Dielectric relaxation of PrFeO3 nanoparticles

PrFeO₃ (PFO) nanoceramic is synthesized by a sol-gel reaction technique. Thermogravimetric study of the as prepared gel is performed to get the lowest possible calcination temperature of PFO nanoparticles. The Rietveld refinement of the powder X-ray diffraction (XRD) pattern shows that the sample crystallizes in the orthorhombic (Pnma) phase at room temperature. The particle size of the sample is determined by scanning electron microscopy. The vibrational properties of the samples are studied by Raman spectroscopy at an excitation wavelength of 488 nm to substantiate the XRD results. Group-theoretical study is performed to assign the different vibrational modes of the sample in accordance with structural symmetry. Dielectric spectroscopy is applied to investigate the ac electrical properties of PFO at various temperatures between 313 and 473 K and in a frequency range of 42 Hz–1.1 MHz. The modified Cole-Cole equation is used to describe the experimental dielectric spectra. The frequency-dependent conductivity spectra are found to follow the power law. The temperature dependent dc conductivity is found to obey the Arrhenius law with an activation energy of 0.280 eV. An analysis of the real and imaginary parts of impedance is performed, assuming a distribution of relaxation times as confirmed by Cole-Cole plot.     

Dielectric relaxation of polydimethylsiloxane

Dielectric studies were performed on crystallized and amorphous polydimethylsiloxane which had been characterized by differential thermal analysis and polarized light microscopy. The crystallized specimen displayed one relaxation near 160 K at 1 kHz while the amorphous specimen showed absorption peaks at 155 and 165 K. For the latter material the high-temperature peak was not due to a true relaxation but resulted from crystal nucleation at 160 K and the subsequent growth of spherulites. The low-temperature peak at 155 K resulted from the relaxation associated with the glass transition. A sharp decrease of dielectric constant was observed for both specimens at the melting point (235 K). For the dielectric relaxation associated with the glass transition in crystallized specimens, the values of the dispersion amplitude, the apparent activation energy at 160 K, and the half-width of the absorption curve are 0.43 and 29 kcal/mole, and 5.6 decades, respectively, which are in marked contrast to the corresponding values of 0.82, 18, and 2.2 for amorphous specimens.     

Dielectric relaxation of collagen and gelatin

Dielectric dispersions of reconstituted collagens and gelatin were measured from 0.1 to 10 kHz and ?160 to +160°C. At 0.1 kHz there is a γ transition at ?80°C which is attributed to the H₂O-coupled local modes. The process has an activation energy of 7.5 kcal. A devitrification process is observed at 10–20°C. Both of these processes have their counterparts in the dynamic mechanical measurements. The tan δ values are up to 3 times as great for the dynamic mechanical dispersions. There is an additional hightemperature dielectric loss transition which does not correspond to any seen with the mechanical experiments. A probable mechanism for this absorption is the Maxwell-Wagner-Sillars effect.     

Dielectric relaxation time of polymers

The temperature dependence of the dielectric relaxation time of amorphous polymers can be described quite satisfactorily by an expression derived from the theory of the relaxation time for local conformational transitions in a polymer chain. This theory was recently developed using the Kramers theory of the rate constant together with the free-volume theory.     

Dielectric relaxation and elasticity during polymerization

A molecular kinetics-elasticity relation has been investigated by using real time dielectric spectroscopy of a diepoxide-triamine liquid mixture polymerizing at 298 K. As the liquid polymerized, the dielectric relaxation time tau increased linearly with the exponential of the known value of the instantaneous shear modulus G(infinity), in agreement with the elastic model for viscous flow but without the effect of temperature. Thus the structure-dependent effect on the Brownian motions are separated from the temperature-dependent effect. In this time-dependent process, increase in G(infinity) may be compensated by an increase in T, thereby keeping G(infinity) and tau constant. In the potential energy landscape paradigm, a polymerizing liquid's state point, like a normal liquid's on cooling, continuously shifts to deeper and lower energy minima of higher curvature, but the shift occurs irreversibly to other parts of the total energy landscape, thus adding a reaction coordinate to the landscape. A minimum in the energy landscape corresponding to a structure formed by polymerization may be identical to a minimum in another landscape corresponding to another structure.     

Dielectric relaxation studies of aromatic polyamides

Dielectric relaxation spectroscopy (DRS) is presented for a family of four aromatic polyamides trying to relate the structure of the lateral groups to the molecular mobility. A prominent sub-T_g absorption is always seen followed in some cases by remanent dielectric activity at room temperature and a subsequent increase of the loss permittivity. The low temperature relaxation is analyzed in terms of a Fuoss–Kirkwood equation to obtain the broadness and the strength of these relaxations as well as the activation energy (ranging from 10 to 11 Kcal/mol). The low frequency conductive peak shows in each case a half-width higher (1.30) than those corresponding to a single relaxation time peak (1.144). These values of the half-width are an indication of the complex character of these phenomena. A final discussion of the rotational barriers of the lateral chains rules out that such motions are the only molecular origin for the gamma relaxation. Instead, some kind of motion involving the main chain and where the interchain interactions play a significant role should be considered as responsible for that relaxation. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 919–927, 1997     

Dielectric and depolarization thermocurrent study of the γ relaxation in polyethylene isophthalamide

The depolarization thermocurrents (DTC) from polyethylene isophthalamide are studied below the glass-transition temperature. The “entire” DTC spectrum is analyzed in terms of a continuous distribution of relaxation times; the corresponding distribution function is very close to that which fits data deduced from bridge measurements. The complex DTC spectrum can be experimentally resolved into single relaxation time processes. The analysis of each “elementary” DTC peak yields dielectric dispersion and relaxation time which allow one to calculate the frequency variation of the complex dielectric constant. The corresponding Cole–Cole plot is in good agreement with that deduced from studies of the “unresolved” γ relaxation.     

Dielectric γ relaxation in poly(2-hydroxyethyl methacrylate)

The dielectric constant and loss were measured for poly(2-hydroxyethyl methacrylate) from ?90 to 34°C over the frequency range 30 to 3 × 10⁵ Hz in the γ relaxation region. The relaxation was also characterized using activation energies.     

Dielectric relaxation of polystyrene with side-chain- and chain-end-labels

Polystyrenes with different concentrations of side groups with cyano groups were prepared and complex dielectric constants were measured in the range of the glass transition temperature and the frequency range of 10^–2–10⁷ Hz.The GPC and DSC measurements showed that the molecular weight of these polystyrenes was about 10500 g/mole and the glass transition temperatures were 89.5°C for all samples.The dielectric relaxation spectra obtained for the side group polystyrene labels and also the chain-end polystyrene labels prepared before [9] were analyzed to find out the degree of coupling of the chain-end and side-group labels with the cooperative reorientation of the polymeric matrix. The analysis of the spectra was carried out using the analysis method developed by Mansour and Stoll [6].The results obtained showed that both end- and side-group labels are strongly coupled with the segmental reorientation and relax with relaxation times longer than that of the segments.The value of logf _m = (logf _m(label)) – logf _m(matrix)) was obtained from the recently designed comparison diagram suggested by Mansour and Stoll [6, 14]. The value of logf _m depends on the label length in the case of chain-end labels.It was surprising to find that the side groups relax slower than the segments by only 0.9 decades. These results obtained implied that the label relaxes through a multistep relaxation mechanism of the side and end groups and not through a diffusion mechanism of the whole chain. In addition, the effective lengths of the relaxing units were determined using the empirical equation obtained before in the case of rodlike molecules in polyisoprene [7].     

Dielectric relaxation and structure of liquid trialkylphosphine sulfides

Theoretical and Experimental Chemistry -     

Dielectric relaxation and glass transition temperature of copolymers

Dielectric relaxation measurements were made on methyl methacrylate—styrene and methyl methacrylate–p-chlorostyrene copolymers at temperatures higher than the glass transition temperature T_g. It was found that the temperature dependence of the relaxation time can be described satisfactorily by an expression derived recently for chain motion in amorphous polymers. The temperature T_g obtained from the expression agrees well with that determined by differential thermal analysis.     

Dielectric relaxation in concentrated solutions of poly(γ-benzyl-L-glutamate)

Measurements of dielectric constant and loss, broad-line nuclear magnetic resonance, and differential thermal analysis of concentrated solutions of poly(γ-benzyl-L -glutamate) (PBLG) were carried out to determine the effects of intermolecular interactions on the mobility of the side group and the solvent (dichloromethane or dioxane). Only one dielectric loss peak due to the cooperative motion of the side group and the solvent was found. The activation energy of this relaxation process varied from 3 to 47 kcal/mole with increasing concentration of PBLG from 20 to 100% by weight accompanied with steep increases at about 40 and 80%. This result is explained as due to entanglement of neighboring side groups. In NMR, narrowing of the line was observed near the temperature where the dielectric loss was observed. The glass transition was also observed by differential thermal analysis. From these results it was concluded that the relaxation observed in PBLG solution and in pure PBLG have the nature of primary or α relaxations.     

Dielectric relaxation processes in PVDF composite

The present study aims at the detailed elaboration of the dielectric relaxation behavior in PVDF composites using broadband dielectric spectroscopy and the Havriliak – Negami method. The composites with multi-wall nanotube carbon and zirconium dioxide in PVDF is fabricated using a simple melt mixing method. The polarization behavior in PVDF composites are investigated on the different frequency region with various temperature. The complex dielectric constants are calculated with the aid of the Havriliak – Negami equation. The characteristic parameters in Havriliak – Negami equation were in excellent agreement with the experimental complex dielectric constants. The results of utilizing these calculated parameters to analyze the origination of the polarization relaxation are given. The purposes of this work expect to give a deeper insight into the impact of different fillers on the dielectric relaxation behavior, and it could provide the technique for the discrepancy with the dipolar for interfacial polarization and the filler effect on the dielectric relaxation.     

Dielectric relaxation in some polyacrylate solutions

The dielectric constant ?′ and loss factor ?″ of poly(butyl acrylate), poly(butyl methacrylate), and poly(isobutyl methacrylate) solutions are reported in the frequency region of 1 kHz to 24.42 GHz at four different temperatures of 27, 40, 50, and 60°C. Cole–Cole plots are plotted to obtain the distribution parameter and relaxation time. The activation energies are evaluated assuming dielectric relaxation to be a rate process in these solutions. A possible relaxation mechanism is discussed.     

Dielectric relaxation pattern of dilute colloidal suspensions

An immediate method of analysis of the relaxation characteristics of a colloidal suspension, like of any dielectric, is based on the so-called Cole-Cole representation (imaginary part versus real part) of its complex dielectric constant in a wide frequency range. In this work, we show theoretical plots calculated according to the models developed by DeLacey and White (J Chem Soc Faraday Trans 2 77:2007–2039), and by Rosen et al. (J Chem Phys 98: 4183–4194; this model uses the dynamic Stern layer theory). Both theoretical approaches to the dielectric relaxation pattern of a colloidal suspension are compared to each other, and to experimental data obtained on polystyrene suspensions. Although no significant differences are found between the theoretical predictions of the relaxation patterns (except for the values of the dielectric constant; the DSL model yields higher polarizabilities of the suspensions), none of the models can exactly reproduce the frequency dependence of the dielectric constant of a colloidal system. We propose a modification of DeLacey and White's model to include the possibility that the ionic drag coefficients depend on the ion position in the double layer. The final results show that the general trends of the frequency dependence of the quantities involved are not modified, irregardless of the changes in ionic drag coefficients.     

Dielectric relaxation of electrolyte solutions in acetonitrile

Complex permittivity spectra in the frequency range 0.95v (GHz)89 for acetonitrile and its solutions of LiBr, NaI, NaClO₄, and Bu₄NBr at 25°C show one Debye equation for the neat solvent whereas the superposition of a Debye process for the solute and a Cole-Cole distribution for the solvent is necessary to account for the dielectric relaxation behavior of the solutions. The reorientation of bulk acetonitrile is diffusive and only weakly coupled to viscosity. The number of solvent molecules irrotationally bound to the electrolyte is in good agreement with conventional solvation numbers for all electrolytes, when kinetic depolarization is assumed to be negligible. The solute relaxation process is dominated by the formation kinetics and reorientation of contact ion pairs. There is evidence for solvent-shared ion pairs in dilute NaClO₄ solutions.