S. Abdul Jawad

Frequency and temperature dependence of ac electrical properties of graphitized carbon‐black filled rubbers

The a.c. electrical properties of graphitized carbon-black filled rubbers were investigated in the temperature range 25-85°C and in the frequency range 10-10 6 Hz. The volume fraction of the filler varied from 15 to 60%. The a.c. impedance was found to decrease dramatically with increasing filler content for volume fractions higher than 30%. A relaxation transition with an activation energy of 26.4-50 kJ/mol was observed only in the samples containing 45% and 60% of the fillers.

Activation of kaolin with minimum solvent consumption by microwave heating

Abstract A kaolin clay was activated with 1.0 ᴍ H2SO4 solution at minimum liquid to solid ratio (L/S) using microwave heating. The optimum experimental conditions for activation were L/S ratio 3.0 mL 1 ᴍ H2SO4 per gram kaolin, microwave input power 500-600 W, and heating time 5-10 min. Activation at L/S < 3.0 mL/g using 1.0 ᴍ H2SO4 was not efficient, indicating the influence of solvent for absorbing microwaves more intensively and thus improving activation. Significant physicochemical changes were observed by the proposed procedure with smaller volumes of activator compared to the conventional heating method. Microwave input power and heating time have a strong influence on the quality of the final material; activation at high input power (>700 W) and longer heating times (>10 min.) are not recommended since they cause dissolution of kaolinite structure. Microwave-heated kaolin manifested better adsorption for tartrazine dye due to improvements in textural and chemical properties of kaolinite. Moreover, irradiation of used kaolinite has significantly improved dye desorption, increasing the importance of microwaves in regeneration/recycling studies. Detailed dielectric measurements of kaolin-acid mixtures recorded at frequencies much lower than 2.45 GHz revealed that absorption of radiation is highly dependent on the activator solution in the mixture. For 3.0 mL/g mixtures, high dielectric constant Ɛ’ 5223, dielectric loss factor Ɛ’’ 5083, tangent loss tan δ 1.30, penetration depth dp 0.57 cm at (103 Hz), and AC-conductivity σ 0.032 Ωm-1 were determined at 105 Hz. Filling the pores of kaolin by acid solution increased the microwave absorption and hence de-alumination of kaolinite.

Dielectric spectroscopy of polycarbosilazane-based CuCl2 metallopolymers

ABSTRACT A series of poly (carbosilazane-CuCl2) metallopolymers was prepared by the reaction of varying amounts (5% to 30%) of anhydrous CuCl2 with polycarbosilazane, [–(CH3)2SiNH(CH2)2NH–]n, matrix in tetrahydrofuran under continuous sonication. Dielectric measurements were performed at room temperature (25°C) in the frequency range 1 Hz to 106 Hz. It was found that the ac conductivity increases with increasing the CuCl2 up to 16% and then starts to dramatically decrease. The results reveal that all the dielectric parameters such as ac impedance, dielectric permittivity, and electric modulus behave similar to that observed in ac conductivity. (i.e., maximum or minimum depends on the considered dielectric parameter). It is suggested that at low concentration of metal content, all the CuCl2 is uniformly bound to the backbone of the polymer chains, where the conduction mechanism in the system is by electrons jumping (carrier hopping between the chains), whereas at high metal content the coordination capacity of the backbone is exceeded and CuCl2 is present as molecular clusters that lead to an increase in the interaction distances, and make hopping between chains more difficult, and hence, resulting in a reduction of conductivity.

Dc electrical properties of graphytized carbon-black filled rubbers

The dc electrical behavior of graphytized carbon-blacks filled rubbers was investigated in the temperature range 25°C to 125°C. The volume fraction of the fillers varied from 30% to 60%. The observed conductivity increases with increasing filler contents. At a temperature higher than 25°C for 60% filler concentration the behavior is ohmic, where at lower concentration two well defined regions were observed to indicate two types of conduction mechanism. The activation energy for conduction process increases from 0.3 eV for 60% to attain about 0.98 eV for 30% fillers concentration. At 60% fillers concentration, fillers form a conductive network which is ohmic in nature.

Dielectric Spectroscopy of Pure and KBr Dispersed Polypyrrole

Abstract The effect of frequency at room temperature of the ac impedance, permittivity and electric modulus of pure Polypyrrole and KBr Polypyrrole powders with different ratios was investigated in the frequency range 0.1 Hz. to 106 Hz. The frequency dependence of the ac impedance is similar to that observed in RC network in parallel. The real component of ac impedance is frequency independent in low frequency range and becomes proportional to the inverse of frequency at high frequency range. The equivalent bulk resistance decreases with increasing the KBr content while the equivalent capacitance remains nearly unchanged. The relaxation transition in KBr dispersed Polypyrrole is assigned to a summation of relaxations occurring within the bulk.

Dynamic mechanical and dielectric relaxations in liquid crystalline copolymers formed from hydroxynaphthoic acid, terephthalic acid and p-aminophenol

Abstract Dynamic mechanical and dielectric measurements were carried out in the temperature range −120 to 160°C on nematic random copolymers formed from 60% hydroxynaphthoic acid, 20% terephthalic acid and 20% p -aminophenol. Only two relaxation transitions were observed both mechanically and dielectrically: the β and α relaxations. The virtual absence of the low-temperature γ relaxation is attributed to the existence of hydrogen bonding anticipated in this copolymer. The appearance of a pronounced dielectric α relaxation confirms that some electric dipoles were inactive at low temperatures and become active when hydrogen bonds are broken at high temperatures.

DIELECTRIC BEHAVIOR OF ANNEALED POLY(VINYLIDENE FLUORIDE)

This study deals with the effect of annealing temperature of Polyvinylidene Fluoride quenched from melt into ice-water on the dielectric behavior. Measurements were carried out in the temperature range 25°C to 130°C and in the frequency range 1 kHz to 10 MHz. It was found that the dielectric permittivity and dielectric strength decreases with increasing annealing temperature. This is a result of morphological changes including defects creation, recrystallization and additional crystallization at expense of the amorphous phases.

Electrical β‐relaxation of die‐drawn isotactic polypropylene

Measurements of a.c. impedance were carried out in the frequency range 1 Hz to 10 6 Hz at room temperature on isotropic and die-drawn isotactic polypropylene. It was found that the a.c. impedance decreases with increasing draw ratio in the frequency range 1 Hz to 10 4 Hz and then becomes independent of draw ratio with increasing frequency. A dramatic drop in the a.c. impedance was observed in the frequency range 10 4 to 10 6 Hz, which is associated with a β-relaxation process. The intensity of the β-relaxation peak decreases with increasing draw ratio and nearly disappears for a draw ratio of 16.8. The Cole-Cole plots show good semicircles for isotropic and die-drawn samples indicating that a certain polarization mechanism is dominant in polypropylene.

Impedance behavior of liquid crystalline copolymers in the solid phase

Abstract The measurements were carried out as a function of temperature and frequency on nematic random liquid crystalline copolymers of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). The temperature was varied from 20 to 95°C while the frequency could vary from 100 Hz up to 100 kHz. It was found that the dispersion relation of ac-impedance shows a strong dependence on frequency and temperature. It falls from about 56 MΩ at 100 Hz to a value of about 13 MΩ at 100 kHz. The Cole-Cole plot shows a semicircle indicating that a certain polarization mechanism with a single relaxation time is dominant in the liquid crystalline copolymer material.

AC Impedance of Liquid Crystalline Copolyesters in the Solid Phase

Abstract This work deals with the effect of composition on the ac impedance of liquid crystalline copolyester of 4-Hydroxybenzoic Acid (HBA) and 2-Hydroxy 6-Naphthoic Acid (HNA) in the solid phase. The results show that the ac conductivity increases with increasing the content of HBA residues. The Cole-Cole plot yields good semi-circles for liquid crystalline copolyesters specimens. The intensity of the observed relaxation in the plot of the imaginary component of the ac impedance versus frequency increases with the content of HNA residues. This was interpreted to be due to a rotational motion of the identical ester linkages.