conductivity channels, this field can induce currents in such systems. When investigating the colloid system of the human venous blood, it is found that the internal field causes mesoscopic inhomogeneity in the structure of the aqueous components of the plasma and cytoplasm. It is shown that the concentration of free water molecules in the liquid phase of the system is maximal in group I blood and the physically nonuniform properties of combined water show up to the greatest extent in group IV blood. This finding is corroborated by the dispersion of the dielectric polarization in blood of different groups.     

温度相关的微波频率下氯化钠水溶液介电特性

针对NaCl水溶液(0.001–0.5 mol/L)介电特性, 实验调查了频率(200–6.25 GHz), 温度(293–353 K)、 浓度相关复介电常数. 结果表明: 频率增大的过程中虚部呈逐渐减小的趋势, 高温使离子扰动增大, 破坏了溶液内部水分子四面体结构和氢键构象而使介电常数实部减小. 与纯水相比, 溶液的损耗角正切在高温353 K低频区下降明显. 同时发现2.45和5.8 GHz的复介电常数随温度变化的温度窗效应, 温度窗效应导致微波加热时耗散功率的振荡变化, 温度分布不均匀现象在实验中得以证实. 关键词： 微波 复介电常数 介电特性 高温探头     

Dielectric properties of pyridine N-oxide aqueous solution under the static electric field and microwave field

Molecular dynamics simulations of the pyridine N-oxide aqueous solution have been performed in the canonical ensemble macroscopic canonical ensemble (NVT) both in the absence and presence of an external electromagnetic field. It extracts the radial distribution function for each concentration solution, dielectric constant and other information on dielectric properties. Analysing the microscopic dielectric information of the aqueous solution under the static electric field (0–3×10⁹ V/m) and microwave frequencies (2.45G, 0–3×10⁹ V/m), and comparing the dielectric information between the different concentrations and different field strengths, we can get the dielectric properties of two kinds of polar aqueous solution under microwave irradiation. Thus, this project can provide the data of the sample to other correlation studies.     

Intra- and intermolecular proton transfer in methyl-2-hydroxynicotinate

Spectral characteristics of methyl 2-hydroxynicotinate (MEHNA) have been studied using absorption, fluorescence excitation and fluorescence spectroscopy, as well as, using single photon counting nanosecond spectrofluorimeter. MEHNA is present as enol in less polar solvents and keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to phototautomer, formed by excited state intramolecular proton transfer (ESIPT), whereas fluorescence is only observed from keto form in polar solvents. In aqueous and polar solvents monocation (MC) is formed by protonating the exo carbonyl oxygen atom in the ground state (S₀) and in the first excited singlet state (S₁), MC is obtained by protonating carbonyl oxygen atom of the ester. It is formed by ESIPT from exo carbonyl proton to carbonyl oxygen atom of the ester. Dication is formed by protonating both the oxygen atoms. Two kinds of monoanions formed by deprotonating phenolic proton or >N-H proton of keto suggest the presence of enol and keto in aqueous solution. In cyclohexane MC is formed by protonating carbonyl oxygen in both S₀ and S₁ states. The electronic structure calculations were performed on each species using semi-empirical quantum mechanical AM1 method and density functional theory B3LYP with 6-31G^** basis set using Gaussian 98 program, along with potential energy mapping, to characterize the particular species.     

Excited-state inter- and intramolecular proton transfer in methyl 3-hydroxy-2-quinoxalinate: effects of solvent and acid or base concentrations

Absorption, fluorescence excitation and fluorescence spectroscopy, combined with time-dependent spectroscopy and semi-empirical (AM1) and density functional theory using Gaussian 98 program calculations have been used to study the effects of solvent and acid or base concentration on the spectral characteristics of methyl 3-hydroxy-2-quinoxalinate (M3HQ). M3HQ is present as enol in less polar solvents and as keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to the phototautomer, formed by the excited-state intramolecular proton transfer, whereas fluorescence is only observed from keto in the polar solvents. In aqueous and polar solvents the monocation (MC5/MC6) is formed by protonating the carbonyl oxygen atom in the ground (S₀) and the first excited singlet states (S₁). Dication is formed by protonating one of N- atom of MC5/MC6. Monoanion is formed by deprotonating the phenolic proton of enol in the basic solution. pK_a values for different prototropic equilibriums were determined in S₀ and S₁ states and discussed.     

Study on the Electrospinnability of Polyvinyl Alcohol Solutions by Using Water/N,N-dimethylacetamide or Water/N,N-dimethylformamide as Solvents

The electrospinning of poly(vinyl alcohol) (PVA) (99% hydrolysis degree) aqueous solution with added organic solvents N, N-dimethylacetamide (DMAc) or N, N-Dimethylformamide (DMF) was investigated. After the addition of the organic solvents to the PVA aqueous solutions, the surface tension and conductivity decreased and the viscosity significantly increased, which caused an improved electrospinnability of the PVA solutions. The micro-structures of the three solutions were investigated by dynamic light scattering (DLS), differential scanning calorimetry (DSC) and dynamic viscoelastic measurements. The DLS data revealed that the swelling of the PVA coils was slightly increased but the overlaps of PVA coils decreased greatly after one of the organic solvents was added to the aqueous solution. The DSC data showed both the water-rich phase and PVA-rich phase were destroyed and the solution became more homogenous after the addition due to the interaction between the organic solvent and water. Viscoelastic data showed there was an obvious difference in the storage modulus behavior between the aqueous solutions and the water/solvents solutions. These changes in the micro-structure and properties were the reason for the improved electrospinnability of the PVA solution. According to scanning electron microscopy (SEM), the average diameter of the electrospun PVA nanofibers was about 308 nm for the DMF/water system, and 255 nm for the DMAC/water system, as compared with uneven diameter nanofibers for the water system.     

Electrical and electrochemical properties of nanocrystalline LiFePO4 cathode

Lithium iron phosphate (LiFePO₄) cathode material has been prepared by hydrothermal synthesis. The XRD spectrum exhibited different characteristic peaks along with (311) predominant orientation corresponding to orthorhombic crystal structure with Pnma space group. Electric and dielectric properties were studied over a frequency range of 1 Hz–1 MHz at different temperatures. The conductivity was found to be increased with increasing temperature following Arrhenius relation with an estimated activation energy of 0.44 eV. The dielectric properties were analyzed in the framework of complex dielectric permittivity and complex electric modulus formalisms. The complex permittivity as a function of frequency and temperature was investigated. Several important parameters, such as activation energy, ionic hopping frequency, carrier concentration, ionic mobility, and diffusion coefficient, etc., were determined. The electrochemical characteristics of LiFePO₄ are examined in aqueous region. It exhibited a good reversible cyclic voltammogram on sweeping the potential upward and downward with discharge capacity of 140 mAh/g.     

A temperature-jump design for conventional NMR probes

The design and performance of a simple probe insert for temperature-jump experiments in conventional NMR probes is described. The insert uses the output from conventional NMR amplifiers for heating conductive aqueous samples with a rate of 30-80 K/s for 200 W radiofrequency power. The observed dependence of the heating rate on sample conductivity is explained by the dominance of dielectric heating. Factors governing the temperature gradient within the sample are discussed.