S. Sakkopoulos

Conductivity degradation due to thermal aging in conducting polyaniline and polypyrrole

Abstract The decrease of the electrical conductivity of polyaniline and polypyrrole due to thermal aging is reported. The d.c. conductivity is measured in the temperature range 300-80 K for aging times from 0 to 11 h at 120°C in room atmosphere. The conductivity of polyaniline decreases with aging time according to the law σ = σ 0 exp [ − ( t τ ) 1 2 ] . Polypyrrole diverges either from the above or σ 0 − σ ∝ t 1 2 . Moreover, polyaniline follows a σ = σ 0 exp [ − ( T 0 T ) 1 2 ] law with T0 increasing with aging time. This can be explained by a conduction mechanism consisting of electron tunnelling between conducting grains embedded into an insulating matrix. Polypyrrole follows a σ = σ0exp[ − T1/(T + T0)] law until about 5 h of aging time. For longer heating its behaviour diverges from the predictions of known models of conduction in polymers. The above can be attributed to differences of the aging process in the two compounds. In polyaniline, aging is accompanied simply by a decrease of the grain size, the separation of which increases from 27 to 54 A after 10 h of heating at 120°C. In polypyrrole, for short aging times, the grain size remains constant, their separation being about 60 A. Longer aging leads to a thermally activated conductivity whose mechanism is obscure.

Aqueous precipitation and electrical properties of In2S3: characterization of the In2S3/polyaniline and In2S3/polypyrrole heterojunctions

The stability of indium sulphide aqueous supersaturated solutions at pH 2.50 and 25°C was investigated. Spontaneous precipitation proceeded at rates proportional to the solution supersaturation via a polynuclear mechanism and the phase formed was identified as In2S3. The absorption spectrum of In2S3 was measured from 190–800 nm and from the absorption threshold the optical energy gap was estimated to be E0 = (1.8 ± 0.1) eV. The thermal energy gap Et = (1.6 ± 0.2)eV was determined from resistivity against temperature measurements and a thermopower coefficient S = −100μVK−1 at room temperature was found. Finally, In2S3/polyaniline and In2S3/polypyrrole heterojunctions were prepared and from the investigation of their I–V characteristics, the values of the ideality factor, n, the saturation current density, J0, and the effective barrier height, ϕB, were determined to be n = (15 ± 2), J0 = (38 ± 7) A m−2 and ϕB = 0.56 eV for the polyaniline and n = (64 ± 8), J0 = (13 ± 2) A m−2 and ϕB = 0.59 eV for the polypyrrole heterojunction.

Thermal degradation of the electrical conductivity in polyaniline and polypyrrole composites

Abstract The thermal aging in various conducting polyaniline and polypyrrole composites protonated with hydrochloric acid, containing polymers with sulphonic or phosphoryl groups is investigated. The DC conductivity was measured in the temperature range 300–80 K for aging times from 0 to 300 h at 70°C in room atmosphere. The conductivity decreases with aging time according to the law σ = σ 0 exp[−( t / τ ) 1/2 ], indicating an inhomogeneous structure of the granular metal type. Time τ , which specifies the aging rate, varies with the porosity and the composition of the composites from 40 to 3000 h. The porosity and the presence of sulphonic or phosphoryl groups retards the aging process. The conductivity of the composites with temperature T follows the σ = σ 0 exp[− T 1 /( T + T 0 )] law predicted by Shengs model of fluctuation-induced tunneling (FIT), which assumes conductive grains separated by insulating barriers in accordance with the thermal aging law mentioned above. Thermal degradation is attributed to the release of HCl from the samples, which reduces the protonated-conducting phase.

Low frequency dielectric relaxation phenomena in conducting polypyrrole and conducting polypyrrole-zeolite composites

The dielectric properties of polypyrrole-zeolite composites up to 50%w∕w zeolite are studied in the frequency range from 10−2to2×106Hz from room temperature to liquid nitrogen temperature. The complex permittivity formalism reveals a temperature dependent relaxation in all samples except for the 25%w∕w zeolite composite. The frequency fmax where a maximum of a loss peak is located varies with temperature by the Williams-Lander-Ferry law. The values of the activation energy of the relaxation process (which are of the order of polaronic dc conductivity) have the tendency to reach a minimum in the 25%w∕w composition, which is a loss-free composite. The 50%w∕w zeolite behaves as a dielectric where ionic relaxation dominates. The temperature variation of the strength of the dielectric mechanism follows a Curie law, apart from 50%w∕w zeolite where the dielectric strength is practically constant. The frequencies, where loss peaks are maximum, as well as dc conductivity follow qualitatively the same temperature l...

Novel pH-sensitive films constructed from polypyrrole composites

A new type of solid state chemiresistors sensitive to pH-changes is described. The chemical-sensitive layer consists of polypyrrole incorporated into a polyvinyl alcohol polymer matrix by polymerization under vacuum. Preliminary testing has shown these sensors to be stable in aqueous solutions at temperatures up to 50°C with response times in seconds. At 25°C the sensor sensitivity was 47Ω/pH. Interferences appear to be acceptably small.

Antiferromagnetism and metal‐semiconductor transition in iron sulfides FeSx, 1≤x<1.25

An explanation of the semiconductor‐to‐metal transition in iron sulfides FeSx1≤x Tβ, the antiparallel arrangement of the atomic magnetic moments ceases to exist and the impurity band as a whole bridges completely the energy gap between the valence and conduction bands, giving metallic properties to the sulfides.

Electrical conductivity and TSDC study of the thermal aging in conductive polypyrrole/polyaniline blends

The thermal aging of conducting polypyrrole/polyaniline (PPy/PA) blends heated at 70°C for up to about 600 hours was studied by d.c. conductivity measurements and by thermally stimulated depolarization current (TSDC) spectroscopy in the temperature range from 80 to 300 K. The composition of the samples varied from pure polypyrrole to pure polyaniline with the PA content increasing in steps of 10%. Although the σ = σ(T) curves seem smooth, the corresponding Δσ/ΔT = f(T) curves exhibit systematically ascattering of points in the temperature ranges from 100 to 140 K and from 225 to 320 K approximately for all the samples before and after the heat treatment. TSDC measurements with the MISIM (metal-insulator-sample-insulator-metal) configuration, show a low temperature peak at 100–120 K and a high temperature peak at 280–300 K. The first disappears when the polypyrrole content exceeds 70% and for all the samples after heat treatment, although the high-temperature peak persists. The explanation given to this correspondence between d.c. conductivity and TSDC signals is based on the destruction of conformons and the mobility change of the polymer chains with rising temperature.

An insight into the localization of charge carriers in conducting polyaniline by analyzing thermally stimulated depolarization signals

A dielectric relaxation mechanism, which is attributed to the localized motion of trapped polarons, has been recorded in conducting polyaniline by employing the thermally stimulated depolarization current technique. The signal was analyzed within the frame of the normal distribution in the activation energy value. The experimental dielectric relaxation results were manipulated in order to evaluate the length of the jump distance that the trapped polarons transfer along and the concentration of trap centers. The concentration of trapped carriers is calculated from two different viewpoints: the pair approximation that assumes phonon-assisted tunneling through the barrier separating two adjacent sites and the pinning model that considers the trapped polaron oscillating around its pinning point. Both models provide compatible results.

Cadmium sulfide precipitation in aqueous media: Spontaneous precipitation and controlled overgrowth on polyaniline

Abstract The removal of cadmium from aqueous media in the form of its insoluble sulfide is a process of interest for polluted wastewater. Moreover, the controlled deposition of cadmium sulfide on conducting polymers, such as polyaniline, may lead to the development of new photovoltaic elements. In the present work the stability of the cadmium sulfide aqueous, supersaturated solutions at pH 2.0 and 25°C was investigated. The spontaneous precipitation proceeded at rates proportional to the solution supersaturation via a polynuclear mechanism and the only phase forming was identified as α-CdS. The same salt was precipitated upon suspension, in stable cadmium sulfide supersaturated solutions of sulfonated polyaniline. The rates of precipitation increased with increasing sulfur content of polyaniline while smaller rates were obtained when using ammonium persulfate instead of potassium perchlorate as oxidant during polymerization. In all cases of overgrowth a first-order dependence of the rate of crystallization on the solution supersaturation was found, suggesting a surface controlled mechanism. Increasing sulfonation of polyaniline not only increased the rates of precipitation of α-CdS but it also resulted in increased conductivity of the polymers.

Effect of hydrostatic pressure on the d.c. conductivity of fresh and thermally aged polypyrrole-polyaniline conductive blends

In this paper we investigate the effect of hydrostatic pressure on the electrical conductivity of polypyrrole-polyaniline conductive (protonated) blends of various compositions. Results are presented for thermally aged blends as well. The modification of the conductivity due to pressure is more pronounced in the fresh samples than in the aged ones. The phenomenon is discussed within the frame of the granular model. The percentage variation of the conductivity upon pressure change for the polypyrrole rich blends is not influenced by the ageing process. This feature suggests that these blends could possibly be used as pressure sensors, which are insensitive to thermal ageing.