Dielectric Relaxation Modes Research Articles

Dielectric spectroscopy of a binary mixture of liquid crystals showing wide temperature range twisted grain boundary phase with re-entrant cholesteric phase

There are only few materials, which have shown long temperature range twisted grain boundary (TGB) phases. One such material is the chiral binary mixture of 7OCB and 5* CBB (mole ratio 0.8 and 0.2) which shows unique phase sequence of cholesteric ( N*) , a wide temperature range TGBA (approximately 31 degrees C) and reentrant cholesteric (N*re) phases. In the present work we are reporting the dielectric spectroscopy of the above mixture with a chiral analog of earlier reported nematic (N) , smectic- A (SmA) , and reentrant nematic (Nre) phase sequences [Phys. Rev. A 46, 7733 (1992)] for different conditions of molecular orientations. Two modes of dielectric relaxations have been detected in a homeotropically aligned sample with unusually low relaxation frequencies for one of them. Planar oriented molecules in the TGBA phase show a soft mode of relaxation and support the recently proposed theory for the soft mode relaxation of the TGBA phase [Phys. Rev. E 65, 11701 (2001)]. By applying the dc electric field on planar oriented molecules in the TGBA phase, it has been possible to obtain a helix free homeotropically aligned TGBA phase.

Two new siloxanic proton conducting membranes: Part II. Proton conductivity mechanism and NMR study

The synthesis and structural characterization of two types of membranes with formulas {Si(CH 3) 3O[Si(CH 3)HO] 21.26-[Si(CH 3)((CH 2) 3SO 3H)O] 1.8-[Si(CH 3)((CH 2) 3Si(CH 3) 2O-)-O] 14-Si(CH 3) 3} n ( A) and {Si(CH 3) 3O[Si(CH 3)HO] 21.26-[Si(CH 3)((CH 2) 3SO 3H)O] 1.8-[Si(CH 3)((CH 2) 3(Si(CH 3) 2O- ) w )-O ] v [Si(CH 3)((CH 2) 3Si(CH 3) 2O-)-O ] 14 − v Si(CH 3) 3} n ( B), ( w = 20.31 ), were previously proposed. The ac electrical response of A and B was fully characterized in the 40 Hz–2 MHz frequency region by studying the impedance spectra in the medium and low frequency regions by equivalent circuits and complex dielectric spectra at high frequency in terms of dielectric relaxation modes. Results demonstrated that A and B conduct ionically by means of a proton exchange event which occurs via a vehicular mechanism between neighboring water clusters formed by water molecules aggregated around each sulfonic acid group of the siloxane side chains. The proton conductivities at 115 °C of ca. 1.9 × 10 −3 and 1.8 × 10 −4 S cm −1 of fully hydrated membranes A and B, respectively, classify these silicone networks as good proton conductors. Membrane B was chosen for a closer investigation using NMR spectroscopy. Solid state 29Si MAS NMR experiments gave further insight about the three-dimensional structure. Proton diffusion measurements provided some encouraging results about proton dynamics of this membrane signaling the great potential of siloxanic based proton conductors.

Study of different modes of dielectric relaxation in the ferro- and antiferroelectric phases of a newly-synthesized highly tilted chiral liquid crystal material 6H6B

Frequency- and temperature-dependent dielectric spectroscopy of a newly-synthesised AFLC compound in its planar orientation has been carried out in order to determine the different dielectric parameters and analyse the various modes of dielectric relaxation of the different phases. The compound is (S)-(+)-4′-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-(6-heptanoyloxyhex-1-oxy)benzoate (acronym 6H6B). It possesses paraelectric SmA* and ferroelectric SmC* phases, in addition to a wide-temperature-range antiferroelectric phase (∼23°C). The SmA* phase shows a soft mode of relaxation related to the amplitudons. Its relaxation frequencies are highly temperature-dependent and decrease with decrease in temperature. The dielectric strength is small (∼4–5), but increases with decrease in temperature, and follows the Curie law. The ferroelectric SmC* phase shows a Goldstone mode of relaxation related to the phasons. Its relaxation frequency is almost invariant (∼1–2 kHz) with temperature. The dielectric strength of the Goldstone mode is very large (∼130) and slowly increases with decrease in temperature; it shows a maximum just above the SmC*–SmCA* transition temperature. Two modes of dielectric relaxation have been clearly observed in the antiferroelectric phase. The lower-frequency mode observed in the kHz range has been assigned as the PL mode, whereas the higher-frequency mode observed at about 100 kHz has been assigned as the PH mode. Most of the observed relaxations follow a generalised Cole–Cole equation.

Phase-transition temperature and character ofCd2Nb2O7

The complicated dielectric spectrum of ${\mathrm{Cd}}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}$ around the ``paraelectric-ferroelectric'' (PF) phase transition is greatly simplified due to the suppression of the multiple dielectric relaxation processes under a dc electric field. The PF transition is revealed under a dc electric field and characterized by the Curie-Weiss relation. The results show that the PF phase transition is of second order. The dc electric field influence on the dielectric relaxation modes and PF phase transition for both single crystals and ceramics is discussed.

Observation of dielectric relaxation modes in chiral smectic C liquid crystal mixture

The dielectric parameters in the frequency range 50 Hz to 1 MHz have been measured in a SmC* liquid crystal mixture in planar alignment. The relaxation frequency ( f r ), distribution parameter ( α ) and dielectric strength ( δε ) have been evaluated and their corresponding values were found to be 206.62 Hz, 0.06 and 114.6 respectively. The Goldstone mode, domain mode, soft mode and a new relaxation mode have been observed.

Dielectric relaxor behavior of Cd2Nb2O7

The dc electric-field dependence of the dielectric properties of cadmium niobate Cd2Nb2O7 single crystals is studied, and characteristics of the dielectric behavior of Cd2Nb2O7 under dc bias are revealed. The temperature (Tm) where the dielectric relaxation mode occurs is independent of dc bias levels, while its intensity is greatly suppressed and finally eliminated with increasing dc bias from 0 to 15kV∕cm. The observed dielectric relaxation features of Cd2Nb2O7 can be attributed to “dielectric relaxor” behavior, rather than “ferroelectric relaxor” behavior previously designated in literature. The physical nature of the dielectric relaxor behavior is briefly discussed.

Study of different modes of dielectric relaxation in a newly synthesized material exhibiting ferro-, ferri- and antiferro-electric phases

Employing thermodynamical, optical and comprehensive dielectric studies, we have investigated a freshly synthesized chlorinated MHPOBC analogue antiferroelectric liquid crystal; it exhibits different chiral SmC sub-phases, viz. SmCα*, SmC* and SmCγ*, at considerably lower temperature in comparison with MHPOBC, and a wide range room temperature antiferroelectric SmCA* phase. Dielectric spectroscopy investigations under planar molecular anchoring were carried out in the frequency range 1 Hz to 10 MHz. The data measured for low and high frequency regions were corrected by the method of fitting operation of the theoretical equations. Seven different modes of dielectric relaxation in the different phases were identified. Soft mode relaxation was identified in the SmA* and SmCα* phases. The Goldstone mode, characteristic of the ferroelectric phase, was observed in the SmC* and SmCγ* phases. Two characteristic peaks are seen in the spectrum of the SmCA* phase: one, at high frequency, corresponds to fluctuation where the molecules in neighbouring layers are rotating in opposite phase; the other, at low frequency, arises due to phase fluctuation in the helicoidal superstructure.

Dielectric spectroscopy of an antiferroelectric liquid crystal showing an antiferroelectric–ferrielectric transition

We report the dielectric relaxation behaviour in the antiferroelectric SmCA* and ferrielectric SmCγ* phases of the antiferroelectric liquid crystal 4-[5-(4-octloxyphenyl)-2-pyrimidinyl]phenyl 4,4,4-trifluoro-3-(methoxyphenyl)butanoate which shows an antiferroelectric transition at around 88±0.1°C. In the SmCA* phase, two dielectric relaxation modes have been found, namely the usual antiferroelectric Goldstone mode and another arising from molecular rotation around its short axis. In the SmCγ* phase, one dielectric relaxation mode has been observed due to the ferrielectric Goldstone mode. Dielectric increments and relaxation frequencies of the antiferroelectric and ferrielectric phases are estimated from the fits of the Cole–Cole function of the dielectric spectrum. The dependence of the bias field in the ferrielectric phase is also discussed.

Low frequency chain dynamic of cross-linked poly(acrylic acid)

Cross-linked poly(acrylic acid) (PAA) synthesized by radical polymerization in inverse suspension is a swelling gel. The physical structure of PAA has been analyzed using low frequency chain dynamic given by the analysis of thermo stimulated currents (TSC). The α primary dielectric relaxation mode observed around the glass transition temperature ( T g=+38 °C) corresponds to the slowest dynamic. The relaxation times of the constituting processes show that it is due to a delocalized cooperative molecular mobility involving nanometric sequences of the hydrocarbon skeleton. The β secondary dielectric relaxation mode observed at lower temperature ( T β=−35 °C) corresponds to a higher frequency molecular mobility. It has been assigned to the cooperative mobility of hydrogen bonded COOH groups. In fact, the hydrogen bonded side chains behave as an hydrophilic matrix in which nanometric domains constituted by sequences of the main chain are embedded. Such a picture might explain the specific swelling properties of cross-linked PAA.

Molecular dynamics study of the thermal and the density effects on the local and the large-scale motion of polymer melts: scaling properties and dielectric relaxation.

Results from a molecular dynamics simulation of a melt of unentangled polymers are presented. The translational motion, the large-scale and the local reorientation processes of the chains, as well as their relations with the so-called "normal" and "segmental" dielectric relaxation modes are thoroughly investigated in wide temperature and pressure ranges. The thermodynamic states are well fitted by the phenomenological Tait equation of state. A global time-temperature-pressure superposition principle of both the translational and the rotational dynamics is evidenced. The scaling is more robust than the usual Rouse model. The latter provides insight but accurate comparison with the simulation calls for modifications to account for both the local chain stiffness and the nonexponential relaxation. The study addresses the issue whether the temperature or the density is a dominant control parameter of the dynamics or the two quantities give rise to comparable effects. By examining the ratio /alphatau//alphaP between the isochronic and isobaric expansivities, one finds that the temperature is dominant when the dynamics is fast. If the relaxation slows down, the fluctuations of the free volume increase their role and become comparable to those of the thermal energy. Detectable cross-correlation between the "normal-mode" and the "segmental" dielectric relaxations is found and contrasted with the usual assumption of independent modes.

Low frequency dielectric spectroscopy of two room temperature chiral liquid crystal mixtures

Dielectric spectroscopy of two room temperature chiral liquid crystal mixtures (W-96 and W-97) have been carried out in the frequency range of 100 mHz–10 MHz. Low frequency dielectric relaxation modes corresponding to collective behavior of molecules (Goldstone- and soft-modes) in the SmC* phase have been found to be masked by the ionic conductance. Two slow modes of dielectric relaxation due to the ionic conductance have been detected (below 15 Hz) in planar-aligned samples. It has been observed that the effect of ionic conductance decreases with the number of thermal annealing cycles on the materials. With large number of thermal annealing cycles it has been possible to wipe out the effects of ionic conductance and then to detect other weak modes of dielectric relaxation which are otherwise masked.

Dielectric Relaxation Behavior and High Tunability in Cd 2 Nb 2 O 7

Dielectric behavior of Cd 2 Nb 2 O 7 has been studied from room temperature down to 10 K with and without dc electric fields. Three dielectric relaxation modes I, II, and III were reexamined. By using the Cole - Cole equation fitting of the frequency dependence of the dielectric constant and loss of Cd 2 Nb 2 O 7 , we have obtained more precise relaxation time data, compared to the data reported in the literature. The results indicate that the relaxation time follows both the Arrhenius law and Vogel - Fulcher relation in the frequency range of 10 2 - 10 5 Hz. With increasing dc bias, the relaxation modes were suppressed and eventually eliminated. The results show a significant suppression of the dielectric constant and loss by the application of dc fields. At above 10 kV/cm, only a dielectric peak remains. The high tunability (37 - 64%) under 15 kV/cm at 5 kHz in the wide temperature range of 50 - 180 K and very low dielectric loss (0.4 - 6 2 10 m 3 )) are observed.

Dielectric Relaxation Behavior and High Tunability in Cd 2 Nb 2 O 7

Dielectric behavior of Cd 2 Nb 2 O 7 has been studied from room temperature down to 10 K with and without dc electric fields. Three dielectric relaxation modes I, II, and III were reexamined. By using the Cole - Cole equation fitting of the frequency dependence of the dielectric constant and loss of Cd 2 Nb 2 O 7 , we have obtained more precise relaxation time data, compared to the data reported in the literature. The results indicate that the relaxation time follows both the Arrhenius law and Vogel - Fulcher relation in the frequency range of 10 2 - 10 5 Hz. With increasing dc bias, the relaxation modes were suppressed and eventually eliminated. The results show a significant suppression of the dielectric constant and loss by the application of dc fields. At above 10 kV/cm, only a dielectric peak remains. The high tunability (37 - 64%) under 15 kV/cm at 5 kHz in the wide temperature range of 50 - 180 K and very low dielectric loss (0.4 - 6 2 10 m 3 )) are observed.

Dielectric investigation of electrically oriented ferroelectric smectic mixture CS-1013.

From dielectric spectroscopic study, a first-order ferroelectric phase transition has been observed in ferroelectric smectic mixture CS-1013 having the phase sequence Cr-SmC*-SmA-N*-Iso. Frequency (100 Hz-10 MHz) and temperature-dependent dielectric measurements have been performed on an electrically aligned sample (thickness 15+/-1 microm) gold coated on glass plates. In the unidirectionally aligned sample, two dielectric relaxation modes (Goldstone mode and soft mode) have been clearly observed in the ferroelectric SmC* phase while only one relaxation mode (soft mode) is visualized in the paraelectric SmA phase. Low-frequency molecular relaxation was also observed in the smectic phases. The experimental results have also been analyzed at different temperatures and biasing voltages for an understanding of the dynamics of dielectric processes in the ferroelectric phase. Finally, we proposed the "pseudospin" model for understanding the ferroelectric-antiferroelectric transition in liquid crystals. We associate the tilt angle straight theta and the pitch of the helix, respectively, with biaxial (b) and uniaxial (u) anisotropy parameters as fluctuating parameters around their stability limit (corresponding to the crystalline values). Here, the director acts as the pseudospin variable. This gives rise to a transverse Ising type (or anisotropic Heisenberg model under the mean-field approximation). It is then shown that such a model with fluctuations of (b) and (u) would explain the ferroelectric and antiferroelectric phase transitions in such liquid crystals. Using Landau theory and the stability conditions, we have also shown, in brief, the feasibility of different types of phase transitions in the ferroelectric liquid crystal system.

MOLECULAR AND COLLECTIVE PROCESSES IN THE ANTIFERROELECTRIC PHASE OF A CHLORINATED COMPOUND

New liquid crystalline substance, (S) 4-(1-methylheptyloxycarbonyl)-3-chlorophenyl 4′-(4-butanoiloxybut-1-oxy)biphenyl-4-carboxylate, exhibiting ferroelectric and antiferroelectric phases have been studied by means of frequency domain dielectric spectroscopy. DSC calorimetric and electrooptic measurements were done to study its phase transitions and electrooptic behaviour. In the antiferroelectric SmCA* phase one of the dielectric relaxation modes exhibits a non-Arrhenius behaviour. The plots of critical frequencies and dielectric increments vs. temperature show an anomalous behaviour of the relaxation processes in the neighbourhood of the ferroelectric — antiferroelectric phase transition. Reversal current method reveals one current peak in the ferroelectric phase and two in the antiferroelectric one. Temperature dependencies of tilt angle, spontaneous polarization, switching time and transmittance are also presented and discussed in scope of the mean-field theory.

A phenomenological model for dielectric relaxation processes in the B2 phase

A simple model based on the Landau theory is used to describe the dielectric relaxation modes of the antiferroelectric B2 phase. The model allows us to explain the occurrence of two separated dielectric modes based on particle rotation about the molecular long axes.

Characterization of structural heterogeneity of polyurethane coatings

AbstractThe thermal analysis techniques—Differential Scanning Calorimetry and ThermoStimulated Current—have been used to characterize a polyurethane high solid coating. The glass transition temperature, as determined by DSC, is 60°C. Below this glass transition temperature, an αsub dielectric relaxation mode has been observed; it corresponds to cooperative movements precursor of the glass transition. The αss dielectric relaxation mode, located at ≈ Tg has been attributed to movements of soft sequences of the amorphous phase liberated at the glass transition temperature. The analysis of the fine structure shows that they are constituted of elementary processes characterized by relaxation times following a compensation law. Above Tg, the αhs dielectric relaxation of hard sequences has been shown. It corresponds to hard sequences hydrogen bonded in polyurethane. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2786–2790, 2001

Dielectric Relaxation Behavior of Cationic Micellar Solutions: 2

Dielectric relaxation behavior was examined, in the frequency range from 1 MHz to 20 GHz, for an aqueous micellar system of cetyltrimethylammonium bromide (CTAB) with the concentration (CD) from 2 to 1000 m mol kg-1. The system exhibited three dielectric relaxation modes. The fastest mode, with a relaxation time around 1 ns, had relaxation strength proportional to CD up to CD values of 100 m mol kg-1, and showed a constant relaxation strength independent of CD above that concentration. The fastest relaxation mode attributed to the rotational relaxation mode of ionic pairs formed between CTA+ and Br- in spherical micelles is considerably depressed in contact areas on the micellar surface of two micelles in contact with each other at CD values higher than 100 m mol kg-1. This is likely the reason for the leveling-off in the magnitude of the fastest relaxation at high CD and the appearance of the slowest relaxation mode, which has a relaxation time of about 20 ns and increases in strength with increasing CD. The intermediate relaxation mode (with a relaxation time around 8 ns, and strength which decreases with increasing with CD) is attributed to the migration of electrically bound Br- ions around spherical micelles. The average number of Br- ions bound into an ionic cloud covering a micelle was estimated to be less than unity (from the values of direct current electric conductance and Br- concentration determined by use of a Br- selective ion electrode), and a small number of micelles possessed ionic clouds which showed dielectric relaxation behavior.

Study of dielectric relaxation modes in poly(ε-caprolactone): Molecular weight, water sorption, and merging effects

The behavior of the various relaxation modes in poly(ε-caprolactone) (PCL), is studied by Broad Band Dielectric Spectroscopy in the frequency range from 3×10−3 to 1.8×109 Hz and from 133 to 313 K. The experimental trace of the dielectric loss as a function of the angular frequency, ε″(ω), is analyzed by best fitting a sum of Cole–Cole distributions corresponding to the γ and β local modes and to the α relaxation which is the dielectric manifestation of the dynamic glass transition. The kinetic parameters of the three predominant relaxations are determined and relaxation plots describing the temperature dependencies of the relaxation times are given as a function of 1/T. These relaxation plots are insensitive within experimental errors, either to the molecular weight or to the water concentration. The hydration level (<1%) only affects the intensities of the local processes and no plasticization effect is observed. At temperatures higher than those recorded for the α mode a fourth intense process, α′ is observed as a shoulder in the conductivity rise. The position of this peak is the only one that changes with the molecular weight but not as much as it should if it were only caused by a normal mode. As the temperature increases the local β relaxation and the cooperative α mode merge into an αβ mode. The traces can be fitted below the merging temperature with three Cole–Cole distributions which after 219 K reduce to 2, the αβ and γ modes. This decomposition is confirmed by extracting the relaxation time distribution with the Simulated Annealing Direct Signal Analysis. In PCL, there is no necessity of invoking the existence of a change in the relaxation mechanism, a simple superposition of the α and β modes is adequate. This α–β crossover affects the intensity and shape of the distant γ relaxation. At still higher temperatures this αβ mode merges with the more localized γ mode, thus evidencing a new crossover and the persistence of the γ mode in the whole temperature range. The activation energy of the γ process changes after the crossover temperature, together with the α Cole–Cole shape parameter and its dielectric strength.

Oxygen vacancy related dielectric relaxation in (Sr1-1.5xBix)TiO3

This paper reports the dielectric polarization process in (Sr1-1.5xBix)TiO3 (x=0.0267) ceramics in the temperature range 10 – 300K. The main attention was focused on the oxygen vacancy related dielectric relaxation behavior. The results show that the dielectric relaxation mode displays a symmetric frequency dependence, and Debye like relaxation behavior. The dielectric polarization strength follows the modified Debye-Langevin model .