Exchange Process in the Dielectric Loss of Molecular and Macromolecular Ionic Conductors in the Interfacial Layers Formed by Electrode Polarization Effects

Here we report a first analytical approach to determine the spectral dependence of the complex permittivity function of molecular and macromolecular ionic conductors in the interfacial layers formed by electrode polarization effects. We show that a previous model of electrode polarization effects that was successfully applied for describing the dielectric behavior of ionic liquids ( Serghei, A.; Tress, M.; Sangoro, J. R.; Kremer, F. Electrode polarization and charge transport at solid interfaces. Phys. Rev. B 2009 , 80 , 184301 ) can be now generalized and applied for polymer/ionic liquid blends as well as for poly(ionic liquid)s. The determined dielectric function of the interfacial layers reveals a dramatic change in the charge transport process manifested by a large decrease in conductivity. Our approach brings the first evidence for a relaxation peak detected in the dielectric loss of the interfaces, which is attributed to an exchange process between the interface and the bulk. This study gives new insights into the mechanism of charge transport at interfaces and could thus contribute to a better correlation between the dielectric properties of ion conducting materials and their electrochemical behavior at interfaces.

Electrode polarization vs. Maxwell-Wagner-Sillars interfacial polarization in dielectric spectra of materials: Characteristic frequencies and scaling laws

The characteristic frequencies of electrode polarization and of interfacial polarization effects in dielectric spectra of ionic liquids and of polymer bi-layers are determined and systematically analyzed, based on dielectric measurements by means of broadband dielectric spectroscopy, numerical simulations, and analytical calculations. It is shown that, to a large extent, identical scaling laws can be derived for these two dielectric phenomena taking place at external and internal interfaces. Surprisingly, a fundamentally different behavior concerning the interrelation between the characteristic frequencies is found. This brings direct evidence that different manifestations of the phenomenon of electrical polarization can be discriminated by examining the inter-relation governing their characteristic frequencies, which can be of significant importance in disseminating the nature of different contributions appearing in the dielectric spectra of complex materials. Based on our analysis, we derive a new formula, valid for both electrode polarization and interfacial polarization effects, that allows one to determine the conductivity value from the frequency position of the Maxwell-Wagner-Sillars peak. An excellent agreement between experiment and calculations is obtained. The formula can be used, furthermore, to estimate the thickness of the interfacial layers formed due to electrode polarization effects. Values in the order of several nanometers, increasing with decreasing temperature, are reported.

[Peripartum cardiomyopathy: incidence, pathogenesis, diagnosis, treatment and prognosis]

INTRODUCTION

Peripartum cardiomyopathy (PPCM) is a rare and life-threatening disease of unknown aetiology. The primary objective of this review was to analysed aetiopathogeneses, clinical presentation and diagnosis, as well as pharmacological, perioperative and intensive care management and prognosis of this pathology.

METHODS

We undertook a systematic review of the literature using Medline, Google Scholar and PubMed searches.

RESULTS

Unlike other parts of the world in which cardiomyopathy are rare, dilated cardiomyopathy is a major cause of heart failure throughout Africa. Its aetiopathogenesis is still poorly understood, but recent evidence supports inflammation, viral infection and autoimmunity as the leading causative hypotheses. This diagnosis should be limited to previously healthy women who present with congestive heart failure (CHF) and decreased left ventricular systolic function in the last month of pregnancy or within 5 months after delivery. Recently, introduction of echocardiography has made diagnosis of PPCM easier and more accurate. Conventional treatment consists of diuretics, vasodilators, and sometimes digoxin and anticoagulants, usually in combination. Patients who fail to recover may require inotropic therapy. In resistant cases, newer therapeutic modalities such as immunomodulation, immunoglobulin and immunosuppression may be considered. Prognosis is highly related to reversal of ventricular dysfunction. Compared to historically higher mortality rates, recent reports describe better outcome, probably because of advances in medical care. Based on current information, future pregnancy is usually not recommended in patients who fail to recover normal heart function.

CONCLUSION

PPCM is a rare but serious form of cardiac failure affecting women in the last months of pregnancy or early puerperium. Its aetiopathogenesis is still poorly understood. Introduction of echocardiography has made diagnosis of PPCM easier and more accurate. Prognosis is highly related to reversal of ventricular dysfunction.