A.N. Lachinov

On the mutual influence of uniaxial pressure and electric field on the electronic instabilities in polydiphenylenephthalide

It is established that electric field and uniaxial pressure are interdependent, in particular in the vicinity of their threshold values and result in the alternation of the conduction state of the sample. There are three types of states: dielectric, semiconductive and metal-like. However their properties do not depend on the way these states were initiated. Pressure diminishes the threshold voltage of the electrical switching. Thus two kinds of switching which may affect each other are studied in polydiphenylphtalide. The term «switching» in this context implies that the dielectric-metal phase transition takes place not in the entire volume of the polymer, but in its separate regions, i.e. channels

A theoretical study of the chemical structure of the non-conjugated polymer poly(3,3′-phthalidylidene-4,4′-biphenylene)

Abstract The equilibrium geometry of an isolated chain of poly(3,3′-phthalidylidene-4,4′-biphenylene), or PPB, has been studied using quantum chemical calculations based upon the Austin Model 1 (AM1) method. The band structure of an isolated PPB chain has been calculated, using the AM1 geometry as input, with the help of the valence-effective Hamiltonian (VEH) model. From the electronic structure results, the density of valence states has been derived for comparison with experimental data. Both the neutral and the n-doped polymer chains have been simulated. The results are compared with features of the polymer deduced through other measurements reported in the literature.

Thermostimulated switching in thin polymer films

Abstract The paper presents results of a study on a new kind of stability observed in thin poly(3,3′-phthalidylidene-4,4′-biphenylene) films, namely, thermostimulated switching. This occurs at 265 K as a result of heating at a certain rate. After switching the sample transits to a metallic state. Investigations of the polymer dielectric loss and IR spectrum under similar heating regimes show the existence of relaxation excitations at this temperature. Another range of excitations is detected at 190 K. According to IR spectra, in the low temperature range, excitation of nonvalence aromatic distortion oscillations of the CH bonds take place, whereas CC double bonds are excited in the high temperature range. Dependence of the intensity of excitations on film thickness and heating regime allows us to suggest that they are associated with the thermostimulated ionization of trapping states. The model of polaron lattice formation is discussed as a possible explanation of the observed dielectric-metal transition.

Electron-microscopic analysis of polymer thin films capable of switching to the conductive state

Abstract This paper presents results of the electron-microscopic analysis of poly(3,3′-phthalidylidene-4,4′-biphenylylene) thin films with anomalously high spontaneous conductivity. The conductive state of the polymer is shown to be due to the formation of areas of high conductivity about 50–100 nm in size which join into larger clusters. It is established that the microscopic relief of the polymer film and its inherent texture exert no effect on the processes of switching and generation of the conductive structure.

On the influence of trapping states on electronic instabilities in polydiphenylenephthalide

Abstract A phenomenon of switching with memory in polydiphenylenephthalide (PDP) films is studied. The existence of a practically continuous spectrum of the states of polymer film from a dielectric ‘off’ to a metal-like ‘on’ is established. The switching time is less than 0.1 μs. It is shown that highly conducting channels appear as a result of switching. Their diameter is 0.1 μm or less and conductivity ranges from 10 to 10 5 Ω −1 cm −1 . This effect is only observed in pristine PDP films less than 3 μm thick. By virtue of the investigations of thermostimulated currents and optical absorption spectra it is established that thin polymer films differ from thick ones in their much higher concentration of trapping states. Simulation of such states in thick PDP films by doping showed that a critical concentration of these states must be achieved in order to realize the switching effect, no matter what the nature of the states. Analysis of the results obtained allows us to make a suggestion about the electronic character of instabilities observed in PDP.

Thin poly (3,3′-phthalidylidene-4,4′-biphenylene) films studied by scanning force microscopy

Abstract Thin 2–150 nm spin-coated films of poly(3,3′-phthalidylidene-4,4′-biphenylene) (PPB) on gold and silicon substrates with native oxides were investigated with scanning force microscopy and ellipsometry. The film morphology and film thickness were found to depend strongly upon the solvent (chloroform and cyclohexanone), the PPB concentration in the solvent, as well as the substrate.

On the high conductivity of nonconjugated polymers

The mechanism of charge transfer in a metal-electroactive polymer-metal structure has been experimentally studied near the threshold of the uniaxial-pressure-induced transition into a high-conductivity state in the polymer. The dynamics of the I-V curve is investigated as a function of the applied pressure. The data obtained are analyzed in terms of the model of injection currents using the concepts of possible scanning of a quasi-Fermi level near an injection level. Our estimates suggest that a narrow band made of deep trap states located near the Fermi level forms in the polymer film in the pretransition pressure range. In the immediate vicinity of the transition range, a narrow band of coherent charge transfer appears from these states; this band can be responsible for the high metal-type conductivity of thin polymer films, which has been repeatedly observed by many researchers.

Some chemical and electronic structures of the non-conjugated polymer poly(3,3′-phthalidylidene-4,4′-biphenylene)

Abstract The valence band structure of poly(3,3′-phthalidylidene-4,4′-biphenylene) has been studied using ultraviolet photoelectron spectroscopy (UPS). This polymer exhibits a dielectric-to-metal phase transition under the influence of external forces and is therefore a good candidate for studying charge instabilities in conjugated polymers. Interpretation of the UPS spectra is facilitated using quantum chemical calculations based on the semi-empirical Austin Model 1 (AM1) method, for geometry optimization, and the valence effective Hamiltonian (VEH) method, for calculating the energy band structure diagram, from which the density of valence states (DOVS) in the solid material is derived. The calculated DOVs is in good agreement with the experimental UPS spectra, enabling an identification of features in the experimental data in terms of specific features in the energy band structure.

Giant magnetoresistance in a ferromagnet-polymer system

Giant magnetoresistance in a ferromagnet-electroactive polymer-nonmagnetic metal structure is studied. The insulator-to-metal transition in the conductivity of the system is attributed to the change in the shape of the current-voltage characteristics of the sample in a magnetic field. It is concluded that the giant magnetoresistance observed in the experiment may be of injection nature, and this is why the effect is realized at the ferromagnet-electroactive polymer interface alone.

Metal phase in electroactive polymer induced by uniaxial pressure

Abstract The paper considers a metal state induced by a weak uniaxial pressure in electroactive polymers —polyphthalidilidenarylenes. The comparison with polyanyline is performed.