Humidity sensitive properties and sensing mechanism of π-conjugated polymer p-diethynylbenzene-co-propargyl alcohol

A novel thin film resistive humidity sensor based on soluble FeCl₃-doped copolymer of p-diethynylbenzene with propargyl alcohol (p-diethynylbenzene-co-propargyl alcohol, DEB-co-OHP) was prepared. Its humidity sensitive properties were investigated by electrochemical methods (complex impedance spectra, chronoamperometric and voltammetric measurements), and sensing mechanism presented. The impedance of the sensor changed from 10⁷ to 10⁴ Ω in the range of 33–95% relative humidity (RH), showing high sensitivity. Comparison of steady-state current–RH plots of film with and without DEB-co-OHP indicated that conjugated polymer plays a dominant role in the realization of conductivity change with humidity.     

New synthetic routes to poly (isothianaphthene) I. Reaction of phthalic anhydride and phthalide with phosphorus pentasulfide

A new route for the formation of the low bandgap polymer poly(isothianaphthene) is presented. The route comprises the reaction of phthalic anhydride or phthalide with phosphorus pentasulfide at elevated temperatures (T ⩾ 120 °C), and leads to the formation of poly(isothianaphthene) in one single step. The product obtained was analysed by elemental analysis, IR, Raman and solid state NMR spectroscopy. Chemical doping and dedoping of the material were investigated, and the maximum conductivity obtained was 10 S cm⁻¹ by doping with NOSbF₆. Both doped and undoped samples of poly(isothianaphthene) prepared by this new route were thermally very stable in air or in an inert helium atmosphere as shown by thermogravimetric analysis. Furthermore, the conductivity of a doped sample remained unchanged up to 250 °C in air. Preliminary results showed that the reaction with phosphorus pentasulfide can also be used to obtain nitrogen analogues of poly(isothianaphthene).     

Stable polyaniline dispersions prepared in nonaqueous medium: synthesis and characterization

Peroxodisulfate-induced polymerization of aniline at 10°C in acidic (HCl) nonaqueous medium using dimethyl sulfoxide (DMSO) as the solvent, readily produced polyaniline (PANI) in a stable dispersion form. The stability of the dispersion of PANI in the nonaqueous (DMSO) medium is much enhanced when the same is synthesized in the presence of a support polymer, poly(vinyl alcohol) (PVA) dissolved in the solvent. Results of studies on HCl-doped PANI prepared in DMSO medium by UV–VIS and FTIR spectroscopy support that the doped PANI so obtained is structurally similar to that of doped PANI prepared likewise in acidic aqueous medium. PANI and PANI–PVA composite as prepared in DMSO medium and the relevant isolated dry products were further characterized thermally, employing differential scanning calorimetry (DSC) and thermogravimetry (TG) and morphologically, employing transmission electron microscopy (TEM). HCl-doped PANI prepared separately in DMSO and aqueous media shows electrical conductivity values of 1.07 and 12.0 S cm⁻¹, respectively, the polymer prepared in nonaqueous medium (DMSO) being measurably poorer in electrical conductivity,     

Erosion–corrosion behavior of aluminum in a 50°C ethylene glycol aqueous solution simulating cooling water in HVDC transmission

The erosion–corrosion of aluminum in a 50 °C ethylene glycol aqueous solution (EGAS) was studied. Compared with that in deionized water, the corrosion of aluminum in an EGAS was inhibited to a certain extent. The corrosion potential, corrosion current, and charge transfer impedance of aluminum changed from −1.5776 V, 380.4 nA cm⁻², and 1.924 × 10⁻⁴ Ω cm⁻² in deionized water to −1.3127 V, 285.5 nA cm⁻², and 4.041 × 10⁻⁴ Ω cm⁻² in a 45.3 vt% EGAS, respectively. Ethylene glycol did not ionize in deionized water and the ionic conductivity of the EGAS was low, effectively restraining the corrosion of aluminum. However, a test with aluminum in an EGAS after long-term storage (9 days) showed that ethylene glycol gradually oxidized to glycolic acid, oxalic acid, and other substances, which slowly corroded the aluminum surface. Analysis results showed that the corrosion products on the surface of aluminum were Al(OH)₃ and Al₂O₃. The pitting hole formation mechanism of aluminum occurs via an aluminum–alcohol phase formed on the aluminum surface, which can inhibit the dissolution of the oxide film. Therefore, a suitably concentrated EGAS with a high heat capacity and low ionic conductivity similar to that of deionized water can be used as a coolant in airtight valve cooling systems for high-voltage direct-current transmission.     

Spectroscopic studies of CSA-doped poly[C-hydroxyl-(4-N-dimethylamino)phenyl]dithienylmethine and doping effects on ionic conductivity

Protonation of poly[C-hydroxyl-(4-N-dimethylamino)phenyl]dithienylmethine with (1S)-(+)-10-camphorsulfonic acid (CSA) induced a structural change on the pendant group of the polymer and resulted in the formation of a cationic quinoid iminium moiety. This chemical transformation changed the polymer from a non-conducting polymer into a semi-conducting polymer with the maximum ionic conductivity of 2 × 10⁻⁴ S cm⁻¹ at 10 mol% CSA-doping level. The polydithienylmethines doped with various CSA loadings were characterized by UV–vis-NIR, FT-IR and Raman spectroscopy. An ion-hopping mechanism was suggested as the conduction mechanism for this CSA-doped polymer.     

An in situ ESR study on poly(3-methylthiophene): charge transport due to polarons and bipolarons before the evolution of metallic conduction

In situ ESR measurements with ClO₄⁻-doped poly(3-methylthiophene) (PMT) are carefully performed over a wide range of doping level from 0.02% to 23% in order to correlate a drastic increase of carrier mobilities by doping with the nature of charged species in the π-conjugated polymer. Either or both of a Gaussian and Lorentzian components with different g-factors are observed, depending on the doping level. The broad Gaussian signal ascribable to structural defects in the polymer remains almost unchanged at various doping stages while the Lorentzian signal varies greatly with the doping level. From spin concentrations for the Lorentzian signal, molar fractions of polarons and diamagnetic species (bipolarons) are evaluated as a function of doping level. Combination of the molar fractions and the mobility data obtained earlier demonstrates that the conduction process in the lightly doped PMT film (0.02%–1%) can be expressed by independent driftings of polarons and bipolarons under electric field, whose mobilities are 2×10⁻⁵ and 10⁻³ cm² V⁻¹ s⁻¹, respectively. At higher doping levels of 1%–23%, conductivity is much more enhanced than expected from this model and the line width of the Lorentzian signal increases from 1.2 to 7.5 G, suggesting the evolution of a metallic conduction.     

E.S.R. of BF4−-doped polythiophene

We have studied poly(3-methylthiophene) doped electrochemically with BF₄⁻ by electron spin resonance. The spin concentration increases linearly by about one spin per injected charge up to quite high doping levels (≈ 10 mol% at room temperature). The magnetic susceptibility χ_m(T) gradually changes from a Curie- to a Pauli-type behaviour with increasing doping concentration. Simultaneously, an increasing degree of delocalization of the corresponding spin centres is inferred from linewidths, line shape analysis and spin-lattice relaxation rates. The metallic behaviour observed by e.s.r. at high doping levels is attributed to a microscopic electrical conductivity, which is higher than the macroscopic one observed by transport measurements by a factor of about 50. The present results demonstrate that polarons are the dominant charge states in poly(3-methylthiophene) doped with BF₄⁻.     

Optical,electrical and electrochemical characterization of electrosynthesized polythieno(3,2-b)thiophene

A new conductive polymer is prepared by electropolymerization of thieno(3,2-b)thiophene. Electrochemical characterization by cyclic voltammetry shows that it can be repeatedly driven between the doped and the undoped species. The FT-IR spectra of both materials are compared and the electrical conductivity is shown to be 3 × 10⁻⁶ S cm⁻¹.     

Conducting polymers from anodic coupling of dithienylacetylenes. Electrochemistry and potential-driven conductive and magnetic properties

Anodic coupling of dithienylacetylene and dithienylacetylenes substituted with alkyl and alkoxy groups has been investigated in acetonitrile. The resulting polymers have been characterized by cyclic voltammetry, UV-Vis spectroscopy, electrochemical quartz crystal microbalance, in situ ESR and in situ conductivity. The degree of conjugation of the polymers is increased as the monomer oxidation potential is lowered, which occurs with alkyl-substitution and even more with alkoxy-substitution. Alkoxy-substitution in position 3 of the thiophene ring produces the dimer, whereas 4-substitution yields an extensively crosslinked polymer. A regularly coupled polymer is obtained by 3,4-disubstitution with the ethylenedioxy group. The low conductivity (1 × 10⁻³ S cm⁻¹) and the high unpaired spin density in the oxidized state of this polymer are explained by localization of charge and spin operated in the dithienyl moieties by the triple bonds.     

Humidity sensitive properties of substituted polyacetylenes

Thin film resistive humidity sensors have been prepared based on a series of homogeneously doped soluble substituted polyacetylenes synthesized using palladium acetylide complex catalyst. The humidity sensitive properties of the polymers (poly(propiolic acid) (PPA); poly(propiolic acid-co-ethynylbenzene) (PA-co-EB); poly(propiolic acid-co-p-diethynylbenzene) (PA-co-DEB); poly(propiolic acid-co-propargyl alcohol) (PA-co-OHP)) doped with FeCl₃, H₂SO₄ and HClO₄ have been investigated and compared. The chemical structures of polymers and the doping agents have great influence on the sensing properties of the sensors. A sensor based on PA-co-OHP doped with HClO₄ shows the best response. Its logarithm of impedance varies linearly with relative humidity (RH) for four orders of magnitude (10⁷–10³ Ω) over a wide range of 30–95%RH, and the response time is <6 and <15 s for absorption and desorption, respectively. Furthermore, the effect of temperature on the sensing behaviour of the polymers has also been described.     

Electrical properties and spectroscopic studies of HClO4-doped poly(p-diethynylbenzene)

Electrical properties of poly(p-diethynylbenzene) (PDEB), chemically doped with perchloric acid (HClO₄), were investigated as a function of dopant concentration and relative humidity. The doped polymer was characterized by X-ray photoelectron spectroscopy (XPS), UV—Vis, IR, electron spin resonance (ESR) and scanning electron microscopy (SEM). A possible structure of the HClO₄-doped PDEB was proposed on the basis of spectroscopic studies. In the presence of humidity, HClO₄-doped PDEB follows the proton conduction mechanism.     

Low-frequency electrical conductivity of poly(phenylacetylene)-iodine in tetrahydrofuran solutions: effect of iodine concentration

The electrical conductivity of poly(phenylacetylene)-iodine in tetrahydrofuran solution has been measured at the frequency of 10 kHz, in the temperature range from −15 to 40 °C as a function of iodine concentration. The data have been analysed on the basis of the counterion fluctuation model developed for highly charged polyelectrolyte aqueous solutions. The dependence of the electrical conductivity on the iodine concentration suggests that the conduction mechanism is governed by the interaction of the polymer with iodine molecules, resulting in a charge-transfer complex in solution, the effectiveness of which is maximum at some well-defined concentrations corresponding to six polymer units for each I₅⁻ ion.     

Electrical properties of 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl] benzene

The increase of the electrical conductivity of the arylenevinylene (AV) system 1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl] benzene was measured as a function of the doping level of iodine. Compressed powders and films blended with polystyrene (PS) containing up to 30 wt.% of the oligomer were used. At the highest doping level the conductivity of the compressed powder reaches a value of 3.3×10⁻⁴ Ω⁻¹ cm⁻¹. The conductivity, measured between 90 and 300 K on the powder with the highest conductivity, is thermally activated with an activation energy of ∼0.1 eV. The thermopower is positive in agreement with a charge transfer from the organic chain to an iodine atom. Its value is small (25 μV K⁻¹) and temperature independent. The results are interpreted in terms of small-polaron hopping.     

Electrical and optical properties of highly conducting charge-transfer complexes of poly(p-phenylene)

Electrical conductivity, Hall effectm and spectroscopic measurements have been made on AsF₅-doped poly(p-phenylene). Doping increases the conductivity of the parent polymer by as much as 14 orders of magnitude to values as high as 5 × 10⁴ S/m. Hall effect measurements indicate p-type conduction with a Hall mobility approaching 10^?4 m²/V s for doping levels between 0.24 and 0.42 moles of AsF₅ per mole of monomer. Doping with an electron donor, K, has increased the conductivity to about 10³ S/m for a doping level of 0.57 moles of K per mole of monomer. Using this conductivity value, with the assumption of total charge transfer from the donor, suggests a drift mobility for electrons which is significantly less than that for holes. The assumption of an intercalant structure analogous to that of polyacetylene and graphite leads to the conclusion that the presently achieved AsF₅-doping levels in poly(p-phenylene) correspond to a compound which is not wholly stage 1.     

Study of IF5 chemical doping of aluminium polyfluorophthalocyanine: (PcAlF)n

Chemical IF₅ doping of aluminium polyfluorophthalocyanine, (PcAlF)_n, has been studied by thermogravimetric analysis, infrared spectroscopy, elemental analysis and electrical conductivity measurements. An optimum electrical conductivity (σ = 6 × 10⁻² ω⁻¹ cm⁻¹) is achieved for a doping rate lying in the range 0.5 - 0.8 mole of IF₅ per (PcAlF) ring. The decrease of electrical conductivity of the doped material measured for higher doping rates is shown to be related to the disappearance of the eclipsed configuration in the stacking of the rings, induced by excess inserted dopant species. These results confirm those previously published concerning the AsF₅ doping of the polymer and can be explained with the help of the same structural model.     

Characterization of polyaniline doped with diphenyl phosphate

UV–Vis spectroscopy and conductivity of the polyaniline (PANI) doped with diphenyl phosphate (DPHP), in comparison with that of HCl-doped polyaniline, show that the decrease in polaron delocalization and structural order results from the steric hindrance imparted by the longer chain counter ion. PANI protonated with DPHP is soluble in common organic solvents such as toluene, chloroform, dimethyl sulfoxide, etc. The conductivity of PANI/DPHP pressed pellet (at 25°C, 1.66 S cm⁻¹) increases with temperature from −40°C (1.18 S cm⁻¹) to +140°C (3.07 S cm⁻¹) due to thermal activation, and decreases with temperature from 140 to 180°C (0.25 S cm⁻¹) due to thermal undoping accompanied by the loss of some polarons. The temperature dependence of spin density is consistent with those of conductivity. After the heating scan, the conductivity (0.52 S cm⁻¹) at room temperature reduces to nearly one-third of the original value.     

Studies of photoexcitations in conducting polymers mixed with C60

We have studied photoexcitations in mixed compounds of C₆₀ with derivatives of poly (p-phenylenevinylene) and poly(phenyleneacetylene) using a variety of CW and picosecond (ps) transient spectroscopies. The CW techniques used include photoinduced absorption (PA), photoluminescence (PL) and optically detected PA (ADMR), whereas the ps transient techniques used were time-resolved PA, PL and stimulated emission (SE). Compared with the pristine polymers, all mixed C₆₀ compounds have shown enhancement of PA bands associated with charged photoexcitations. These PA bands have shown a single spin-half ADMR resonance, identified as due to spins on the polymer chains; no spin-half ADMR signal from photogenerated C₆₀⁻ has been observed. In the ps time domain we have observed a pronounced reduction of the transient PL and SE in the mixed compounds. Due to the similarity found between the ps transient and CW PA bands, we have identified the ps photoexcitations in the mixed compounds as polarons (P⁺−P⁻ ) on the polymer chains, which are indirectly photogenerated as a result of exciton dissociation in C₆₀⁻ related defect centers. The ultrafast charge transfer, claimed to exist between the polymer chains and C₆₀ molecules, has not been observed in the ps time domain.     

Synthesis and characterization of polyazopyrrole. A novel narrow-bandgap polypyrrole with proton-exchange properties

Anodic coupling of 2,2′-azopyrrole, N,N′-dimethyl-2,2′-azopyrrole and pyrrolyl-2-aldazine has been performed in acetonitrile. The resulting polymer films were investigated by cyclic voltammetry (CV), UV-Vis-NIR spectroscopy, FT-IR reflection-absorption spectroscopy and in situ conductivity. Polyazopyrrole is the first reported narrow-bandgap pyrrole-based polymer (E_g ≈ 1.0 eV). It undergoes reversible oxidation without significant changes in the visible region but with strongly enhanced absorption in the NIR region. The CV of polyazopyrrole displays a two-step oxidation process with a finite potential window of conductivity (maximum conductivity 2 × 10⁻² S cm⁻¹). In the oxidized form polyazopyrrole easily dissociates its NH protons. Its acidic properties are no longer displayed in its N,N′-dimethyl-substituted homologue and in poly (pyrrolyl-2-aldazine).     

Structural changes during annealing and during acceptor doping of oriented poly(p-phenylene sulfide)

Thermally-annealed, melt-formed poly(p-phenylene sulfide), PPS, has a folded-chain, lamellar structure with a repeat period varying from 90 Å to 160 Å, depending upon the annealing temperature. Local crystalline order develops in the polymer before the onset of extensive lamellar ordering. On doping oriented PPS with AsF₅, the polymer swells and loses both crystallinity and chain orientation. The absence of crystalline and lamellar reflections in doped PPS probably results from dopant-induced structural disorder and chemical reactions (formation of dibenzothiophene linkages and crosslinking). These structural changes may explain the relative insensitivity of the conductivity of the doped polymer to the degree of crystallinity and orientation in the precursor polymer.     

Electrochemical synthesis and electronic properties of poly(3,4-dibutyl-α-terthiophene)

Electrochemically prepared poly(3,4-dibutyl-α-terthiophene) has been investigated and the relationship between the electronic properties, the chemical structure of the monomer and the electrolyte used during the electropolymerization was studied. Doping studies were carried out with various electrolytes. We have electrochemically oxidized (p-doping) and reduced (n-doping) thin films of this polymer on platinum electrode under the same electrochemical conditions. We find that the films show a reversible oxidation wave yielding an electrically conducting polymer and a reversible reduction wave which is sensitive to the nature of the electrolyte. A value for the band gap derived electrochemically compares well with that obtained by optical absorption measurements and X-ray photoelectron spectroscopy. The electrical conductivity varies by 10¹² S cm⁻¹ between the doped and undoped states of the polymer. The temperature-independent magnetic susceptibility above 100 K is consistent with the conducting properties of this polymer. The electrochemically prepared material is compared with the chemically prepared one using the same monomer.