Electrical conductivity and dielectric constant of heat-treated polyacrylonitrile in an AC regime

The electrical conductivity σ(ω) and the dielectric constant ?(ω) of heat-treated polyacrylonitrile (PAN) samples obtained by pyrolysis at temperatures up to 550°C, were measured in the frequency range from 500 Hz to 250 MHz at room temperature. For samples treated at temperatures below 300°C and for frequencies up to 200 MHz, the conductivity obeys a ω^s law (s = 1) while ? does not change significantly. For heat-treatment temperatures above 300°C, the behaviour of σ(ω) is greatly modified. In fact, ?(ω) greatly increases at low frequencies for increasing heat-treatment temperatures, and is strongly dependent frequency. Our results can be explained with the help of two randomly distributed phases in the medium and by introducing percolation theory.     

Crosslinked Cholesteric Network from the Acrylic Acid Ester of (Hydroxypropyl)Cellulose

The acrylic acid ester of (hydroxypropyl)cellulose was prepared from (hydroxypropyl)cellulose and acryloyl chloride. The resultant polymer, with 2.2 ester groups per anhydroglucose unit, formed a thermotropic cholesteric mesophase with visible reflection hands at temperatures between ambient and 60°C. By exposing a thin layer of the mesophase to UV light, the mesophase structure was stabilized to give a crosslinked cholesteric film.     

Charged defects in liquid chalcogenides

The conductivity of liquid chalcogenides has been measured at frequencies 2 Hz - 2×10⁵ Hz and 10¹⁰ Hz in the temperature range 500 K to 830 K. While in liquid Se the conductivity increases with increasing frequency no frequency dependence of the conductivity is found in liquid As₂Se₃. A simple dielectric model explains the change of conductivity with frequency and temperature.     

Dielectric Spectroscopy of Ferronematics Based on 6CHBT Liquid Crystal

In our study, the dielectric behaviour of the rod-like liquid crystal (6CHBT) doped with magnetic nanoparticles of spherical shape was investigated by means of dielectric spectroscopy in the frequency range from 20 Hz to 2 MHz. A low frequency dielectric dispersion in the nematic and isotropic phases of the pure liquid crystal (LC) has been assigned to the space charge polarization. After doping the host LC with the magnetic nano particles, a nearly Debye-like relaxation process was observed with the temperature dependence obeying the Arrhenius law. Considering a possible electric double layer formation on the particle surfaces, the detected relaxation process in the doped LC can be associated with the electric double layer polarization. The experimental results point out that in the measured frequency range the space charge and interfacial effects constitute the main dielectric response. Any anchoring effects were not observed and are therefore expected to appear in higher frequencies.     

Cholesteric Pitch Near the Smectic-C Phase

The temperature dependence of the cholesteric pitch of the chiral ester 4-n-hexyloxyphenyl-4'-(2”-methylbutyl)biphenyl-4-carboxylate has been measured in detail using the method of selective reflection. This compound has a cholesteric to smectic-C transition at 79°C. Because of the first-order nature of the transition, the pretransitional unwinding of the pitch is considerably less pronounced than that in cholesteryl esters near the smectic-A transition. The wavelength of maximum reflection increases from 440 nm at 110°C to 560 nm at the transition. When fitted to a power-law temperature dependence, the data imply an exponent of 0.82 ± 0.15, which is consistent with de Gennes’ theory in the meanfield approximation.     

Electric relaxation in chemically synthesized polyaniline: study using electric modulus formalism

A method of dielectric spectroscopy was used to determine the electric properties of chemically synthesized polyaniline (PANI) with special emphasis on the influence of acid and water doping levels on the polymer properties. The dependence of the dielectric loss factor (tan δ) on the temperature (in the range from − 100 to + 100°C) and the frequencies of the ac voltage (F = 0.1–30 000 Hz; amplitude of the voltage 1 V) were measured. They were subsequently transformed into the temperature and frequency dependences of a complex electric modulus, M*. Tan δ(F, T) and M(F, T)) presentations were both used in the analysis of the electric properties of PANI samples to compare their informative possibilities. At least two modes of the polymer response to varying temperature and frequency were resolved. Each mode corresponds to a certain area in an electric map of the system (log F versus 1/T plane) with their positions displaced towards higher frequencies and lower temperature with increases of both acid and water concentration in PANI. This effect is interpreted in terms of a unified mechanism of interaction of water and acid molecules with polymer backbone.     

Lasing Spectra of Dye Doped Cholesteric Liquid Crystals Under Reversible Phototuning of the Frequency

We investigate the lasing frequency phototuning in a distributed feedback (DFB) laser based on cholesteric liquid crystals (CLC). To improve the CLC planar texture quality, we use new materials such as nematic ZhK-654 and an analog of cholesteryl oleate as a twisting additive. The selective excitation of the cis-form molecules of the azoxy-nematic is carried out with the use of a combination of an interference filter with maximum transmission at 436 nm and a cut-off filter. Using the orienting substrates with SnO₂ allows us to reduce the threshold pump power by more than an order of magnitude. The proposed combination of filters for the selective excitation enables us to align the ranges of lasing wavelengths in both directions.     

Ac conductivity and dielectric relaxation in ionically conducting soda-lime-silicate glasses

The frequency dependent conductivity and dielectric relaxation of alkali ions in some soda-lime-silicate (Na₂O-CaO-SiO₂) glasses are investigated over a frequency range from 50 Hz to 1 MHz and in a temperature range from room temperature to 603 K by using alternating current impedance spectroscopy. The conductivity isotherms show a transition from frequency independent dc region to dispersive region where the conductivity continuously increases with increasing frequency. The electric modulus representation has been used to provide comparative analysis of the ion transport properties in these glasses. The scaling behavior of imaginary part of electric modulus indicates that all dynamical processes occurring at different frequencies give the same activation energy.     

Effect of LiCF3SO3 on L-Chitosan/PMMA Blend Polymer Electrolytes

This paper focuses on the effect of lithium triflate (LiCF₃SO₃) on the structural and conduction properties of lauroyl (L)-chitosan/poly(metylmethacryalate) (PMMA)-based polymer electrolytes. Films of L-chitosan/PMMA blends and its complexes were prepared using a solution-casting technique. The ionic conductivity of the system was measured over a wide range of frequency between 50 Hz-1 MHz. Impedance plot for the samples demonstrates two well-defined regions. The disappearance of the high frequency semicircular region led to a conclusion that the current carriers are ions. Sample with 30 wt% of LiCF₃SO₃ showed the highest conductivity of 7.59 ± 3.64 × 10^?4 Scm^?1 at room temperature. This is consistent with the results obtained from infrared spectroscopy.     

Internal friction of mixed alkali metaphosphate glasses: (I). Results

The internal friction of LiNa, LiK, LiCs, LiAg, NaCs and NaAg metaphosphate glasses was measured at 0.5 Hz and 2 kHz. The dielectric losses were also measured from 40 to 160°C, at frequencies of 300, 3 000 and 30 000 Hz. The densities of the glasses were determined and the molar volume of oxygen was calculated. In general, the mixed alkali behaviour of metaphosphate glasses is very similar to the mixed alkali behaviour of silicate glasses. Silver behaves in this respect like an alkali ion with approximately the same size as a sodium ion.     

Dielectric monitoring of curing process for some epoxide-amine thermosets

Three recently synthesized epoxy monomers were studied. All the monomers form a nematic phase in temperatures above 80 °C. This paper presents the dielectric response of pure monomers in the frequency range of 10⁶–10⁹ Hz as well as its changes occurring in the course of crosslinking with 4,4′-diaminodiphenylmethane used as curing agent. A relaxation process related to molecular motions is clearly visible in this frequency range for each of the monomers, though its characteristics are somewhat different in particular cases. The evolution of the relaxation process is visible in the course of curing. The dynamics of curing observed through dielectric data is compared with the results of differential scanning calorimetry measurements.     

Two-photon actuation of crosslinked liquid-crystalline polymers utilizing energy transfer system

ABSTRACT

We prepared crosslinked azotolane liquid-crystalline polymer (LCP) films doped with a stilbene derivative (two-photon chromophore) utilizing an interpenetrating polymer network (IPN) structure. The IPN films bend toward the light source upon irradiation with femtosecond laser pulses at 600 nm, which can excite the stilbene derivative by two-photon absorption. The bending speed of the IPN films increases with the square of the laser pulse intensity, which is compelling evidence for the two-photon processes.     

Low-frequency mechanical relaxation in CoO crystals

Internal friction Q ⁻¹ was studied in nominally pure and Li-doped CoO crystals at frequencies of about a few Hz in the temperature range from −180 to 20°C. Doping Co crystals with Li results in a new relaxation maximum of Q ⁻¹ at −115°C at a frequency of 3 Hz. The occurrence of this maximum is related to the reorientation of the Co³⁺-Li¹⁺ elastic dipole under external stress.     

Dielectric Relaxation Spectroscopy of Liquid Crystal in Nematogenic Mesophase

In the present study, we report the dielectric behavior of a high temperature nematic liquid crystal (NLC) in both the planar as well as homeotropic alignments. To understand the mechanism of such high temperature NLC, we have performed dielectric spectroscopy in frequency range 100 Hz to 10 MHz. The different dielectric parameters, viz., effective dielectric permittivity and dielectric anisotropy of the sample have been determined with the variation of frequency and temperature. It is found that the dielectric parameters depend on dipole moment and rotation of side chain of molecule. The phase transition of this high temperature NLC has been confirmed by the DSC study.     

Synthesis and properties of new fluorene-containing copolymer via the catalytic dehydrocoupling reaction of 9,9-dipropargyl fluorene and 4,4′-diphenyl-2,6-dibromo-dithieno-[3,2-b:2′,3′-d]silole

ABSTRACT

New fluorene-containing copolymer was prepared via the catalytic dehydrocoupling reaction of 9,9-dipropargylfluorene and 4,4′-diphenyl-2,6-dibromo-dithieno-[3,2-b:2′,3′-d]silole in 44% yield. The resulting polymer was completely soluble in common organic solvents. The thermal behaviors and optical properties of the resulting polymer were measured and discussed. The chemical structure of polymer was characterized by NMR, IR, and UV-visible spectroscopies. The polymer showed characteristic wide UV-visible absorption band and blue PL maximum value at 440 nm, which is corresponding photon energy of 2.82 eV. The quantum yield (6.44%) of poly(DPF-DTS) was found to be 5 times higher than that of poly(DPF-bithiophene)).     

Polystyrene Blend Alignment Layers for High Pretilt Angle Control

We prepared blend alignment layer based on 4-ethylphenoxymethyl-substituted polystyrene (P4EP) and polystyrene containing 7-hydroxycoumarin (P7COU) for liquid crystal (LC) alignment. Stable and uniform vertical LC cells having pretilt angles adjustable from 90° to 81.2° were obtained from these polystyrene films having molar contents of P4EP and P7COU in the ranges of 100–70 and 0–30 mol%, respectively. The LC alignment behavior was well correlated with the wettability of the polymer films. These polystyrene blends having good solubility in many organic solvents and their films having low process (curing) temperature can be candidates for LC alignment layer in the flexible display. This study contributes to the latest efforts to develop new alignment layers for pretilt angle control.     

Dielectric behaviour in a vanadium phosphate glass

The dielectric constant and conductivity of 80% V₂O₅: 20% P₂O₅ glass has been measured in the frequency range 10² to 10⁹Hz and in the temperature range 80 to 350°K. It is shown that the dielectric behaviour over these ranges is described by a Debye type relaxation process with distribution of relaxation times. A method is proposed to determine the width of distribution from the data at fixed frequencies and different temperatures. The width of distribution increases at frequencies ω > 10/τ, which leads to an a.c. conductivity at these frequencies almost linearly proportional to frequency and independent of temperature. The estimated value of the static dielectric constant of about 30 was found to decrease with temperature while the infinite frequency dielectric constant of 10 was independent of temperature. The carrier concentration calculated from the dielectric relaxation time and the d.c. conductivity through a thermal diffusion model shows reasonable agreement with direct measurement using electron paramagnetic resonance.     

Low-frequency dielectric properties of DMAAS crystals in the vicinity of 110 K

Dielectric properties in “pure” and partly deuterated DMAAS crystals have been studied within the frequency range 40^?2–10⁷ Hz in the vicinity of 110 K. It is established that the crystals possess relatively high conductivity, 10^?4–10^?7 Ω^?1 m^?1, which is explained by their crystal structure. The frequency dependence of the complex dielectric constant has two linear segments, which indicate the change in the charge-carriers motion in the vicinity of 110 K. It is revealed that at low temperatures, conductivity increases at higher frequencies.     

Dielectric properties and AC conductivity studies of novel NR/PVA full-interpenetrating polymer networks

Fully interpenetrating polymer networks (IPN) based on natural rubber (NR) and polyvinyl alcohol (PVA) were prepared by using Glutaraldehyde as the common crosslinking agent. Crosslinking reactions were confirmed by Fourier Transform Infra-red Spectroscopy. The electrical properties of full-IPN have been studied in the frequency range of 10²–10⁶ Hz. The dielectric constant, dielectric loss and ac conductivity were analyzed as a function of frequency, temperature and blend composition. It was found that the dielectric constant and dielectric loss increased with the addition of PVA into NR. Interpenetrated system showed a significant reduction in dielectric constant and dielectric loss and almost frequency independent. The dielectric constant and dielectric loss were increased with increase in temperature. The change in these parameters with temperature in the IPN system was found too low as compared to the pure blends. The ac conductivity study revealed that the IPN materials exhibited a considerable reduction in conductivity.     

Synthesis and Characterization of Violet Light Emitting Polymer based on Fluorene and Dimethylsilane

New fluorene based light emitting polymer, poly[(4-(9,9-didecyl-9H-fluoren-2-yl) phenyl)dimethyl(phenyl)silane] (PFDPS), was synthesized by palladium-catalzed Suzuki coupling reaction. The obtained copolymer was characterized by ¹H-NMR, and IR-spectroscopy. The polymer showed good solubility in common organic solvents and weight average molecular weight of 16,300 with polydispersity index of 1.4. The maximum photoluminescence of the solution and film of the polymer was observed at 392 nm and 410 nm, respectively.

The double-layered device with the configuration, ITO/PEDOT/PFDPS/LiF/Al structure has a turn-on voltage at about 5.5 V and maximum brightness of 9.40 cd/m², and emitted violet light at 414 nm.