J. Kroh

Some aspects of the radiation processing of conducting polymers

Abstract Recently, in our laboratory (IARC), complex studies have been made on conducting polymers such as polyaniline, polypyrrole and polythiophene. Our research programme has been directed toward a better molecular understanding of the structural properties controlling electronic conductivity of polymers. The most important issue of our studies within conducting polymer family includes effect of γ and electron beam irradiation on their electrical conductivity. The irradiation was performed on the samples in the form of pellets onto which aluminium electrodes had been vacuum-deposited. The relative conductivities of irradiated pellets prepared from highly conducting polyaniline, polypyrrole and polythiophene powders decreased with increase of irradiation dose under vacuum. On the other hand in the case of irradiation under dopant atmosphere conductivity of pellets was increased. The dramatic increase in conductivity was observed after irradiation of insulating polyaniline form (even under air atmosphere). The ESR studies suggest that the charge-transport in low doped conducting polymer can be explained by paramagnetic polaron migration along and between the polymer chains. At higher doping levels, in the case of polythiophene and polypyrrole the energetically favourable is a bipolaron state as charge carrier.

Kinetic model for electron tunneling in frozen irradiated systems

A kinetic model for tunneling of the trapped electron e−t from its trap to a scavenger S has been developed. The probability of the transition of e−t through the three‐dimensional potential barrier has been calculated based on the theory of quasistationary states. The transparency of the barrier has been expressed in terms of (1) the distance e−t−S and (2) the energy level in the acceptor molecule. In order to calculate the kinetics of e−t decay, the distribution of e−t−S distances, and the energy levels of electrons in the traps have been taken into account. The model has been numerically applied to the decay of electrons trapped in the glassy 8M NaOH in the presence of NO−3 used as an electron acceptor. The presented model does not allow one to calculate the density of energy levels in the scavenger accesible to electron. Therefore, this parameter has been chosen to provide the best fit of the theory to the experimental data. The observed time dependence as well as scavenger concentration dependence are...

Electron tunnelling in frozen aqueous solutions at 77 K

Abstract The decrease in concentration of stabilized electrons in frozen aqueous solutions at 77 K is described and assigned to tunnelling.

Excimer formation in irradiated high density polyethylene doped with aromatics

Abstract Luminescence from γ-irradiated high density polyethylene doped with naphthalene and pyrene was investigated. Excimer emission of the aromatic admixture was observed in the temperature range (200–230 K) of the high temperature radiothermoluminescence peak. During nanosecond pulse radiolysis, traces of excimer emission were recorded in the same temperature range. Pyrene excimer emission was observed from pulse-irradiated samples at room temperature. The mechanism of excimer formation is discussed.

Effect of ionizing radiation on polyaniline solutions

Abstract This communication presents the optical studies associated with transition doped (metallic)-neutral (semiconductor or insulator) state for conducting polymers. Special attention is focused on the electronic properties of polyaniline. The interconversion of different oxidation states of polyanilines has been studied by chemical and radiolytic methods. The polyaniline system is described by three sets of chromophores of three different oxidation states: fully reduced leucoemeraldine base (LB), partially oxidized emeraldine base (EB), and fully oxidized pernigraniline (PB). Each oxidation state can exist in its protonated form by treatment with an acid. All members of polyaniline family are spectroscopically distinguishable. The radiolytic study presents evidence that the polyaniline can exist in a continuum of oxidation states. The highly conducting form of polymer, i.e. emeraldine salt can be converted by using ionizing radiation into leucoemeraldine salt. The leucoemeraldine base is the final product of radiolysis of emeraldine base solution. The fully oxidized form of polyaniline can also be obtained by the irradiation of EB in the presence of CCl4 or chlorobenzene.

Concentration dependence of pyrene excimer formation in doped polyethylene

Abstract Pyrene-doped low density polyethylene films were studied by steady state and time-resolved fluorescence emission spectroscopy at room temperature for dopant concentrations up to about 1.5×10−2 mol dm−3. The excimer fluorescence observed in these films for a high pyrene concentration (about 10−2 mol dm−3) was proposed to result either from pyrene molecular pairs close to the excimer configuration or from pyrene aggregates which satisfy the geometrical requirements for excimer formation.

Some aspects of electron tunnelling in radiation chemistry of glassy matrices

Abstract The paper presents a review of the interpretative applications of long-range electron tunnelling in relation to the scavening phenomena in glassy solids. The theory of tunnelling is briefly discussed. The decay of trapped electrons, e−t, as a function of time is illustrated for aqueous and hydrocarbon glasses. The influence of scavenger concentration and its nature on the efficiency of tunnel transfer of electrons are also discussed. The nature of stabilizing matrix, the trap depth and the temperature are the other factors which are shown to affect tunnelling. It is indicated that in order to explain the observed relationships the more sophisticated model of tunnelling is required. The interpretation of some other phenomena in radiation chemistry on the basis of tunnelling is also briefly exemplified.

Pulse radiolysis study of charge transfer processes in polyethylene film containing two solutes

Abstract Intermolecular charge transfer reactions were studied by the pulse radiolysis of polyethylene containing two solutes, i.e. dimethyldiphenyl (DMD) and pyrene (Py). Positive and negative charges can be transfered from DMD ions to Py. Kinetic measurements of Py ion growth and DMD ion decay allowed us to calculate the activation energies ( E a ) for these charge transfer processes. E a was found to be temperature dependent and equalled 83.1–88.9 and 1.8–4.4 kJ mol −1 for T > T g and T T g respectively.

Do trapped electrons tunnel to scavengers in irradiated alcohol glasses

It is suggested that electrons trapped in irradiated 95% alcohol glasses (+ 5% H2O) do not react with the scavenger molecules and probably recombine with geminate cations. The scavenging has been interpreted in terms of dry electron reaction.

On the mechanism of decay of trapped electrons at very low temperatures

Abstract To explain the decay of electrons trapped in water-ethanol glasses, it is proposed that at liquid helium temperature the electrons decay by proton tunnelling and at higher temperatures by migration of depply trapped electrons.