Transport in polyaniline networks near the percolation threshold

Effect of face-to-face and side-to-side interchain interactions on the electron transport in emeraldine salt polyaniline

Polyaniline (PANI) is a conductive polymer that has been studied intensively due to its high conductivity, ease of synthesis, fascinating doping mechanism, and a broad spectrum of applications. Polyaniline doped HCl was synthesized by a common direct-oxidation method of aniline using ammonium persulfate as the oxidant in HCl solution at various temperatures. This study focused on conductivity alteration of PANI-ES (emeraldine salt) due to the interchain interaction observed at different reaction temperatures from room temperature down to -15 °C. The molecular structure of PANI-ES was determined by FTIR and Raman spectroscopy. At low reaction temperature, the electronic transport properties improve significantly as reflected by its conductivity. X-ray diffraction (XRD) analysis shows that the value d_{(110)} and β play an important role in electron transport through face-to-face and side-to-side interactions, respectively. Scanning electron microscopy (SEM) analysis shows that the morphology of the synthesized PANI-ES consists of granules that are interconnected by nanofibers. Here, the correlation between electronic transport properties, structure, and morphology induced by reaction temperature was analyzed and discussed in detail. Moreover, PANI ES synthesized at 0 °C was applied as an electrocatalytic active layer in the DSSC's counter-electrode with a promising result.

Metal-polymer interface influences apparent electrical properties of nano-structured polyaniline films

The interface between the conductive polymer, polyaniline (PAn-Cl), and gold, platinum, or an interceding layer of electrodeposited platinum on gold or platinum, markedly influences the apparent electrical properties and the electronic to ionic transition in physiological buffers. Polyester-supported, sputter-deposited gold and platinum thin films were laser patterned to yield co-planar Thin Film Electrodes (TFEs) suitable for platinization and deposition of PAn-Cl nanofibers. Electrodeposition of platinum from chloroplatinic acid (50 mC cm^-2) onto gold produced larger feature sizes and larger surface roughness (23.5 nm) when compared to platinization of platinum (15.2 nm) and both similarly reduced interfacial impedance in water and physiologically relevant buffers, PBS and HEPES. UV-Vis characterization produced absorption edges (DI water 2.36 eV, PBS 2.64 eV, and HEPES 2.66 eV) reflective of the ionic strength of the medium. Thin films (23 ± 2 μm) of PAn-Cl nanofibers were deposited onto Au, Pt, Au|Pt, Pt|Pt TFEs and each characterized by Electrical Impedance Spectroscopy (EIS) over the range 10⁶-10^-1 Hz at RT in air, DI water, PBS, and HEPES buffers and by multiple scan rate cyclic voltammetry (MSRCV) in PBS. Platinized gold and platinized platinum decorated with PAn-Cl behaved quite differently in these test environments confirming a role for the contacting surface roughness/nano-topography in influencing apparent electrical properties. Equivalent circuit modeling of EIS data revealed a modified Randles circuit (R(QR)) of low chi-square values (<0.05) that rationalized the capacitance and membrane resistance and confirmed that platinization of gold served to increase the PAn-Cl apparent resistance while platinization of platinum served to decrease the PAn-Cl apparent resistance.

New nanocomposites of polystyrene with polyaniline doped with lauryl sulfuric acid

This work is concentrated on synthesis and investigation of new core-shell nanocomposites of polystyrene (PS) with doped polyaniline (PANI). The latex containing PS nanoparticles with sizes of 15-30 nm was prepared by microemulsion polymerization of styrene in water media. The PS/PANI nanocomposites were synthesized by chemical oxidative polymerization of aniline in the PS latex media in a presence of lauryl sulfuric acid (LSA), which served as both dopant and plasticizer. The real content of PANI in the synthesized nanocomposites was determined by UV-Vis spectroscopy method. The composition of the nanocomposites and oxidation state of the doped polyaniline were characterized by FTIR spectroscopy. The core-shell morphology of the nanocomposite nanoparticles was proved by transmission and scanning electron microscopy. It was found that conductivity and thermal behavior in air of these nanocomposites not only nonlinearly depended on the doped polyaniline content but also were strongly effected both by plasticizing properties of the acid-dopant and presence of the polyaniline shell. A possibility of application of these nanocomposites as sensor materials has been demonstrated.

Strain induced insulator-to-conductor transition in conducting polymer composites from the auxetic behaviour of hierarchical microstructures

The auxetic behaviour of the hierarchichal microstructure present in polyaniline composites is shown to result in an insulator-to-conductor transition and a reduction in the percolation threshold upon the application of strain.

Decoupling order and conductivity in doped conducting polymers

Herein it is demonstrated that the high level of interchain ordering of pEDOT is not necessary for the polymer to have efficient charge transport. Resistance and order are compared during the manufacturing process, where the polymerisation step and ordering step are decoupled as separate stages of the processing. GIWAXS experiments measuring interchain order are correlated to resistivity measurements at multiple stages of the manufacturing process on single films, and it is shown that for an individual film, where percolation is achieved, having a long range ordered system offers no reduction in resistance compared to having a highly disordered state of the same film. For this system, once the chains of pEDOT are formed, it is experimentally demonstrated that for percolation to be achieved, a remarkably low 4.5% volume fraction pEDOT is required. The apparent lack of necessity for significant interchain ordering allows for a meaningful measurement of development of the charge transport during the chemical polymerisation process.

Nanoscale electrical and mechanical characteristics of conductive polyaniline network in polymer composite films

The presence and characteristics of a connected network of polyaniline (PANI) within a composite coating based on polyester acrylate (PEA) has been investigated. The bulk electrical conductivity of the composite was measured by impedance spectroscopy. It was found that the composite films containing PANI have an electrical conductivity level in the range of semiconductors (order of 10(-3) S cm(-1)), which suggests the presence of a connected network of the conductive phase. The nanoscopic distribution of such a network within the cured film was characterized by PeakForce tunneling atomic force microscopy (AFM). This method simultaneously provides local information about surface topography and nanomechanical properties, together with electrical conductivity arising from conductive paths connecting the metallic substrate to the surface of the coating. The data demonstrates that a PEA-rich layer exists at the composite-air interface, which hinders the conductive phase to be fully detected at the surface layer. However, by exposing the internal structure of the composites using a microtome, a much higher population of a conductive network of PANI, with higher elastic modulus than the PEA matrix, was observed and characterized. Local current-voltage (I-V) spectroscopy was utilized to investigate the conduction mechanism within the nanocomposite films, and revealed non-Ohmic characteristics of the conductive network.

Highly conductive multifunctional graphene polycarbonate nanocomposites

Graphene nanosheet-bisphenol A polycarbonate nanocomposites (0.027-2.2 vol %) prepared by both emulsion mixing and solution blending methods, followed by compression molding at 287 °C, exhibited dc electrical percolation threshold of ∼0.14 and ∼0.38 vol %, respectively. The conductivities of 2.2 vol % graphene nanocomposites were 0.512 and 0.226 S/cm for emulsion and solution mixing. The 1.1 and 2.2 vol % graphene nanocomposites exhibited frequency-independent behavior. Inherent conductivity, extremely high aspect ratio, and nanostructure directed assembly of the graphene using PC nanospheres are the main factors for excellent electrical properties of the nanocomposites. Dynamic tensile moduli of nanocomposites increased with increasing graphene in the nanocomposite. The glass transition temperatures were decreased with increasing graphene for the emulsion series. High-resolution electron microscopy (HR-TEM) and small-angle neutron scattering (SANS) showed isolated graphene with no connectivity path for insulating nanocomposites and connected nanoparticles for the conductive nanocomposites. A stacked disk model was used to obtain the average particle radius, average number of graphene layers per stack, and stack spacing by simulation of the experimental SANS data. Morphology studies indicated the presence of well-dispersed graphene and small graphene stacking with infusion of polycarbonate within the stacks.

Transport mechanisms in metallic and semiconducting single-wall carbon nanotube networks

A fundamental understanding of the conduction mechanisms in single-wall carbon nanotube (SWCNT) networks is crucial for their use in thin-film transistors and conducting films. However, the uncontrollable mixture state of metallic and semiconducting SWCNTs has always been an obstacle in this regard. In the present study, we revealed that the conduction mechanisms in nanotube networks formed by high-purity metallic and semiconducting SWCNTs are completely different. Quantum transport was observed in macroscopic networks of pure metallic SWCNTs. However, for semiconducting SWCNT networks, Coulomb-gap-type conduction was observed, due to Coulomb interactions between localized electrons. Crossovers among a weakly localized state and strongly localized states with and without Coulomb interactions were observed for transport electrons by varying the relative content of metallic and semiconducting SWCNTs. It was found that hopping barriers, which always exist in normal SWCNT networks and are serious obstacles to achieving high conductivity, were not present in pure metallic SWCNT networks.

Electrical Transport Mechanism in Polyaniline/Formvar Blend Films

Polyaniline (PANI) was synthesized chemically and doped with camphor sulfonic acid. New PANI/formvar blend films were prepared using solution blending. These blend films were characterized by UV-visible, Fourier transform infrared spectra and scanning electron microscopy to investigate their optical, structural and morphological properties. It was found that the percolation threshold of these blend films is 4.4 wt.% of PANI. The dc and ac conductivities of these blend films were measured at a temperature range from 70 to 300 K in the frequency range of 10 kHz—1 MHz. The temperature dependence of the dc conductivity of the PANI/formvar blend films followed the three-dimensional variable range hopping. Temperature variation of frequency exponents in this blend suggests that ac conduction is attributed to be correlated to barrier hopping.

The Optimal Structure−Conductivity Relation in Epoxy-Phthalocyanine Nanocomposites

Phthalcon-11 (aquocyanophthalocyaninatocobalt (III)) forms semiconducting nanocrystals that can be dispersed in epoxy coatings to obtain a semiconducting material with a low percolation threshold. We investigated the structure-conductivity relation in this composite and the deviation from its optimal realization by combining two techniques. The real parts of the electrical conductivity of a Phthalcon-11/epoxy coating and of Phthalcon-11 powder were measured by dielectric spectroscopy as a function of frequency and temperature. Conducting atomic force microscopy (C-AFM) was applied to quantify the conductivity through the coating locally along the surface. This combination gives an excellent tool to visualize the particle network. We found that a large fraction of the crystals is organized in conducting channels of fractal building blocks. In this picture, a low percolation threshold automatically leads to a conductivity that is much lower than that of the filler. Since the structure-conductivity relation for the found network is almost optimal, a drastic increase in the conductivity of the coating cannot be achieved by changing the particle network, but only by using a filler with a higher conductivity level.