Subir Kumar Das

Electrochemical preparation and study of transport properties of polypyrrole doped with unsaturated organic sulfonates

Electrochemical synthesis of polypyrrole films containing vinyl sulphonate and styrene sulphonate dopants were carried out in a three-electrode one-compartment cell employing galvanostatic conditions at constant charge density. Various parameters like current density, dopant concentration, pH and temperature during synthesis were varied for both the dopant anions. Room temperature conductivity for each sample was measured and their surface morphologies were analysed from the scanning electron microscope pictures. Dependence of conductivity and surface morphology on different synthetic conditions were discussed and probable reasons for the dependence were indicated. For each of the dopant anions, resistivities were measured down to 25 K for samples synthesized at 0°C and room temperature. From the resistivity data, charge transport behaviour in these films were analysed in the light of Motts variable range hopping model. Ageing effects of conductivity for films containing both the dopant anions were studied by storing them in ambient atmosphere and measuring their conductivities from time to time.

Study of Triton X-100 surfactant-UO2+2 aqueous system by positron annihilation technique

Abstract Positron annihilation parameters have been used as a chemical probe to study the microphase transitional behaviour and microstructural changes in the aqueous surfactant system of Triton X-100. The interactions of positronium atom (Ps) in the system are discussed in the presence of a polar solubilizate e.g. UO2+2, in terms of the kinetic model. From this investigation, the microenvironment of UO2+2 is elucidated. An independent fluorescence spectroscopic assay of the UO2+2 ions in the micellar phase was made. These findings corroborate the information drawn from positron annihilation data.

Modification of the bubble model for positronium atoms in high surface tension liquids

The bubble model is widely used for the analysis of the decay characteristics of the positronium atom in liquids. However, according to some authors this description is inappropriate in the case of polar liquids with high surface tension. It has been advocated that for such media, rather than employing the value of the bulk surface tension (which appears as a parameter in the model), one should incorporate the notion of a transition layer between the liquid phase and the cavity that encloses the positronium. Accordingly, the usual bubble model with sharp boundaries is modified in the present work through the introduction of a diffusivity in the bubble surface, thus developing the proper setting (without involving any extra free parameters) such that liquids with high surface tension may also be meaningfully discussed in this context and confronted with experimental data.

Effect of temperature on the synthesis of FeCl3-doped polypyrroles studied by positron annihilation technique

Abstract FeCl 3 -doped polypyrrole samples have been synthesized at different temperatures, at 308 K and below. The induced defects in the doped structure were analyzed by positron lifetime and Doppler broadening of the annihilation radiation. The magnitude of the conductivity measured in the samples was found to increase monotonically with the induced defect concentration.

Self association of sodium salicylate system

Abstract The aggregation behavior of sodium salicylate (SS) in aqueous solutions has been studied by positronium annihilation technique. From the change of the pick-off life time as well as the intensity of the ortho -positronium component, it has been concluded that the minimal hydrotropic concentration (MHC) of SS lies in the range 0.9–1.0 M concentration. To support the above contention, the molecular association of SS has also been investigated by the changes in light scattering intensity, conductivity as well as NMR data. These experiments conjointly infer that the stacking of SS molecules appears to start beyond 0.9 M concentration.

Dependence of positronium formation on critical micelle concentration: A phenomenological approach

Employing and enlarging the scope of the diffusion-recombination model, proposed by Byakov and Stepanov (1993), for positronium formation in aqueous solutions of surfactants, the general trend of the dependence of positronium formation on Critical Micelle Concentration (CMC) has been successfully explained.

Positronium reactions in the triton X-100-p-nitro phenol system

Abstract The positronium reactions have been used as a probe to study the solubilization of p-nitro phenol within the micellar phase of a Triton X-100 solution and the corresponding changes in the molecular association phenomenon. The presence of p-nitro phenol resulted in an enhancement of micellization which has been corroborated by surface tension measurements. This paper also lays emphasis on the secondary aggregation phenomenon of Triton X-100 molecules at the far post-micellar stage (≈10–15 mM). The solubilization of p-nitro phenol at various stages of aggregation has been discussed through the interaction with positronium atoms by setting up a kinetic model and reaction equilibria.

Guest molecules in Triton X-100 microemulsion: studied by positron annihilation technique and absorption spectroscopy

Abstract Positron annihilation lifetime measurements have been performed in water in oil (w/o) microemulsion (ME) of Triton X-100/butanol, n -heptane system in the presence of additives, UO +2 2 and p -nitro phenol (PNP). The microenvironment of the guest molecules in the ME phase was studied by the positronium (Ps) reaction-rate constant. The Ps reaction rate constant values with UO +2 2 in ME changed gradually as a function of water content in ME phase where as a considerable enhancement of Ps reaction rate constants with PNP was observed in the same phase. Furthermore, the solubilization of the probe molecules in the different microphase of the ME system was investigated through the UV-VIS absorption spectra. The results show that UO +2 2 ions were preferentially occupied in the polar water pool of ME but PNP molecules were distributed in the interfacial region (intermediate polarity) of the ME phase.

Positron annihilation study on γ-irradiated cellulose acetate matrix

Abstract A cellulose acetate matrix was irradiated in the dose range 0–250 kGy by y radiation from a 60 Co source under an argon atmosphere; positron lifetime experiments were performed under the same atmosphere at room temperature. The results reveal a dominant intermediate component τ 2 (∼430 ps) attributable to a crystalline macrocellulosic domain. Important changes are shown by the long-lived pick-off component τ 3 due to the degradation in the polymer matrix.

Positronium formation in a reverse micellar system

Abstract The observed decrease in intensity of the long-lived component in positronium decay at a critical surfactant concentration in non-polar solvents, concommitant with the formation of reverse micelles, is described through a diffusion-recombination model. It is emphasized that phenomenological studies, such as the present one, are important for the development of an understanding of positronium formation in liquids for which as yet no satisfactory theory exists, and strengthens the use of positron annihilation as a tool, for the study of the aggregational behaviour of surfactant molecules, the characteristics of which are different for polar and non-polar solvents.