A density functional study on dielectric properties of acrylic acid grafted polypropylene

Influence of acrylic acid grafting of isotactic polypropylene on the dielectric properties of the polymer is investigated using density functional theory (DFT) calculations, both in the molecular modeling and three-dimensional (3D) bulk periodic system frameworks. In our molecular modeling calculations, polarizability volume, and polarizability volume per mass which reflects the permittivity of the polymer, as well as the HOMO-LUMO gap, one of the important measures indicating the electrical breakdown voltage strength, were examined for oligomers with various chain lengths and carboxyl mixture ratios. When a polypropylene oligomer is grafted with carboxyl groups (cf. acrylic acid), our calculations show that the increase of the polarizability volume α' of the oligomer is proportional to the increase of its mass m, while the ratio α'/m decreases from the value of a pure polymer when increasing the mixture ratio. The decreasing ratio of α'/m under carboxyl grafting indicates that the material permittivity might also decrease if the mass density of the material remains constant. Furthermore, our calculations show that the HOMO-LUMO gap energy decreases by only about 15% in grafting, but this decrease seems to be independent on the mixture ratio of carboxyl. This indicates that by doping polymers with additives better dielectric properties can be tailored. Finally, using the first-principles molecular DFT results for polarizability volume per mass in connection with the classical Clausius-Mossotti relation, we have estimated static permittivity for acrylic acid grafted polypropylene, assuming the structural density keeping constant under grafting. The computed permittivity values are in a qualitative agreement with the recent experiments, showing increasing tendency of the permittivity as a function of the grafting composition. In order to validate our molecular DFT based approach, we have also carried out extensive three-dimensional bulk periodic first-principles total-energy calculations in the frameworks of the density functional theory and density functional perturbation theory (DFPT) for crystalline acrylic acid grafted polypropylene. Interestingly, the computed electronic and dielectric properties behave very similarly between the simplified molecular DFT modeling and the more realistic 3D bulk periodic DFPT method. In particular, the static permittivity values [ε(r)(0)] from the molecular DFT-Clausius-Mossotti modeling are in excellent agreement with the high-frequency dielectric constant values (ε(∞)) from the DFPT method. This obviously implies that the chain-to-chain interaction to dielectric and electronic properties of acrylic acid polypropylene, to a first approximation, can be neglected, therefore justifying the usage of molecular DFT modeling in our calculations.     

Dipole moments of poly(ethylene oxide) and poly(hexamethylene oxide) chains

The mean-square dipole moments of poly(ethylene oxide) and poly(hexamethylene oxide) chains have been determined from dielectric constant measurements on dilute solutions of the polymers in benzene. The values obtained are in good agreement with those predccted using the rotational isomeric state models for these chains. In addition, the unperturbed dimensions of poly(hexamethylene oxide) have been calculated as a function of molecular weight, using the isomeric state theory.     

Thermodynamical study of the thermoelectric effect for magnesium silicide

The thermoelectric effect of magnesium silicide is studied by using a thermodynamical method in the presence of an electric field. The thermoelectric potential is evaluated from the partial derivative of free energy with respect to charge in which the free energy is calculated at the B3LYP/6-31G(d,p) level of density functional theory. This free energy is also utilized to determine the average dipole moment from which the polarizability, alpha; molar polarization, Psi; and dielectric constant can be computed. The present calculation for the dielectric constant (approximately 24-20) is in very good agreement with the experimental value (20). This accurate dielectric constant can be used to derive the relation of the thermoelectric potential with respect to temperature, from which the thermoelectric power or the Seebeck coefficients are calculated. The present result shows good agreement with experiment measurement for the Seebeck coefficients. In comparison, that calculation from the energy band structure theory is far off from the experimental values.     

Mode-coupling study on the dynamics of hydrophobic hydration II: Aqueous solutions of benzene and rare gases

The dynamic properties of both the solute and solvent of the aqueous solution of benzene, xenon and neon are calculated by the mode-coupling theory for molecular liquids based on the interaction-site model. The B-coefficients of the reorientational relaxation and the translational diffusion of the solvent are evaluated from their dependence on the concentration of the solute, and the reorientational relaxation time of water within the hydration shell is estimated based on the two-state model. The reorientational relaxation times of water in the bulk and within the hydration shell, that of solute, and the translational diffusion coefficients of solute and solvent, are calculated at 0-30 degrees C. The temperature dependence of these dynamic properties is in qualitative agreement with that of NMR experiment reported by Nakahara et al. (M. Nakahara, C. Wakai, Y. Yoshimoto and N. Matubayasi, J. Phys. Chem., 1996, 100, 1345-1349, ref. 36), although the agreement of the absolute values is not so good. The B-coefficients of the reorientational relaxation times for benzene, xenon and neon solution are correlated with the hydration number and the partial molar volume of the solute. The proportionality with the latter is better than that with the former. These results support the mechanism that the retardation of the mobility of water is caused by the cavity formation of the solute, as previously suggested by us (T. Yamaguchi, T. Matsuoka and S. Koda, J. Chem. Phys., 2004, 120, 7590-7601, ref. 34), rather than the conventional one that the rigid hydration structure formed around the hydrophobic solute reduces the mobility of water.     

Molecular simulation of the high-pressure phase equilibria of binary atomic fluid mixtures using the exponential-6 intermolecular potential

Molecular simulation results using the exponential-6 intermolecular potential are reported for the phase behaviour of the atomic binary mixtures of neon+xenon, helium+neon, helium+argon and helium+xenon. These binary mixtures exhibit both vapour–liquid and liquid–liquid phase equilibria up to very high pressures. Comparison with experiment indicates good overall agreement. The results indicate that the exponential-6 intermolecular potential is a useful generic potential for molecular simulation.     

Electrostatic calculation of linear and non-linear optical properties of ice Ih,II, IX and VIII

Macroscopic first- and third-order susceptibilities of ice Ih, ice II, ice IX and ice VIII are calculated using static and frequency-dependent electronic and static vibrational molecular (hyper)polarizabilities at the MP2 level. The molecular properties are in good agreement with experiment and with high-level ab initio calculations. Intermolecular electrostatic and polarization effects due to induced dipoles are taken into account using a rigorous local-field theory. The electric field due to permanent dipoles is used to calculate effective in-crystal (hyper)polarizabilities. The polarizability depends only weakly on the permanent field, but the dipole moment and the hyperpolarizabilities are strongly affected. The calculated linear susceptibility is in good agreement with available experimental data for ice Ih, and the third-order susceptibility for a third harmonic generation experiment is in reasonable agreement with experimental values for liquid water. The molecular vibrational contributions have a small effect on the susceptibilities. The electric properties of a water tetramer are calculated and used to estimate the effect of non-dipolar interactions on the susceptibilities of ice Ih, which are found to be small.     

Analysis of anisotropy in dielectric absorption due to defects in the polyoxymethylene crystal

The dielectric absorption due to terminal polar groups localized in defect regions of polyoxymethylene crystals shows an anisotropy relative to an applied field. An attempt to interpret this anisotropy has been made by applying the barrier theory of Hoffman. The potential energy map for the rotation of the terminal polar group calculated from conformational analysis gives two minima. The angle between the dipole moment of the terminal OH and the fiber axis calculated in terms of the two conformations corresponding to the potential energy minima is in good agreement with results from infrared dichroism. This agreement is considered as evidence that the conformational analysis is reliable for estimating the potential energy minima required in the calculation according to the barrier theory. The barrier height obtained by the conformational analysis and the anisotropy calculated from the components of the dipole moments for the terminal polar group in terms of the barrier theory are in qualitative agreement with the experimental data.     

Thermostimulated depolarization currents in thermorheologically simple materials

A theory is presented which makes possible the calculation of the dielectric parameters for a distributed dipole relaxation from thermostimulated depolarization current (TDC) data. The theory is applicable to dielectrics which obey the time–temperature superposition principle, i.e., for thermorheologically simple materials. The shift factor, the activation energy, the dielectric relaxation strength, the density of the isothermal displacement current, and the distribution function of relaxation times of the β relaxation in poly(methyl methacrylate) are calculated. The TDC investigations were carried out over the temperature range of ?136 to 90°C. The values for the activation energy U = 26.4 kcal/mole and the dielectric relaxation strength Δ_?β = 2 are in good agreement with values obtained from dynamic measurements. A criterion for checking the validity of the time–temperature superposition principle by TDC is suggested.     

Calculation of the bulk modulus of simple and complex crystals with the chemical bond method

The relation between the lattice energies and the bulk moduli on binary inorganic crystals was studied, and the concept of lattice energy density is introduced. We find that the lattice energy densities are in good linear relation with the bulk moduli in the same type of crystals, the slopes of fitting lines for various types of crystals are related to the valence and coordination number of cations of crystals, and the empirical expression of calculated slope is obtained. From crystal structure, the calculated results are in very good agreement with the experimental values. At the same time, by means of the dielectric theory of the chemical bond and the calculating method of the lattice energy of complex crystals, the estimative method of the bulk modulus of complex crystals was established reasonably, and the calculated results are in very good agreement with the experimental values.     

High-capacitance organic nanodielectrics: effective medium models of their response

Molecular and macromolecular high-permittivity organic gate dielectric materials have been the focus of recent experimental research as a consequence of their promising properties for organic and inorganic field effect transistor (FET) applications. Two types of molecular thin films, self-assembled nanodielectrics (SANDs) and cross-linked polymer blends (CPBs), have been shown experimentally to afford high capacitances and low FET operating voltages. In an effort to design optimized nanostructures having even larger capacitances, lower leakage current densities, and further reduced FET operating voltages, we discuss approaches for computing the effective permittivities of each nanodielectric motif and investigate how molecular arrangements impact overall device capacitance. The calculated frequency-dependent capacitances, derived from Maxwell-Wagner theory applied to the Maxwell-Garnett effective medium approximation, agree fairly well with the experimental values for the two types of nanodielectrics. Predictions of larger capacitance SANDs are made with the two-capacitors-in-series equivalent circuit, where the layered, self-assembled structure is viewed as two different capacitors. The Maxwell-Garnett and Polder-Van Santen effective medium approximations are used to predict the dielectric response of higher permittivity polymer cross-linked blends. In calculations showing good agreement between theory and experiment, and with all parameters being equal, it is found that greater capacitances should be achievable with cross-linked composites than with layered composites.     

Structure and electric properties of Sn(N) clusters (N = 6-20) from combined electric deflection experiments and quantum theoretical studies

Electric deflection experiments have been performed on neutral Sn(N) clusters (N = 6-20) at different nozzle temperatures in combination with a systematic search for the global minimum structures and the calculation of the dielectric properties based on density functional theory. For smaller tin clusters (N = 6-11), a good agreement between theory and experiment is found. Taking theoretically predicted moments of inertia and the body fixed dipole moment into account permits a quantitative simulation of the deflected molecular beam profiles. For larger Sn(N) clusters (N = 12-20), distinct differences between theory and experiment are observed; i.e., the predicted dipole moments from the quantum chemical calculations are significantly larger than the experimental values. The investigation of the electric susceptibilities at different nozzle temperatures indicates that this is due to the dynamical nature of the tin clusters, which increases with cluster size. As a result, even at the smallest nozzle temperature of 40 K, the dipole moments of Sn(12-20) are partially quenched. This clearly demonstrates the limits of current electric deflection experiments for structural determination and demonstrates the need for stronger cooling of the clusters in future experiments.     

Theoretical Studies on Dielectric Breakdown Strength Increasing Mechanism of SF6 and Its Potential Alternative Gases

A theoretical investigation on the dielectric insulation mechanism of sulfur hexafluoride(SF₆) and its potential alternative gases at the atomic and molecular levels was made. The electronic structures of the molecules of them were calculated at the B3LYP/6-311+G(d,p) level. The HOMO-LUMO energy gaps, ionization potentials, electron affinities, and dipole moments of the studied molecules at the ground state were obtained. The 11 isomerization reactions, with the harmonic vibration frequencies of the equilibrium geometries and the minimum energy path by the intrinsic reaction coordinate theory, were also obtained at the same level. The results show that the insulation gas, with the larger HOMO-LUMO energy gap, the higher ionization potential and the stronger electron affinity, can increase the dielectric breakdown strength efficiently, which is in good agreement with the available experimental finding. We suggested that the molecule with isomerization reaction occurring can dissipate the energy of hot electrons availably, which is favorable to the dielectric breakdown strength increasing for the SF₆ potential alternative gas.     

Relation between dielectric behavior and structure in some solid polypeptides

An attempt to interpret the prominent dielectric relaxations of poly(γ-benzyl L-glutamate) and poly(γ-methyl L-glutamate) in terms of their structures was made by applying the barrier theory of Hoffman. Potential energy maps for the rotation of the polar side group, which are required in this application, were calculated by taking account of molecular environment of the polar side group. The dichroic ratios from infrared measurements were satisfactorily calculated, based on the maps. This provides evidence that the maps are reliable. In applying the barrier theory, it was modified by the assumption that the conformations of the side groups are distributed according to the Boltzmann law. On the basis of the maps, the magnitude of the dielectric absorption and the mean relaxation time were calculated in terms of the modified barrier theory; these were in fairly good agreement with the experimental data.     

Structure-electrical properties relationships of polymer composites filled with Fe-powder

Structure-properties relationships of composite materials, consisting of a polymer matrix and metal inclusions, is very important for designing new materials with desirable properties. In the present work the electrical and dielectric properties of several composites, consisting of a polymer matrix and iron (Fe) particles as filler, were investigated. Broadband dielectric relaxation spectroscopy measurements were carried out. The electrical behaviour of the composites is described in terms of the percolation theory. Percolation threshold values were calculated and the values of the dielectric permittivity critical exponent were found in good agreement with the theoretical ones. The influence of using different polymer matrices on the physical properties of the composites was also of particular interest. The results were related to the microstructure of the composites and a schematic model was proposed.     

Investigation of thermal expansion and compressibility of rare-earth orthovanadates using a dielectric chemical bond method

The chemical bond properties, lattice energies, linear expansion coefficients, and mechanical properties of ReVO 4 (Re = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y) are investigated systematically by the dielectric chemical bond theory. The calculated results show that the covalencies of Re-O bonds are increasing slightly from La to Lu and that the covalencies of V-O bonds in crystals are decreasing slightly from La to Lu. The linear expansion coefficients decrease progressively from LaVO 4 to LuVO 4; on the contrary, the bulk moduli increase progressively. Our calculated results are in good agreement with some experimental values for linear expansion coefficients and bulk moduli.     

Molecular interaction studies in hydrogen bonded polar binary mixtures

The molecular interactions between the polar systems of propan-1-ol (1PN) with alkyl benzoates (methyl benzoate and ethyl benzoate) for various mole fractions at different temperatures are studied by determining the dielectric permittivity in radio, microwave and optic frequency regions, respectively. Dipole moment, excess dipole moment, excess Helmholtz free energy, excess permittivity, relaxation time, excess inverse relaxation time and excess thermodynamical values are calculated using experimental data. Hamiltonian quantum mechanical calculations are performed on both pure and equimolar binary systems of 1PN with alkyl benzoates for the measurement of dipole moment from the ab initio Hartree–Fock and density functional theory (B3LYP) methods with 6-31?+?G* and 6-311?+?G** basis sets using Spartan 08 modelling software and these theoretical values are in good agreement with the experimental values.     

Theoretical study of volume changes accompanying xenon-lysozyme binding: implications for the molecular mechanism of pressure reversal of anesthesia

The change in partial molar volume (PMV) accompanying the xenon-lysozyme binding was investigated for elucidating the molecular mechanism of the pressure reversal of general anesthesia, using the three-dimensional reference interaction site model theory of molecular solvation. An increase of the PMV from xenon binding to the substrate binding site of lysozyme was found, and the binding is suppressed by pressure, while the internal site binding did not change the PMV. The PMV change was analyzed by decomposing it into several contributions from geometry and hydration. We also analyzed the hydration change due to the binding. From the results, we draw a molecular picture of the PMV change accompanying xenon-lysozyme binding, which gives a possible mechanism of pressure reversal of anesthesia.     

H-bonded molecular interaction study on binary mixtures of mono alkyl ethers of ethylene glycol with different polar solvents by concentration dependent dielectric analysis

Static dielectric constant measurements on binary mixtures of the homologous series of mono alkyl ethers of ethylene glycol with six different characteristic polar solvents i.e. ethyl alcohol, ethylene glycol, glycerol, water, dimethyl formamide and dimethyl sulphoxide over the entire concentration range were carried out using precision LCR meter and a four terminal liquid dielectric test fixture at 1?MHz and 25°C. The concentration dependent excess dielectric constant and Kirkwood correlation factor were determined for the confirmation of solvent–cosolvent heterogeneous molecular interactions. The values of stoichiometric ratio corresponding to maximum interactions between the mixtures constituents were also estimated from the concentration dependent values of ?^E. It is observed that the behaviour of heterogeneous interactions significantly varies with the increase in molecular size of the homologous series and also with the change in the mixture constituents. Comparative dielectric parameters values of the studied binary mixtures were applied to recognize the dipolar orientation due to heterogeneous interactions of mono alkyl ethers with hydroxyl group(s) containing solvents and non-hydroxyl group containing solvents.