Ronald R. Chance

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

Abstract Electrical conductivity, Hall effectm and spectroscopic measurements have been made on AsF 5 -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 × 10 4 S/m. Hall effect measurements indicate p-type conduction with a Hall mobility approaching 10 −4 m 2 /V s for doping levels between 0.24 and 0.42 moles of AsF 5 per mole of monomer. Doping with an electron donor, K, has increased the conductivity to about 10 3 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 AsF 5 -doping levels in poly( p -phenylene) correspond to a compound which is not wholly stage 1.

Solid‐state synthesis of highly conducting polyphenylene from crystalline oligomers

Paraphenylene oligomers (biphenyl, p‐terphenyl, p‐quaterphenyl, p‐quinquephenyl, p‐sexiphenyl) form electrically conducting complexes with AsF5. Prolonged exposure to AsF5 causes a polymerization of these p‐phenylene oligomers to give highly conducting charge‐transfer complexes of poly(p‐phenylene). Conductivities as high as 50 S/cm have been measured. Powders, thin films, and single crystals of p‐phenylene oligomers have been reacted with AsF5. The undoped oligomers and the doped, compensated, and annealed products have been investigated by means of x‐ray diffraction, and UV‐visible and IR transmission spectroscopy. The x‐ray diffraction studies give evidence for a change in lattice spacings to those characteristic of the crystalline polymer. The spectroscopic measurements during AsF5 doping reveal shifts in absorption bands in the UV and the IR to those characteristic of poly(p‐phenylene). Paraoligophenylenes have also been reacted with elemental potassium in THF solution with trace amounts of naphthale...

Thermochromism in a polydiacetylene crystal

Specular reflection spectra are reported for a thermochromic polydiacetylene crystal, ETCD [substituent group – (CH2)4OCONHC2H5] over the temperature range 23–130°C. A reversible thermochromic phase change occurs at ∼120°C and is accompanied by a 2750 cm−1 blue shift in the reflection spectra. Evidence is presented which strongly suggests that the dramatic change in optical properties of ETCD at the phase transition is due to an acetylene to butatriene transformation in the bonding sequence of the polydiacetylene backbone.

Conducting complexes of polyphenylene sulfides

Poly(p‐phenylene) sulfide, poly(m‐phenylene) sulfide, and the newly synthesized polymer poly(thio‐2,8‐dibenzothiophenediyl) have been treated with strong electron acceptors (AsF5, SbF5) to form conducting complexes with p‐type electronic conductivities up to 3 S/cm. Near IR to UV absorption spectra and temperature‐dependent conductivity measurements suggest a localization of charge carriers even at high doping levels. Elemental analysis and IR spectroscopy demonstrate that heavy exposure to AsF5 causes substantial changes in the backbone structure of these polymers. The dopant appears to predominantly induce the formation of carbon–carbon bonds bridging the sulfur linkages to form thiophene rings. This chemical modification enhances the conductivity of the complex and, in the case of poly(m‐phenylene), is shown to be an actual prerequisite for achieving high conductivity.

Ab initio effective Hamiltonian study of the electronic properties of conjugated polymers

The valence effective Hamiltonian technique is applied to a series of polymers to compute ionization potentials, bandwidths, and band gaps. The polymers considered represent systems of interest to the conducting polymers area and include various derivatives of polyacetylene and polyphenylene, polydiacetylene, polyacene, polybenzyl, and polyyne. The theoretical results for relative ionization potentials are in excellent agreement with available experimental estimates, as well as with the observed behavior of the electrical conductivity of these systems on exposure to weak (I2) versus strong (AsF5) electron acceptors. The bandwidths of the highest occupied band show a qualitative correlation to the conductivities achieved with acceptor doping. Band gaps for the planar systems considered are also in good agreement with experiment.

Theoretical Studies of Charged Defect States in Doped Polyacetylene and Polyparaphenylene

Abstract Defect statecalculations have been per formed for polyacetylene and polybparaphenylene in the fraamework of the Su, schriever, and Heeger Hamiltonian. In polyacetylene, the study of the energetics of the separation of the radical(natural defect)-ion(charged defect) pair induced upon doping indicates that the two defects tend to remain close to each other. This results in the formatn of polarons whose binding energy is estimated to be of the order of 0.05 eV Absorption spectra at low doping levels are consistent with polaron formation. Interaction between polarons leads to the formation of charged solitions. In poly(p-phenylene), defects are always correlated in pairs. Upn doping, polarons are formed (binding energy ∼ 0.03 eV), wit the relaxation of the lattice extending over about four rings. Calculations suggest the possibility of bipolarons (doubly charged defects) that yield conductivity without Pauli susceptibility.

Electronic properties of sulfur containing conjugated polymers

Valence effective Hamiltonian (VEH) calculations are performed on a number of sulfur containing organic conjugated polymers of interest to the conducting polymers area. Theoretical results for parameters related to conductivity such as ionization potentials, bandwidths, and bandgaps are presented. Systems considered include various derivatives of poly (p‐phenylene sulfide), polybenzothiophene, and polythiophene, as well as potentially interesting compounds such as polythieno [3,2‐b] thiophene and polyvinylene sulfide. The electronic structure description afforded by the VEH method for sulfur containing polymers is demonstrated to be of the same quality as that presented previously for hydrocarbon polymers. In particular, for ionization potentials, good agreement with available experimental data on poly (p‐phenylene sulfide) and polybenzothiophene is obtained, after scaling downward the VEH values by a 1.9 eV polarization correction. The comparison between the theoretical and experimental XPS spectra for polybenzothiophene is excellent with use of the same energy scaling factor previously employed for polyacetylene, poly(p‐phenylene), and poly(p‐phenylene sulfide). These results, in conjuction with previous results obtained on hydrocarbon polymers, lend confidence in the predictive capabilities of this purely theoretical technique. Calculations show that polyvinylene sulfide, as yet unsynthesized, should display very promising characteristics as a conducting polymer.

Life Cycle Energy and Greenhouse Gas Emissions for an Ethanol Production Process Based on Blue-Green Algae

Ethanol can be produced via an intracellular photosynthetic process in cyanobacteria (blue-green algae), excreted through the cell walls, collected from closed photobioreactors as a dilute ethanol-in-water solution, and purified to fuel grade ethanol. This sequence forms the basis for a biofuel production process that is currently being examined for its commercial potential. In this paper, we calculate the life cycle energy and greenhouse gas emissions for three different system scenarios for this proposed ethanol production process, using process simulations and thermodynamic calculations. The energy required for ethanol separation increases rapidly for low initial concentrations of ethanol, and, unlike other biofuel systems, there is little waste biomass available to provide process heat and electricity to offset those energy requirements. The ethanol purification process is a major consumer of energy and a significant contributor to the carbon footprint. With a lead scenario based on a natural-gas-fueled combined heat and power system to provide process electricity and extra heat and conservative assumptions around the ethanol separation process, the net life cycle energy consumption, excluding photosynthesis, ranges from 0.55 MJ/MJ(EtOH) down to 0.20 MJ/ MJ(EtOH), and the net life cycle greenhouse gas emissions range from 29.8 g CO₂e/MJ(EtOH) down to 12.3 g CO₂e/MJ(EtOH) for initial ethanol concentrations from 0.5 wt % to 5 wt %. In comparison to gasoline, these predicted values represent 67% and 87% reductions in the carbon footprint for this ethanol fuel on a energy equivalent basis. Energy consumption and greenhouse gas emissions can be further reduced via employment of higher efficiency heat exchangers in ethanol purification and/ or with use of solar thermal for some of the process heat.

Adsorption of Water and Ethanol in MFI-Type Zeolites

Water and ethanol vapor adsorption phenomena are investigated systematically on a series of MFI-type zeolites: silicalite-1 samples synthesized via both alkaline (OH(-)) and fluoride (F(-)) routes, and ZSM-5 samples with different Si/Al ratios as well as different charge-balancing cations. Full isotherms (0.05-0.95 activity) over the range 25-55 °C are presented, and the lowest total water uptake ever reported in the literature is shown for silicalite-1 made via a fluoride-mediated route wherein internal silanol defects are significantly reduced. At a water activity level of 0.95 (35 °C), the total water uptake by silicalite-1 (F(-)) was found to be 0.263 mmol/g, which was only 12.6%, 9.8%, and 3.3% of the capacity for silicalite-1 (OH(-)), H-ZSM-5 (Si/Al:140), and H-ZSM-5 (Si/Al:15), respectively, under the same conditions. While water adsorption shows distinct isotherms for different MFI-type zeolites due to the difference in the concentration, distribution, and types of hydrophilic sites, the ethanol adsorption isotherms present relatively comparable results because of the overall organophilic nature of the zeolite framework. Due to the dramatic differences in the sorption behavior with the different sorbate-sorbent pairs, different models are applied to correlate and analyze the sorption isotherms. An adsorption potential theory was used to fit the water adsorption isotherms on all MFI-type zeolite adsorbents studied. The Langmuir model and Sircars model are applied to describe ethanol adsorption on silicalite-1 and ZSM-5 samples, respectively. An ideal ethanol/water adsorption selectivity (α) was estimated for the fluoride-mediated silicalite-1. At 35 °C, α was estimated to be 36 for a 5 mol % ethanol solution in water increasing to 53 at an ethanol concentration of 1 mol %. The adsorption data demonstrate that silicalite-1 made via the fluoride-mediated route is a promising candidate for ethanol extraction from dilute ethanol-water solutions.

The role of mobile organic radicals and ions (solitons, polarons and bipolarons) in the transport properties of doped conjugated polymers

Abstract We discuss the formation of charged defects such as solitons, polarons and bipolarons, in doped conjugated polymers. We present the results of ab initio quality calculations on the modifications of geometric and electronic structures occurring upon doping. A transport model for spinless conduction through bipolarons applicable to doped polymers with or without degenerate ground state is described.