Dielectric properties of doped polystyrene and polymethylmethacrylate

About 1 μm thick films of polystyrene (PS) and polymethylmethacrylate (PMMA) doped with diphenylsulfoxide (DS) up to 40 wt.% were prepared from solutions using spin-coating method. Glass transition temperature (T _g) of doped polymer films was determined by DSC technique. The depth profile and surface concentration of DS dopant were measured by RBS and XPS methods, respectively. The temperature dependence of relative permittivity of the films was determined from capacitance measurement. The dependence of polarization (P) on electric field (E) was measured using a standard Sawyer–Tower circuit. The glass transition temperature T _g of both composites was found to be decreasing function of the DS concentration. The DS doping leads to an increase of relative permittivity of the PS and PMMA films. RBS and XPS measurements reveal an outward diffusion of DS dopant in PS/DS films at elevated temperature. No such effect was observed in PMMA/DS films. PMMA/DS layers were found to be more thermally stable comparing to PS/DS.     

Dielectric and relaxation behaviour studies in acrylonitrile-methylacrylate copolymer

Poly(acrylonitrile-methylacrylate) copolymer is a semicrystalline material, as defined by X-ray diffraction, and is thermally stable up to 425 K as revealed by infrared investigations. The study of dielectric properties obtained from this copolymer allows us to follow the relaxation processes and showed the difference between three processes; -, - and -relaxations. The first two processes occur in the two phases, amorphous and crystalline, and the third process is associated with the glass transition temperature. The origins of these processes were attributed to local motion of the polymer backbone segments, to a dipole orientation of the chain side groups and to an ionic space-charge relaxation, respectively.     

Dielectric and piezoelectric studies of Fe doped PLZT ceramics

Polycrystalline samples of Pb_0.9(La_1 − zFe_z)_0.1(Zr_0.65Ti_0.35)_0.975O₃ [referred as PLFZT] (where z = 0.0, 0.3, 0.5 and 0.7) have been synthesized by a high-temperature solid-state reaction technique. X-ray diffraction analysis suggests the formation of single-phase compounds with tetragonal structure. The dielectric studies of the compounds as a function of temperature (from room temperature (RT) to 600 K) at a frequency 10 kHz show that the compounds undergo a phase transition of diffuse type. Diffusivity studies of the phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. Measurement of dc resistivity (ρ_dc) as a function of temperature (RT to 600 K) at a constant biasing field dc 10 V/cm suggests that the compounds exhibit negative temperature coefficient of resistance (NTCR). The piezoelectric parameters as planar coupling coefficient, frequency constant and mechanical quality factor have been presented.     

Phase transformation studies on pure and doped strontium carbonate

The orthorhombic-hexagonal transformation of strontium carbonate is a reversible reconstructive transformation. Cationic impurities like K⁺, Ca²⁺, Ba²⁺, Cd²⁺, Y³⁺, La³⁺, Nd³⁺, Sm³⁺, Gd³⁺, Dy³⁺, Ho³⁺, Bi³⁺, Zr⁴⁺ doped in pure strontium carbonate affect the kinetics and energetics of the transformation. Paper presented at the Annual Convention of Chemists held at Andhra University, Waltair, December 1978.     

Mechanical and optical studies on pure and nitro doped 4-hydroxyacetophenone

Single crystals of pure and nitro doped 4-hydroxyacetophenone (NLO material) were grown by slow cooling technique. The single crystals were subjected to studies of Vicker's microhardness. The Vicker's hardness increases with the increase in applied load. The UV cut off wavelength of the grown crystals has been measured as about 280 nm from the optical transmission spectra of the samples. Chemical etching has also been carried out.     

Dielectric relaxation and thermal studies on dispersed phase polymer nanocomposite films

Dispersed phase polymer nanocomposite films (PNC) based on PMMA–LiClO₄+ n-YSZ, has been prepared. The effect of filler concentration on dielectric constant, tanδ and ac conductivity has been observed. For each PNC films the activation energy for relaxation (E_τ) is almost same as the activation energy for ion conduction (E_a). The dc conductivity, the hopping frequency of charge carriers have been obtained at different temperature from the analysis of the ac conductivity data. For all the PNC films, the concentration of charge carriers has been calculated at different temperature using Almond–West formalism. The estimated activation energies for the dc conductivity and the hopping frequency are different, which indicates that the both charge carrier mobility and concentration contribute significantly to the ionic conductivity of polymeric electrolyte. Contribution of charge carrier mobility to the total conductivity has also been confirmed from the differential scanning calorimetry analysis. Improvement in thermal stability has been noticed with filler addition.     

Thermally stimulated discharge current studies of pure and doped polyvinyl alcohol films

Thermally stimulated currents have been studied in solution grown pure and doped polyvinyl alcohol films polarised at different temperatures. The TSC spectra have been used to calculate relaxation time, activation energy, capture cross section, charge and the attempt-to-escape frequency in films doped with different impurities. The electrical conductivity, charge-retaining capability and other parameters are found to be dependent on the doping. The results are explained in terms of the energy levels created by the dopants and charge transfer through a tunnelling mechanism.     

Dielectric relaxation and alternating current conductivity of lanthanum, gadolinium, and erbium-polyvinyl alcohol doped films

Fourier transform infrared (FTIR) spectrum dielectric constant, ε', loss tangent, tan(δ), electric modulus, M*, and ac conductivity, σ(ac), of pure polyvinyl alcohol (PVA) as well as La-, Gd-, and Er-PVA doped samples have been carried out. The dielectric properties have been studied in the temperature and frequency ranges; 300-450?K and 1?kHz-4?MHz, respectively. FTIR measurements reveal that La(3+), Gd(3+), and Er(3+) ions form complex configuration within PVA structure. Two relaxation processes, namely, ρ and α were observed in pure PVA sample. The first process is due to the interfacial or Maxwell-Wagner-Sillers polarization. The second one is related to the micro-Brownian motion of the main chains. For doped PVA samples, α-relaxation process splits into α(a) and α(c). This splitting is due to the segmental motion in the amorphous (α(a)) and crystalline (α(c)) phases of PVA matrix. Electric modulus analysis was discussed to understand the mechanism of the electrical transport process. The behavior of ac conductivity for all PVA samples indicates that the conduction mechanism is correlated barrier hopping.     

Dielectric relaxation studies on PVC-Pb3O4 polymer nanocomposites

Journal of Materials Science: Materials in Electronics - This work studies the FTIR as well as dielectric characteristics of the PVC-Pb3O4 nanocomposite films. FTIR analysis shows the small shift...     

Dielectric relaxation studies of conveyor belt compounds to determine indentation rolling resistance

The accuracy of analytical models used to predict the indentation rolling resistance of belt conveying systems depends on the method of testing the material properties. Due to the limitations and inconsistency of obtaining material properties using mechanical methods, it is becoming more common for viscoelastic behaviour to be represented by non-mechanical models, such as dielectric models. Dielectric models can lead to the same mathematical predictions as mechanical models; however, the experimental work is highly accurate and can be performed over a much greater frequency range than mechanical testing. This paper introduces a dielectric energy loss model which uses dielectric methods to measure the viscoelastic material properties of the bottom cover of conveyor belt. The model is compared to actual indentation rolling resistance data obtained from tests performed at the University of Newcastle on a purpose built test facility.     

Dielectric properties of doped polymethylmethacrylate

Permittivity of about 1 μm thick films prepared from polymethylmetactrylate (PMMA) solution doped with 20% of diphenyl sulfoxide was studied. The film structure was characterized by XRD, RBS, DSC and SEM techniques. The dopant was found to be distributed homogenously in the film volume. Permittivity of the films was measured as a function of the temperature. The measurement of the dependence of polarization on electrical field was performed using a standard Sawyer-Tower circuit. The presence of the dopant increases the composite permittivity namely above the PMMA glass transition temperature. Hysteresis loops observed on the measured polarization vs. electrical field dependence indicate easier and more pronounced polarizability of the composite comparing to pristine PMMA.     

Dielectric relaxation in polychlorotrifluoroethylene

The effect of increasing lamellar thickness in bulk polychlorotrifluoroethylene (PCTFE) by various methods (annealing and increasing time and temperature of crystallisation) on the parameters of the and relaxations has been studied.For the a relaxation consistent increase in temperature of maximum loss and activation energy with increasing lamellar thickness confirms the strong suggestion of earlier work [6] on oriented specimens and shows that the relaxation arises mainly in the interior of the lamellae with little contribution from the chain folds.No systematic changes in these parameters with increasing lamellar thickness was found for the relaxation and this is explained by the presence of _c and _a components in crystalline and amorphous regions respectively. The component _a (as well as the _c one) shows a marked anisotropy in magnitude with respect to the draw direction in oriented specimens relative to the direction of the applied electric field. This implies a lining up of the chain molecules parallel to one another in amorphous regions.The non-existence of analogous short chain compounds and experimental scatter prevented detailed checking of the consistency of the data for the relaxation with the theory for the _c-C _c model [5].     

Dielectric relaxation of vegetable-based polyurethane

Vegetable-based polyurethane (PU) was prepared in the thin film form by spin coating. This polymer is synthesised from castor oil, which can be extracted from the seeds of a native plant in Brazil called mamona. This polymer is biocompatible and is being used as material for artificial bone. The PU was characterised by dielectric spectroscopy in a wide range of frequency (10^–5 Hz to 10⁵ Hz) and by thermally stimulated discharge current (TSDC) measurements. The glass transition temperature (T _g = 39°C) was determined and using the initial rise method the activation energy was found to be 1.58 eV.     

Stress relaxation of high impact polystyrene

The stress relaxation behaviour of high impact polystyrene has been correlated with the microstructural changes observed in tensile tests. The inhomogeneity of plastic deformation, manifested as stress whitening, has been measured using microhardness tests. This method has been found to be sensitive to the amount of crazing in the material. The stress relaxation behaviour changed at the onset of crazing, but did not change appreciably as the volume fraction of crazes increased. An analysis of the relaxation in terms of a site population model based on White's approach suggests the macroscopic stress relaxation is related to the crazes in the boundary regions between the stress whitened and unwhitened material.     

Characterization of polystyrene and doped polymethylmethacrylate thin layers

About 1 μm thick films of polystyrene (PS) and polymethylmethacrylate (PMMA) were prepared from solutions using spin-coating method. The PMMA films were doped with diphenylsulfoxide (DS) up to 45 wt%. Glass transition temperature (T _g ) of doped PMMA films was determined by DSC technique and relative permittivity (ε) as a function of the sample temperature was determined from capacitance measurement. The dependence of polarization (P) on electric field (E) and the temperature was measured using a standard Sawyer-Tower circuit. Spectral dependence of film refractive index was measured using a refractometer. The glass transition temperature T _g of PMMA/DS composite was found to be decreasing function of the DS concentration. Relative permittivity ε of unpolar PS is lower than that of polar PMMA. The PS permittivity does not depend on the sample temperature. For PMMA the permittivity is increasing function of both, DS dopant concentration and sample temperature. The dependence of the polarization on the electric field on PS film does not exhibit a hysteresis and indicate no polarization contrary to PMMA. PMMA/DS composites exhibit easier and larger polaribility and a permanent dipole moment. Resulting polarization is an increasing function of DS concentration. Refractive index of both pristine PS and PMMA decreases with increasing wave length. The refractive index of PMMA/DS composites depends on the DS concentration.     

Lattice vibrations of pure and doped GaSe

The Bridgman method is used to grow especially undoped and doped single crystals of GaSe. Composition and impurity content of the grown crystals were determined using X-ray fluorescence (XRF) method. X-ray diffraction, Raman scattering, photoluminescence (PL), and IR transmission measurements were performed at room temperature. The long wavelength lattice vibrations of four modifications of GaSe were described in the framework of modified one-layer linear-chain model which also takes into consideration the interaction of the selenium (Se) atom with the second nearest neighbor gallium (Ga) atom in the same layer. The existence of an eight-layer modification of GaSe is suggested and the vibrational frequencies of this modification are explained in the framework of a lattice dynamical model considered in the present work. Frequencies and the type of vibrations (gap, local, or resonance) for the impurity atoms were calculated and compared with the experimental results.     

Dielectric relaxation with dipolar screening

The phenomenon of dipolar screening does not appear to have received sufficient attention in the past, in a manner comparable to charged particle screening, and yet it is shown that it is capable of explaining some forms of dielectric response to which no other evident explanation exists. While reorientations of mathematical dipoles of zero “length” do not entail any charge displacements, physical dipoles of finite length, l, do produce charge displacements and therefore screening. A theory of dipolar screening is presented and it is shown that relatively “dense” systems give complete screening so that neighbouring dipoles do not “see” one another, while in more dilute systems many dipoles fall within a screening radius, Rs, of any one dipole. Dipoles within Rs tend to adopt energetically favoured configuration and the entire system “seizes up” in a “domain”, thus reducing the number of dipoles which can be reoriented at finite frequencies. This revised version was published online in November 2006 with corrections to the Cover Date.