Influence of cylindrical submicrometer confinement on the static and dynamic properties in nonyloxycyanobiphenyl (9OCB)

Broadband dielectric spectroscopy (10(2)-1.9 x 10(9) Hz) and specific heat measurements have been performed on nonyloxycyanobiphenyl (9OCB) in the isotropic (I), nematic (N), and smectic A (SmA) phases confined to 200 nm diameter parallel cylindrical pores of Anopore membranes. Untreated and HTBA-treated membranes have been found to obtain axial and radial confinements, respectively. However, structural or configurational transitions in untreated membranes have been reported to exist in the SmA-mesophase of 9OCB. Both confinements clearly affect the N-I and SmA-N phase transitions. In the axial confinement, the analysis of the specific heat and static dielectric permittivity data leads to a second order SmA-N phase transition, which is known to be weakly first order for bulk 9OCB. Dynamic dielectric measurements have accounted for the different molecular motions in both confinements. On both mesophases, either N or SmA, the relaxation processes in axial configuration are faster than in the bulk. However, in radial confinement, they are either equal or slower than in the bulk. Additionally, there are no differences in the energy barrier hindering the molecular motions between the axial and radial confinements and even in relation to bulk. Likewise, dielectric results suggest that the extension inside the pores of the surface pinned molecular layer (proved to be temperature-dependent) persists at high enough temperature as a residual-thin layer adjacent to the pore wall.     

Conductivity and dielectric properties of the monomer and polymer of the bis(p-toluene sulfonate) of 2,4-hexadiyne-1,6-diol

The electrical conductivity of single crystals of the bis(p-toluene sulfonate) of 2,4-hexadiyne-1,6-diol (TS) and its polymer (PTS) was measured in a temperature range 77–373 K (to 353 K for TS), dielectric properties were measured from 123 to 283 K. Anomalies of the conductivity and the real part of the complex dielectric permittivity of TS and PTS along the molecular stacking direction at the temperature corresponding to the low-temperature phase transitions were observed. The dependence of phase transitions on the polymerization conversion of the monomer TS was examined. The phase diagram of the TS-PTS mixed system was constructed.     

Dielectric,viscous and elastic properties of nematogenic 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane

This paper presents the results of measurements of the static and dynamic electric permittivity, the shear viscosity of a freely flowing sample, and the splay and bend elastic constants for 1-(4-trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane (C3H7-CyHx-CH2CH2-Ph-C=N) (3CCPE). The static permittivity of the isotropic phase shows pre-nematic critical behaviour which is discussed in the frame of the fluid-like model of Mukherjee. From the temperature dependence of the dielectric relaxation time (corresponding to the molecular rotation around the short axis) and the shear viscosity, the strength of the nematic potential and the effective length of the 3CCPE molecule (in the isotropic phase), were estimated. The splay and bend elastic constants were determined from the voltage dependence of the capacitance of a planar nematic cell.     

Dielectric, viscous and elastic properties of nematogenic 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane

This paper presents the results of measurements of the static and dynamic electric permittivity, the shear viscosity of a freely flowing sample, and the splay and bend elastic constants for 1-(4- trans -propylcyclohexyl)-2-(4-cyanophenyl)ethane (C 3 H 7 -CyHx-CH 2 CH 2 -Ph-C=N) (3CCPE). The static permittivity of the isotropic phase shows pre-nematic critical behaviour which is discussed in the frame of the fluid-like model of Mukherjee. From the temperature dependence of the dielectric relaxation time (corresponding to the molecular rotation around the short axis) and the shear viscosity, the strength of the nematic potential and the effective length of the 3CCPE molecule (in the isotropic phase), were estimated. The splay and bend elastic constants were determined from the voltage dependence of the capacitance of a planar nematic cell.     

Thermodynamic and dielectric studies concerning the influence of cylindrical submicrometer confinement on heptyloxycyanobiphenyl

Measurements of the specific heat and the static dielectric permittivity of heptyloxycyanobiphenyl (7OCB) confined to the 0.2 microm diameter parallel cylindrical pores of Anopore membranes in the isotropic phase and nematic mesophase, are presented. A comparison between the bulk and the confined 7OCB in treated and untreated pore wall surfaces using a chemical surfactant (HTBA) is performed. Both the treated and untreated membrane confinements seem to affect the nematic-to-isotropic phase transition by a downshift in transition temperature and some rounding at the specific-heat maximum, in a way similar to that which was earlier published for other liquid crystals confined in the same geometry. The static dielectric measurements clearly point out that untreated membrane confinement is axial, with the nematic director aligned parallel to the pore axis being homeotropic bulklike, i.e., with the nematic director aligned perpendicular to the electrode cell surfaces. After chemical surfactant treatment, the nematic director is constrained in a radial alignment being perpendicular to the pore walls. The dielectric measurements are revealed to be specially sensible to analyze the surface-induced nematic order due to the pore wall. The tricritical nature of the nematic-to-isotropic phase transition in bulk 7OCB as well as in treated and untreated Anopore confined geometries is discussed through both the specific heat and the static dielectric data.     

Pretransitional behavior in the isotropic phase of a nematic liquid crystal with the transverse permanent dipole moment

The results presented give the evidence for the quasicritical, pretransitional behavior of dielectric properties in the isotropic phase of a rodlike nematic liquid crystal with the transverse permanent dipole moment. Studies were conducted in 2-cyano-4-pentylbiphenyl 4-(trans-4-pentylcyclohexyl) benzoate, focusing on the static-and ionic-dominated low-frequency (LF) regions. For the static dielectric permittivity [epsilon(')(T)] the application of the derivative analysis revealed the pretransitional anomaly associated with the specific heat exponent alpha approximately 0.5. For the LF domain the contribution to epsilon(')(T) from residual ionic impurities follows a linear temperature dependence on approaching the isotropic-nematic (I-N) transition. This dependence and pretransitional anomalies of electric conductivity and dielectric modulus can be associated with the influence of prenematic fluctuations. "Linear" dielectric studies were supported by the static nonlinear dielectric effect measurements, which delivered reliable estimations of the temperature of the hypothetical continuous phase transition T(*) and the discontinuity of the I-N transition DeltaT approximately 1.7 K.     

Prenematic behavior of the electric-field-induced increment of the basic thermodynamic quantities of isotropic mesogenic liquids of different polarity

Temperature dependences of the static dielectric permittivity and its derivative, obtained for isotropic mesogenic liquids composed of the molecules of different polarity in relation to the basic thermodynamic quantities (internal energy, entropy, and Helmholtz free energy), are analyzed. A role of the molecular polarity in the dielectric behavior of the liquids in the vicinity of the isotropic to nematic phase transition is discussed.     

Pulsed‐laser–deposited and plasma‐polymerized polytetrafluoroethylene (PTFE)‐like thin films: A comparative study on PTFE‐specific properties

Glass‐like and structural first‐order phase transitions are investigated in polytetrafluoroethylene (PTFE) foils and PTFE‐like films prepared by pulsed‐laser deposition (PLD) and plasma polymerization (PP). A structural comparison of the investigated polymers is performed by infrared spectroscopy and dielectric dilatometry. It is shown that dielectric dilatometry (the measurement of the susceptance vs. temperature) provides a simple and elegant means for detecting volumetric transitions in thin nonpolar polymer films. In conventional PTFE foils, the known glass‐like and structural first‐order phase transitions are identified. The structure of pulsed‐laser deposited PTFE strongly depends on the target material, ranging from highly crystalline films showing only structural phase transitions to films strongly deviating from PTFE foils, with structural characteristics comparable to plasma‐polymerized fluorocarbons. The dielectric loss of the highly crystalline PLD films compares favorably with conventional PTFE foils, making the films attractive for new applications in miniature electret devices. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2115–2125, 1999     

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.     

High-temperature behavior and polymorphism in novel members of the perovskite family Pb2LnSbO6 (Ln=Ho, Er, Yb, Lu)

The synthesis, crystal structure, and dielectric properties of four novel members of the family of double perovskites Pb(2)LnSbO(6) are described. The room-temperature crystal structures were refined from neutron powder diffraction (NPD) data in the monoclinic C2/c (No. 15) space group. They contain a completely ordered array of alternating LnO(6) and SbO(6) octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with a a(-)b(-)b(-) tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy highly asymmetric voids with 8-fold coordination due to the stereoactivity of the Pb(2+) electron lone-pair. Several trends are observed for the entire family of compounds upon heating. The Ln = Lu, Yb, and Er oxides display three successive phase transitions in a narrow temperature range, as shown by differential scanning calorimetry (DSC) data, while the Ln = Ho shows only two transitions. Different crystal structure evolutions have been found from temperature-dependent NPD and DSC, following the space-group sequence C2/c → P2(1)/n → R ?3 → Fm ?3m for Ln = Lu and Yb, the sequence C2/c → unknown → P2(1)/n → Fm ?3m for Ln = Er, and C2/c → P2(1)/n → Fm ?3m for Ln = Ho. The Ln/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature from the Curie-Weiss fit of the reciprocal permittivity.     

Dynamics of 4,4'-di-n-heptylazoxybenzene (HAB) studied using dielectric and 2H NMR relaxation measurements

Results of studies of 4,4'-di-n-heptylazoxybenzene (HAB) in the isotropic, nematic and smectic A phases are presented. Two experimental methods were employed: broad band dielectric spectroscopy and nuclear magnetic resonance spectroscopy. The complex dielectric permittivity, ε*(ω)=ε'(ω)-iε'(ω), was measured in the frequency range 1 kHz-4 GHz. This allowed two main relaxation processes to be separated in all the phases studied: the low frequency (l. f.) process connected with molecular reorientations around the short axes, and the high frequency (h. f.) process connected with the rotations around the long axes. The corresponding relaxation times and activation enthalpies were obtained. The l. f. relaxation time changes step-wise at the phase transitions, whereas the h. f. relaxation time passes smoothly through all the phases. The measurement of ²H spin-lattice relaxation times was carried out throughout the mesophase range at 61.38 MHz. These data were analysed together with the relaxation times measured at 10.00 and 46.04 MHz, available from previous studies. Using suitable theoretical models the principal components of the diffusional tensor, D_‖ and D_⊥, as well as the diffusion coefficients D_R relative to the internal rotation of the phenyl rings, were determined. The results of both studies are compared and discussed.     

Odd-Even Effects in the Dielectric Properties of Nematics 4-(Trans-4'-N-Alkylcyclohexyl)Isothiocyanatobenzenes

The anisotropy of the static electric permittivity and conductivity for the homologous series of nematics: 4-(trans-4'-n-alkylcyclohexyl)isothiocyanato-benzenes (n-CHBT) for n = 3-10 have been measured. It is shown that the odd-even effect in the dielectric anisotropy Ac is related to the perpendicular component of electric permittivity. The odd-even effect is found in n-CHBT for both the anisotropy in the conductivity and the activation energy for ionic movement.     

Dielectric relaxational behavior of poly(diitaconate)s containing cyclic rings in the side chain

Dielectric relaxations of several poly(diitaconate)s with cyclobutyl, cycloheptyl, and cyclooctyl groups in the side chain were investigated. The study was performed by determining the dielectric permittivity and loss, depending on the frequency and temperature. Dynamic dielectric measurements indicated several relaxations according to the chemical structure of the polymers. The dielectric behavior of these polymers was compared with those of poly(dicyclohexyl itaconate), previously reported. The α relaxations were analyzed with the Havriliak–Negami equation. Significant differences in the subglass relaxations were observed. A tentative explanation of the molecular origin of each absorption was proposed in terms of the number of carbon atoms of the ring and their conformational versatility. Strong conductive processes were observed in these polymers at low frequencies and high temperatures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1059–1069, 2003     

Force Field Benchmark of Organic Liquids: Density, Enthalpy of Vaporization, Heat Capacities, Surface Tension, Isothermal Compressibility, Volumetric Expansion Coefficient, and Dielectric Constant

The chemical composition of small organic molecules is often very similar to amino acid side chains or the bases in nucleic acids, and hence there is no a priori reason why a molecular mechanics force field could not describe both organic liquids and biomolecules with a single parameter set. Here, we devise a benchmark for force fields in order to test the ability of existing force fields to reproduce some key properties of organic liquids, namely, the density, enthalpy of vaporization, the surface tension, the heat capacity at constant volume and pressure, the isothermal compressibility, the volumetric expansion coefficient, and the static dielectric constant. Well over 1200 experimental measurements were used for comparison to the simulations of 146 organic liquids. Novel polynomial interpolations of the dielectric constant (32 molecules), heat capacity at constant pressure (three molecules), and the isothermal compressibility (53 molecules) as a function of the temperature have been made, based on experimental data, in order to be able to compare simulation results to them. To compute the heat capacities, we applied the two phase thermodynamics method (Lin et al. J. Chem. Phys.2003, 119, 11792), which allows one to compute thermodynamic properties on the basis of the density of states as derived from the velocity autocorrelation function. The method is implemented in a new utility within the GROMACS molecular simulation package, named g_dos, and a detailed exposé of the underlying equations is presented. The purpose of this work is to establish the state of the art of two popular force fields, OPLS/AA (all-atom optimized potential for liquid simulation) and GAFF (generalized Amber force field), to find common bottlenecks, i.e., particularly difficult molecules, and to serve as a reference point for future force field development. To make for a fair playing field, all molecules were evaluated with the same parameter settings, such as thermostats and barostats, treatment of electrostatic interactions, and system size (1000 molecules). The densities and enthalpy of vaporization from an independent data set based on simulations using the CHARMM General Force Field (CGenFF) presented by Vanommeslaeghe et al. (J. Comput. Chem.2010, 31, 671) are included for comparison. We find that, overall, the OPLS/AA force field performs somewhat better than GAFF, but there are significant issues with reproduction of the surface tension and dielectric constants for both force fields.     

Investigations on Binary Mixtures of Propan-2-ol with Methyl Benzoate and Ethyl Benzoate

The molecular interactions between the polar systems propan-2-ol with alkyl benzoates (methyl benzoate and ethyl benzoate), for various mole fractions and different temperatures, were studied by determining the dielectric permittivity using a LF impedance analyzer and Abbe’s refractometer in the static and optical frequency regions, respectively. The effective Kirkwood correlation factor, corrective Kirkwood correlation factor, dipole moment, excess dipole moment, and excess Helmholtz free energy were calculated using the experimental data. Hamiltonian quantum mechanical calculations (ab initio and semiempirical) were performed using PC Spartan and Argus lab Modeling software for both pure and equimolar binary systems of propan-2-ol with alkyl benzoates.     

Thermally-induced dielectric relaxation spectra in three aldohexose monosaccharides

Three aldohexose monosaccharides, d-glucose, d-mannose, and d-galactose, were examined by scanning temperature dielectric analysis (DEA) from ambient temperatures through their melts. Phase transitions, including glass transition (T _g) and melting temperature (T _m), were evaluated by differential scanning calorimetry (DSC). The monosaccharides were found to exhibit thermally-induced dielectric loss spectra in their amorphous-solid phase before melting. Activation energies for electrical charging of each of the monosaccharides were calculated from an Arrhenius plot of the tan delta (e″/e′, dielectric loss factor/relative permittivity) peak frequency versus reciprocal temperature in Kelvin. The DEA profiles were also correlated with the DSC phase diagrams, showing the changes in electrical behavior associated with solid–solid and solid–liquid transitions.     

钛酸铅/聚[偏氟乙烯(70)-三氟乙烯(30)]复合物的非线性介电性质

用自建的非线性介电测试装置研究了陶瓷相被极化 (B1 )以及陶瓷相和共聚物相都被极化 (B2 )的钛酸铅 /聚 [偏氟乙烯 ( 70 ) 三氟乙烯 ( 3 0 ) ](PT/P[VDF( 70 ) TrFE( 3 0 ) ]) 0 3型铁电复合物厚片在不同测试场强下的非线性介电系数 .对于PT含量 >0 3的复合物 ,介电系数ε1随场强的升高明显增大 .在 <0 1时 ,可用Maxwell Garnett方程拟合实验结果 .Bruggeman方程适合于低电场 ( <1MV/m)下复合物介电系数的预测 .通过Lz 的变化 ,用Yamada模型可拟合不同场强下复合物介电系数随的变化关系 .二阶非线性介电系数ε2 随的升高而增大 ,B2的ε2 大于B1 ,说明B2具有较高的剩余极化 .三阶非线性介电系数ε3 随场强的升高而减小 ,且高值的复合物具有较大的ε3 .两相都被极化试样的ε3 小于仅陶瓷相被极化的试样 .     

Dielectric relaxation study of N -methyl formamide with glycols using time domain reflectometry technique

The dielectric relaxation study that is static dielectric permittivity (∈₀) and relaxation time (τ) of amide of N-methyl formamide (NMF) with increasing volume percent propylene glycol (PLG) and BLG has been carried out at different temperatures. The time domain reflectometry (TDR) technique has been used to measure reflection coefficient in frequency range of 10 MHz to 20 GHz. The dielectric parameters have been obtained by fitting experimental data with the Havriliak–Negami equation. The experimental observation shows that the static dielectric permittivity and relaxation time decreases with increasing temperature. The experimental observation also shows that the static dielectric permittivity decreases and relaxation time increases with increasing percentage volume of Propylene glycol (PLG) and Butylene glycol (BLG) in NMF. The nature of (?₀) and (τ) is same for the temperature ranges (20, 30, and 40°C). The thermodynamic parameters enthalpy (ΔH) and entropy (ΔS) of the binary mixture are also reported in this work.     

Dielectric permittivity in a dense layer of the hydration shell of an ion in a strong-electrolyte solution

Based on the experimental data on the dielectric dispersion and the static dielectric permittivity of solutions of strong electrolytes, the effective value of the latter in the dense layer of the hydration shell of an ion has been calculated. The calculations have been carried out in terms of the three-layer model of the hydration complex. The calculations have shown that for the solutions of strong electrolytes the value of the static dielectric permittivity (dielectric constant) in the dense layer of the hydration shell of an ion proves to be close to 2 and is almost independent of the concentration and temperature.     

Dielectric studies of liquid crystals under high pressure III. Low frequency relaxation process in 4-(trans-4'-n-hexylcyclohexyl)isothiocyanato benzene

Dielectric studies of 4-(trans-4'-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) were performed in the pressure range 0·1-150 MPa, the frequency range 1 kHz-13 MHz and the temperature range 295-325 K. The temperature and pressure dependencies of the static permittivity ε_0∥ and of the relaxation time τ_∥ are analysed and compared with the analogous data obtained recently for 4-n-pentyl-4'-cyanobiphenyl (5CB) (Parts I and II of this series). Marked differences in the dielectric properties of the nematic phases of the two substances are observed. They are interpreted as a result of varying degrees of molecular association in particular compounds. It is concluded that in the nematic phase of 6CHBT dipole-dipole correlations do not exist or are very weak, whereas for 5CB they are easily broken by a relatively low pressure.