H. Isotalo

Thin polyaniline films in EMI shielding

Abstract Electromagnetic interference shielding efficiency has been measured for highly electrically conducting Polyaniline-Camphor Sulfonic Acid. The polymer is spin coated from m-cresol solution on an electrically insulating substrate as a 1–30 μm thick layer having a conductivity of 10–100 S/cm. The shielding efficiencies (SE) for these electrically thin polyaniline films have been measured in the near-field with a dual-chamber box and in the far-field with a transmission line method in the frequency range 0.1–1000 MHz. The measurements show that SE depends primarily on the surface resistance both in the far-field and the near-field. An additional effect >10 dB is seen when the two layer structure is measured in the near field. By using layer structures, the SE is >40 dB up to ca. 100 MHz in the near-field and 39 dB at 1 GHz in the far-field.

Melt and solution processable poly(3-alkylthiophenes) and their blends

Abstract We report on the properties of melt and solution processable poly(3-alkylthiophenes), P3AT, and on the fabrication of electrically conducting polymer blends consisting of P3AT, specifically poly(3-octylthiophene), POT, and a thermoplastic matrix polymer made using ordinary melt processing techniques. The POT used in this work was synthesized using two different chemical polymerization techniques. Films made from as-synthesized POT powder, either by solution casting or melt processing techniques were found to be easily doped by electron acceptors to conductivities of 20–30 S/cm. Blending of POT with thermoplastics such as ethylenevinylacetate copolymer, EVA, in the molten state yields polymer blends with excellent mechanical properties that might be doped to conductivities exceeding 1 S/cm.

Synthesis and properties of FeCl4-doped polythiophene

Abstract Electrochemically synthesized polythiophene films containing BF 4 − -anions, PT-BF 4 , have been carefully reduced and re-oxidized with FeCl 4 -anions to obtain highly conducting PT-FeCl 4 -films. The maximum conductivity we have observed for this complex is 50 Scm −1 which corresponds to a maximum dopant concentration of approximately 26 mol-%. Thermoelectric power (TEP) measurements of lightly and heavily doped PT-FeCl 4 show that the positive TEP decreases drastically from 614 μV/K to 10.5 μV/K when the conductivity increases from 1.1 10 −5 to 10.1 Scm −1 . In heavily doped samples TEP is slightly overlinear, a phenomenon which is explained by a model based on conducting strands separated by thin potential barriers. PT-FeCl 4 -films exhibit promising stability properties, the conductivity remaining in the metallic regime for at least 16 months under ambient conditions.

Electrical conductivity of synthetic DOPA-melanin polymer for different hydration states and temperatures

The dependence of the d.c. conductivity on the hydration and temperature (293-343 K) for synthetic DOPA-melanin polymer is presented. The hydration state of the melanin has been changed by varying the humidity conditions around the sample. It has been shown, that in the range of relative humidity values (0-100%), changes in the hydration state of melanin have predominant influence on electrical conductivity (10(-13)-10(-5)S cm-1) in comparison to temperature. The influence of the two forms of water on the conductivity--the first form adsorbed mainly on the melanin surface and easily removed by drying, and the second one incorporated into the inner structure of the polymer, has been investigated. The temperature dependence of the conductivity in vacuum (0.8 and 0.04 mb) and thermal activation energy values (0.49-0.76 eV) for cooling and heating curves have been determined. The relationship between thermal activation energy and preexponential factor sigma 0 (compensation effect) and possible charge transport mechanisms are discussed.

Conductivity, thermoelectric power and field-effect mobility in self-assembled films of polyanilines and oligoanilines

Abstract Thin conducting films of polyanilines and oligoanilines were deposited by using a layer-by-layer self-assembly process. Conductivity, thermoelectric power and field-effect measurements were performed on the films. The highest conductivities were around 1 S cm −1 at room temperature. The conductivity was studied as a function of temperature and was found to obey the T − 1 2 law of variable-range hopping. The thermoelectric power of the films was initially n-type in air at room temperature, but changed its sign to p-type under vacuum or with time. A linear temperature dependence was observed and related to the contribution from metallic polymer chain segments. The field effect, measured using thin film transistor structures, was always n-type and the field-effect mobilities about 10 −6 -10 −3 cm 2 V −1 S −1 . The Coulomb gap model gives a good description of the system still lacking an expression for the field effect.

Heat-induced transition to the conducting state in polyaniline/dodecylbenzenesulfonic acid complex

Abstract Polyaniline (PANI) doped with dodecylbenzenesulfonic acid (DBSA) is a new conducting polymer complex which combines high electrical conductivity with processability. The doping reaction of PANI/DBSA has been found to be a time-dependent process which can be accelerated by heating. This process has been studied by measuring the conductivity during heat treatment in complexes with a molar DBSA ratio, relative to PANI, varying from 0.55 to 1.12. The temperature for the onset of conductivity depends on the mixture ratio. Conductivities on the order of 1 S/cm were achieved.

Lithographic patterning of conductive polyaniline

Abstract Electrically conducting patterns of polyaniline are made by utilizing conventional semiconductor industry process. First polyaniline is spin- or spray-coated on an insulating substrate and has a conductivity of 1–100 S/cm. After that UV resist is spread on top of polyaniline, exposed by UV light, developed and removed. As a result one has a patterned polyaniline layer in insulating and in conducting form on top of the substrate. The conductivity remains essentially unaffected below the resist throughout the process and polyaniline turns insulating at places where the resist is removed. The difference between the electrically conducting part and the electrically insulating part is upto 10 10 . When the linewidth is smaller than 100 μm the square resistance increases slightly, because the deprotonating liquid penetrates somewhat below the resist. Linewidths down to 10 μm have been demonstrated. The process has been utilized in making all-polymer circuit boards having resistors and capacitors made of polyaniline.

Effect of Cu2+-ions on semiconductor properties of synthetic DOPA melanin polymer

The purpose of the present study was to examine semiconductor properties of synthetic DOPA melanin, which are basic for future biological applications. DC conductivity, electron spin resonance (ESR), and atomic absorption spectroscopy (AAS) measurements have been performed to investigate the effect of Cu(2+)-ions on the semiconductor properties of melanin polymer synthesized from DOPA (3,4-dihydroxyphenylalanine). DOPA melanin - Cu2+ complexes examined show the decrease of both thermal activation energy delta Ea and pre-exponential factor sigma o values upon doping. At the same time no substantial changes in conductivity at 293 K have been observed. Formation of bipolaron states due to chelation of copper ions by melanin orthosemiquinones has been postulated. The Meyer-Neldel rule with a characteristic temperature T0 equal to 298 K and possible physiological implication of this fact are discussed. These data suggest, that DOPA melanin polymer could be useful as a type of culture substratum.

Pressure dependence of d.c. conductivity in poly(3-alkylthiophenes)

Abstract Pressure dependence of d.c. conductivity of poly(3-octylthiophene) (POT), poly(3-decylthiophene) (PDT) and poly(3-dodecylthiophene) (PDDT) has been measured under hydrostatic pressure up to 6 kbar as a function of FeCl 3 doping. In all cases conductivity increased with pressure. The increase depends on the ambient pressure conductivity and side chain length. A maximum in the pressure dependence was found in the low conductivity region. With increasing side chain length, the pressure dependence increased and its maximum shifted to higher conductivity. At dilute doping levels Kivelsons theory for intersoliton hopping was applied. The three-dimensional variable range hopping model was used in the intermediate doping region. The Kivelson theory was applied to give numerical values for the bulk compressibility ranging from 2 × 10 −10 m 2 /N (POT) to 6 × 10 −10 m 2 /N (PDDT). In the metallic region the measured pressure effect on conductivity is small and less dependent on the side chain length.

Metallic transport properties of conducting Langmuir-Blodgett films

Abstract The conductivity of the Langmuir-Blodgett (LB) films was examined in terms of a granular structure. The conductivity of bis(ethylenedioxy)tetrathiafulvalene-decyltetracyanoquinodimethane (BO-C 10 TCNQ) LB film was well fitted by the formula, σ = AT − α exp(-E a /kT), where the behavior is determined by the semiconducting domain boundaries in the granular-structured film. The metallic nature of the domains was revealed by thermoelectric power measurements: the temperature dependence of the thermoelectric power was almost linear with T. The temperature dependence of the conductivity in tridecylmethylammonium-Au(dmit) 2 (3C10-Au) LB film is well explained by a linear combination of Shengs model and common metal, R = C 1 exp [ T 1 ( T+T 0 ) ] + C 2 T . The conduction was governed by the fluctuation-induced tunneling between metallic domains.