M. Reghu

Hopping transport in doped conducting polymers in the insulating regime near the metal-insulator boundary: polypyrrole, polyaniline and polyalkylthiophenes

Abstract Transport data for polypyrrole-hexafluorophosphate (PPy-PF6), protonated polyaniline and iodine-doped regio-regular polyalkylthiophenes (PATs), all in the insulating regime near the disorder-induced metal-insulator (M-I) boundary, are presented and analyzed. These doped conducting polymers have the transport properties of a Fermi glass insulator; the disorder being characterized by the resistivity ratio, ρr  ρ(1.4 K)/ρ(300 K). In the regime close to the M-I transition (ρr 103), ρ(T) shows two distinctly different behaviors. In homogeneous material, Motts VRH conduction is recovered. In inhomogeneous samples, where ‘metallic island’ are formed after partial dedoping or by strong morphological disorder, ln ρ ∝ (T 0 ′/T) 1 2 , characteristic of a granular system.

Electrical transport in conductive blends of polyaniline in poly(methyl methacrylate)

The electrical transport properties of solution-processed conducting polyblends of polyaniline-camphor sulfonic acid (PANI-CSA) in poly(methyl methacrylate) (PMMA) are investigated for PANI-CSA over the full range of volume fractions (f) above the percolation threshold. The positive temperature coefficient of resistivity at high temperature and the linear temperature dependence of thermoelectric power indicate that, because of the phase-separated network morphology, the microscopic transport properties of PANI-CSA in the blends remain nearly unchanged by dilution. The low temperature resistivity indicates transport by generalized variable range hopping, ϱ(T) = ϱ0exp[(T0T)γ]. A systematic change in γ is observed as a function of dilution; γ increases from approximately 13 at high concentrations to approximately 23 near the percolation threshold (f ≈ 0.01). Measurements of the magnetoresistance show that the localization length decreases with dilution.

Thermal stability of polyaniline networks in conducting polymer blends

Abstract The thermal stability of polyaniline-camphor sulfonic acid (PANI-CSA) networks in blends with poly (methyl methacrylate) (PMMA) and a low molecular weight polyester (PES ) has been studied by transmission electron microscopy (TEM) supplemented with measurements of the optical and electrical properties. The tenuous interpenetrating fibrillar network of PANI-CSA is robust and remains intact at temperatures above the glass transition temperature (Tg) and the melting point (Tm) of the host polymers (Tg~90 °C for PMMA; Tm~60 °C for PES), indicating that the phase-separated network morphology is a thermodynamically stable phase. Although deprotonation of the PANI gradually starts around 100 °C, the network morphology persists (with crystallites of CSA, released by the deprotonation, distributed within the network). After exposure to increasingly higher temperatures, especially above 200 °C, the fibrils of the network coarsen, and the structure becomes more open. Complete deprotonation and degradation of the PANI are observed at temperatures above 200 °C. The conclusions from the TEM micrographs are consistent with thermal gravimetric analysis, spectroscopic data and electrical conductivity measurements.

Infrared reflectance of polypyrrole: 'metal' with a gap in the spectrum of charged excitations

Abstract We report reflectance (R) measurements of polypyrrole (PPy) films with d.c. conductivity of 350 S/cm at 300 K. The reflectance is metal-like in the IR at all temperatures, with R increasing in the far-IR to about 100% at temperatures below 50 K. The optical conductivity, σ(ω), and the real part of the dielectric function, ϵ1(ω), are not typical of a Drude metal. At 10 K, e1(ω) = − A/ω2 for h ω cm −1 while σ(ω) decreases in the same spectral range, implying a narrow peak in σ(ω) at ħω well below 20 cm−1 with oscillator strength corresponding to about 2% of the total free-carrier oscillator strength. ‘Metallic’ PPy is, therefore, a conductor with a gap in the spectrum of charged excitations; the d.c. conductivity comes from a narrow peak in σ(ω) at or near ω = 0.

Spectroscopic studies of a soluble and stable polyacetylene blend

Abstract Polyacetylene blends with poly(vinyl butyral) (PVB) exhibit strongly enhanced stability under ambient conditions. Isomerization and doping processes are carried out in air and studied in situ by optical spectroscopy of the blends in liquid suspensions and in cast films. Third-order nonlinear optical susceptibility, χ (3) (3 ω ), spectra are studied via third-harmonic generation (THG) for cis -rich and trans -(CH) χ in the PVB blends. The THG spectra are typical of polyacetylene with the previously observed cis-trans differences clearly evident and with the enhanced THG in trans -(CH) χ due to the simultaneous two- and three-photon resonance at 0.6 eV (into the soliton continuum and the electron-hole continuum, respectively). The THG spectra are unaffected by air exposure over a period of a month. Photoinduced absorption spectra (modulated excitation spectroscopy) indicate a strong enhancement of the high energy peak (at 1.5 eV) with no indication of the 0.6 eV band. Exposing the material to air for several weeks causes the low energy peak to grow in intensity implying a correlation of the charged soliton lifetime with defects (trapping sites) in the material. The enhanced stability appears to be intrinsic, rather than simply due to encapsulation by the PVB.

Transports in blends of conducting polymers

Abstract We investigated the transport properties of solution-processed conductive polymer blends of polyaniline-(camhpor sulfonic acid), PANI-CSA, with insulating polymethylmethacrylate (PMMA), as a function of various volume fraction (ƒ) of PANI-CSA. Due to the phase separated morphology and the fibrillar geometry of percolating objects, a self-assembled PANI-CSA network in insulating matrix shows extremely low percolation threshold (ƒ c ≈ 0.003). The characteristic metallic properties of pure PANI-CSA, such as positive temperature coefficient of resistivity at high temperature, linear temperature dependence of thermoelectric power, and frequency independent ac conductivity, are retained in PANI-CSA/PMMA blends down to ƒ c . At low temperature, the hopping transports through the conducting polymer network depends on the volume fraction of PANI-CSA in blends and its network structure.

Pressure and magnetic field dependence of the low temperature resistivity of PF6-doped polypyrrole

Abstract The temperature coefficient of the resistivity (TCR) of doped polypyrrole changes sign below 25 K, from negative to positive, in samples with relatively weak disorder (as inferred from the resistivity ratio, ϱ r = ϱ (1.4 K)/ ϱ (300 K)). We have studied the crossover from negative to positive TCR as a function of the disorder, and as a function of pressure and magnetic field. At ambient pressure, the sign change is systematically observed only for samples having ϱ r ⩽2. For example, when ϱ r decreases from 1.97 to 1.33, the temperature ( T 1 ) defining the transition from negative to positive TCR increases from 7.5 to 24 K. Pressure enhances interchain transport and thereby decreases ϱ r . For samples having ϱ r ≈ 2–10, the TCR transition can be induced by applying high pressure. A magnetic field of 8 T suppresses the TCR transition. This anomalous temperature dependence of the low temperature electrical transport is discussed in terms of correlation corrections to the conductivity in the disordered metallic regime near the metal-insulator transition.

Metal-insulator transition in polyaniline doped with surfactant counter-ion

Polyaniline (PANI) films of superior homogeneity can be prepared by using functionalized protonic acids to protonate the polymer and simultaneously induce solubility, in the conducting form, in common organic solvents. The resistivity ϱ(T), of polyaniline (PANI) complexed with camphor sulfonic acid (CSA) exhibits a positive temperature coefficient for 180K 10, ϱ(T,H=0)exp(T−1/4). For samples with ϱ(1.2K)/ϱ(300K) ∼3–5, ϱ(T,H=0) T−β where β≈0.36, consistent with transport in the critical region near the metal-insulator (M-I) boundary, whereas ϱ(T,H=8Tesla) =ϱ0exp[(T0/T)1/4], indicating variable range hopping between localized states with mean localization length, Lc(H=8T)∼100–140A. Thus PANI-CSA is on the (M-I) boundary.

Doping and conductivity studies on poly(p-phenylene vinylene)

Abstract We have investigated the doping and temperature dependence of the conductivity of poly( p -phenylene vinylene) (PPV) free-standing films. High conductivities and a very weak temperature dependence are obtained in sulfuric acid (H 2 SO 4 ) -doped PPV (10 4 S/cm), indicative of a metallic state, whereas iron trichloride (FeCl 3 )-doped samples are observed to be less conducting (10 3 S/cm) and insulating at low temperatures. Doping of PPV with triflic acid (CF 3 SO 3 H) results in a high conductivity as well. Conductivity as a function of stretching ratio is obtained in H 2 SO 4 -doped PPV. The stability of this system in air and moisture was also investigated. These features of PPV as a highly doped conducting polymer system are discussed and compared with previous studies of PPV and of polyacetylene.

Resistivity and magnetoresistance of metallic polyaniline and polypyrrole at millikelvin temperatures

Abstract The resistivity and magnetoresistance of polyaniline protonated by camphor sulfonic acid (PANI-CSA) and polypyrrole doped with PF 6 (PPy-PF 6 ) have been studied down to 25 mK in magnetic fields up to 16 T.