Helical heterojunctions originating from helical inversion of conducting polymer nanofibers

The helical sense of conducting polyaniline nanofibers was induced using chiral acid as dopant, which can be inversed through a copolymerization of aniline with N-methyl aniline. By delicately controlling the inversion driving force of steric hindrance, we successfully obtained "helical heterojunctions" composed of right- and left-handed helical structures in one helical nanofiber.     

Charge carrier transport in heterogeneous conducting polymer materials

The similarities of conductivity mechanisms of composites and nanocomposites and doped conjugated polymers that are also characteristic of specific heterogeneity are discussed. It is shown that the formulae developed to account for internal heterogeneity of conductive polymers can be applied also for polymer composites in spite of low overall content of the conductive phase. For fully organic nanocomposites (reticulate doped polymers) showing metal-like conductivity and a crossover temperature effective contribution of metallic phase is estimated. Examples of different properties of nanoparticles forming conducting networks as compared with the bulk crystals are discussed.     

Parameters influencing transport across conducting electroactive polymer membranes

The parameters which influence electrochemically facilitated transport of electroinactive ions across conducting electroactive polymer membranes have been investigated. The design of membranes and the materials used as well as transport cells and systems have been addressed to improve selectivity and flux. Polypyrrole-para-toluenesulfonate (PPy-pTS) was compared with the copolymer of pyrrole with 3-carboxy-4-methylpyrrole (PPy/PCMP-pTS) and their different chemistries resulted in different membrane selectivities for ions. Platinum mesh was found to be the most suitable auxiliary electrode material and its placement in the cell chamber(s) facilitated ion incorporation/expulsion at the membrane working electrode. This enhanced the flux of ion transport. The flux can also further enhanced by narrowing the distance between the membrane working electrode and the platinum mesh auxiliary electrode(s), and/or by stirring to improve the hydrodynamics. An alternative cell design, namely a dual membrane flow through cell, also proved to be more efficient for ion transport. Good connection geometry to the membrane as well as the application of a square wave pulsed potential waveform to the membrane was found to be essential for achieving high and sustainable flux in ion transport.     

Modifying thermal transport in electrically conducting polymers: effects of stretching and combining polymer chains

If their thermal conductivity can be lowered, polyacetylene (PA) and polyaniline (PANI) offer examples of electrically conducting polymers that can have potential use as thermoelectrics. Thermal transport in such polymers is primarily influenced by bonded interactions and chain orientations relative to the direction of heat transfer. We employ molecular dynamics simulations to investigate two mechanisms to control the phonon thermal transport in PANI and PA, namely, (1) mechanical strain and (2) polymer combinations. The molecular configurations of PA and PANI have a significant influence on their thermal transport characteristics. The axial thermal conductivity increases when a polymer is axially stretched but decreases under transverse tension. Since the strain dependence of the thermal conductivity is related to the phonon scattering among neighboring polymer chains, this behavior is examined through Herman's orientation factor that quantifies the degree of chain alignment in a given direction. The conductivity is enhanced as adjacent chains become more aligned along the direction of heat conduction but diminishes when they are orthogonally oriented to it. Physically combining these polymers reduces the thermal conductivity, which reaches a minimum value for a 2:3 PANI/PA chain ratio.     

Direct approach for the electron transport through molecules

A very simple, straightforward, and easy to implement fully ab initio procedure for the determination of current-voltage characteristics in molecular junctions is presented. Application of this procedure predicts reasonably well the experimental findings for low bias voltages of a break junction experiment and can help us to characterize its geometry.     

Protonation effects on electron transport through diblock molecular junctions: A theoretical study

Diblock oligomers are widely used in molecular electronics. Based on fully self-consistent nonequilib-rium Green's function method and density functional theory, we study the electron transport properties of the molecular junction with a dipyrimidinyl-diphenyl (PMPH) diblock molecule sandwiched between two gold electrodes. Effects of different kinds of molecule-electrode anchoring geometry and protona-tion of the PMPH molecule are studied. Protonation leads to both conductance and rectification en-hancements. However, the experimentally observed rectifying direction inversion is not found in our calculation. The preferential current direction is always from the pyrimidinyl to the phenyl side. Our calculations indicate that the protonation of the molecular wire is not the only reason of the rectification inversion.     

Ion effects in REDOX cycling of conducting polymer based electrochromic materials

Conducting polymer based electrochromic devices were assembled with various ionic liquid (IL) based electrolytes to probe the role of the ion structure on electrochromic performance. When the IL contained the same anion as the dopant ion used in the conducting polymers an enhanced electrochromic performance was observed providing high photopic contrast at low applied potential.     

Electrochemically controlled transport of small charged organic molecules across conducting polymer membranes

The transport of a range of functionalised sulfonated aromatics across conducting polypyrrole membranes has been considered. In the course of these studies several unique aspects of the chemical selectivity of these conducting materials have been identified. Using electrochemical quartz crystal microbalance (EQCM) the ion-exchange behaviour of these membranes was investigated to further elucidate the transport mechanism.     

From monomeric nanofibers to PbS nanoparticles/polymer composite nanofibers through the combined use of gamma-irradiation and gas/solid reaction

Organic metal-salt (lead dimethacrylate (Pb(MA)2)) nanofibers are prepared, and these Pb(MA)2 monomeric nanofibers are successfully converted into PbS nanoparticles/polymer composite nanofibers through the combined use of gamma-irradiated polymerization and gas/solid reaction. The resulting composite nanofibers have excellent thermal and chemical stability, and the PbS nanoparticles (with diameters of about 4 nm) are well dispersed in the polymer-fiber matrices. This approach could also be extended to methacrylates containing other metal ions. We anticipate that this method would provide a platform for the fabrication of diverse and multifunctional polymer nanocomposite fibers, which would have potential applications in fabricating devices with optical, electric, and magnetic properties.     

Mechanisms of electron transport through bellows-shaped fullerene tubes

A bellows-shaped fullerene tube is featured by diameter modulation along the tube, where C60 molecules polymerize with tubular linkages between the molecules. The electronic structures of two types of bellows-shaped fullerene tube are theoretically found to exhibit band gaps indicating semiconducting characteristics. Although the effective masses of the conduction states of the two tubes are similar, these states have different spatial distributions. One of the two tubes is expected to exhibit thermally assisted electron transport.     

Theoretical analysis of electron transport through organic molecules

We present a theoretical study of electron transport through a variety of organic molecules. The analysis uses the Landauer formalism in combination with complex bandstructure and projected densities of states calculations to reveal the main aspects of coherent electronic transport through alkanes, benzene-dithiol, and phenylene-ethynylene oligomers. We examine the dependence of the current on molecule length, the effects of molecule-molecule interactions from film packing, differences in contact geometry, and the influence of phenyl ring rotation on the conductances of phenylene-ethynylene oligomers such as 1,4-bis-phenylethynyl-benzene.     

Temperature-dependent solid-state electron transport through bacteriorhodopsin: experimental evidence for multiple transport paths through proteins

Electron transport (ETp) across bacteriorhodopsin (bR), a natural proton pump protein, in the solid state (dry) monolayer configuration, was studied as a function of temperature. Transport changes from thermally activated at T > 200 K to temperature independent at <130 K, similar to what we have observed earlier for BSA and apo-azurin. The relatively large activation energy and high temperature stability leads to conditions where bR transports remarkably high current densities above room temperature. Severing the chemical bond between the protein and the retinal polyene only slightly affected the main electron transport via bR. Another thermally activated transport path opens upon retinal oxime production, instead of or in addition to the natural retinal. Transport through either or both of these paths occurs on a background of a general temperature-independent transport. These results lead us to propose a generalized mechanism for ETp across proteins, in which tunneling and hopping coexist and dominate in different temperature regimes.     

Contact atomic structure and electron transport through molecules

Using benzene sandwiched between two Au leads as a model system, we investigate from first principles the change in molecular conductance caused by different atomic structures around the metal-molecule contact. Our motivation is the variable situations that may arise in break junction experiments; our approach is a combined density functional theory and Green function technique. We focus on effects caused by (1) the presence of an additional Au atom at the contact and (2) possible changes in the molecule-lead separation. The effects of contact atomic relaxation and two different lead orientations are fully considered. We find that the presence of an additional Au atom at each of the two contacts will increase the equilibrium conductance by up to two orders of magnitude regardless of either the lead orientation or different group-VI anchoring atoms. This is due to a resonance peak near the Fermi energy from the lowest energy unoccupied molecular orbital. In the nonequilibrium properties, the resonance peak manifests itself in a negative differential conductance. We find that the dependence of the equilibrium conductance on the molecule-lead separation can be quite subtle: either very weak or very strong depending on the separation regime.     

Orbital views of the electron transport through heterocyclic aromatic hydrocarbons

Electron-transport properties of heterocyclic aromatic hydrocarbons are investigated with theoretical methods. The present study is based on a previously derived concept for orbital control of electron transport through aromatic hydrocarbons. The orbital control concept provided crucial basic understanding for the best conductance channels in the aromatic hydrocarbons and was successfully applied in the design of molecular devices. That concept was proven to hold true for small aromatic molecules, large polycyclic aromatic hydrocarbons with different edge structures, and in weak and strong coupling with the electrodes junctions. The polycyclic aromatic hydrocarbons and nanographenes used in the molecular electronics are often immobilized with different types of defects, which require the application of the orbital control concept on heterocyclic aromatic hydrocarbons. In this work, the effect of the heteroatoms in aromatic hydrocarbons on their electron-transport properties and the applicability of the orbital control concept on heterocyclic aromatic hydrocarbons are studied. Effective routes for electron transport are predicted in weak coupling junctions by analyzing the phase and amplitude of the frontier orbitals. The qualitative predictions are made with the nonequilibrium Green??s function method combined with the Hückel approximation. Quantitative, first principle calculations are performed with the nonequilibrium Green??s function method combined with density functional theory. The obtained results are in good agreement with the expectations on the basis of the orbital control concept, which proves its applicability in heterocyclic aromatic hydrocarbons.     

Water transport coefficient distribution through the membrane in a polymer electrolyte fuel cell

The net water transport coefficient through the membrane, defined as the ratio of the net water flux from the anode to cathode to the protonic flux, is used as a quantitative measure of water management in a polymer electrolyte fuel cell (PEFC). In this paper we report on experimental measurements of the net water transport coefficient distribution for the first time. This is accomplished by making simultaneous current and species distribution measurements along the flow channel of an instrumented PEFC via a multi-channel potentiostat and two micro gas chromatographs. The net water transport coefficient profile along the flow channels is then determined by a control-volume analysis under various anode and cathode inlet relative humidity (RH) at 80 °C and 2 atm. It is found that the local current density is dominated by the membrane hydration and that the gas RH has a large effect on water transport through the membrane. Very small or negative water transport coefficients are obtained, indicating strong water back diffusion through the 30 μm Gore-Select^® membrane used in this study.     

Applications of polymer nanofibers in biomedicine and biotechnology

Recent advancements in the electrospinning method enable the production of ultrafine solid and continuous fibers with diameters ranging from a few nanometers to a few hundred nanometers with controlled surface and internal molecular structures. A wide range of biodegradable biopolymers can be electrospun into mats with specific fiber arrangement and structural integrity. Through secondary processing, the nanofiber surface can be functionalized to display specific biochemical characteristics. It is hypothesized that the large surface area of nanofibers with specific surface chemistry facilitates attachment of cells and control of their cellular functions. These features of nanofiber mats are morphologically and chemically similar to the extracellular matrix of natural tissue, which is characterized by a wide range of pore diameter distribution, high porosity, effective mechanical properties, and specific biochemical properties. The current emphasis of research is on exploiting such properties and focusing on determining appropriate conditions for electrospinning various polymers and biopolymers for eventual applications including multifunctional membranes, biomedical structural elements (scaffolds used in tissue engineering, wound dressing, drug delivery, artificial organs, vascular grafts), protective shields in specialty fabrics, and filter media for submicron particles in the separation industry. This has resulted in the recent applications for polymer nanofibers in the field of biomedicine and biotechnology.     

Effect of immobilizing adsorption on mass transport through polymer films

The Freundlich isotherm used in place of the Langmuir isotherm to describe the adsorption mode in the dual sorption theory of glassy polymer—gas systems. Time-lag expressions have been derived and compared with the available corresponding results for the Langmuir case. In the case of large and moderate values of the upstream boundary activity a₀, the value of the time-lag function L^a)(l) calculated with the proposed model is close to that calculated by Paul for the Langmuir isotherm. In the case of small values of a₀, the L^a(l) values predicted by Paul and the proposed model are different and the difference increases when the value of a₀ decreases. The ability of the present model to represent systems exhibiting positive deviations from Henry's law is also described.     

Effects of dissolved polymer on the transport of colloidal particles through a microcapillary

The effect of water-soluble polymer on the transport of latex particles through a microcapillary was investigated. Capillary hydrodynamic fractionation (CHDF) experiments were performed using polystyrene (PS) particles and poly(ethylene oxide) (PEO) solutions as the eluant. Generally, the average particle velocities were greater than those corresponding to a polymer-free eluant. A decrease in the sample axial dispersion was also observed using the PEO solutions. In addition, increasing the polymer molecular weight resulted in lower particle residence times in the capillary tube. The enhanced particle transport arises primarily from an increase in the particle diameter resulting from the adsorption of PEO onto the PS surfaces, and, more importantly, from the migration of particles toward the capillary axis due to the normal stress of the PEO solution.