Molecular Weight Polyaniline Research Articles

Effect of moisture and molecular weight of polyaniline on H2S sensing characteristics

Chemical behavior of some gases, such as H2S, changes with presence of moisture. Sensing of H2S by polyaniline (PANI) has been studied; the sensing characteristics depend upon the specific surface area, concentration of active surface sites, pore size, and surface heterogeneity controlled by the chain configuration, which is further governed by the molecular weight. However, effects of moisture in the environment and molecular weight of PANI on H2S sensing characteristics have not been studied, and this is the motivation of the present work. Here we have studied the H2S sensing by PANI of different molecular weights in the same environment (N2) with different moisture (dry and humid) conditions, and in different environments (N2 and compressed air) at the same humidity level. It was observed that regardless of the environment, (i) with increasing concentration of H2S gas, the resistance of the sensors increased in dry atmosphere and decreased in presence of moisture, and (ii) the response time was lower in moist conditions. The sensitivity and response were non-monotonous with respect to the molecular weight. However, response time decreased monotonically with increase in band gap of PANI. New correlations were developed to get surface site kinetics from the response data.

POLYMERIZATION OF ANILINE IN DIFFERENT SBA-16 SILICA HOSTS

Polyaniline-silica composites made with mesoporous silica have been reported to improve the mechanical properties of the free polymer. However, there are not many studies about the effect of the chemical and physical properties of the inorganic material used as hosts over the final properties of polyaniline and the composites. Here, polyaniline-silica composites, exhibiting electrical conductivity, were synthesized using different types of SBA-16 mesoporous silica as hosts. The effects of the characteristics of the hosts in the polymerization kinetic, molecular weight and electrical conductivity of the final polymer were studied. The composites showed higher electrical conductivity in comparison with polyaniline synthesized under similar conditions of this work but in the absence of silica. The use of silica allowed to obtain composites with defined morphology, which did not occur in the absence of these hosts. From the results of this work, it can be concluded that SBA-16 mesoporous silica hosts have a bifunctional role in the formation of polyaniline-silica composites: the hosts behaved like nucleation sites, which favors polymerization of high molecular weight polyaniline and they catalyzed the polymerization reaction, especially when aluminum is present in the structure of the host.

Electrical field influence on molecular mass and electrical conductivity of polyaniline

We have described the primary studies on the conductivity and molecular weight of polyaniline in an electric field as it is used in a field effect experimental configuration. We report further studies on doped in-situ deposited polyaniline. First we have chemically synthesized polyaniline by ammonium peroxodisulfate in an acidic solution, with aqueous, organic and emulsion conditions at different times. Next, we measured mass and conductivity and obtained the best time of polymerizations. Then, we repeated these reactions under different electrical fields in constant time and measured mass and conductivity. The polyaniline is characterized by gel permeation chromatography (GPC), UV-visible spectroscopy and electrical conductivity. Polyanilines with high molecular weight are synthesized under electric field M w = (5.2–6.8) × 105, with M w/M n = 2.0–2.5. The UV-visible spectra of polyanilines oxidized by ammonium peroxodisulfate and protonated with dodecylbenzenesulfonic acid (PANi-DBSA), in N-methylpyrolidone (NMP) show a smeared polaron peak shifted into the visible. Electrical conductivity of polyaniline has been studied by four-probe method. The conductivity of the films of emeraldine protonated by DBSA cast from NMP is higher than 500 S/cm under (10 kV/cm2 of potential) electric field and shows an enhanced resistance to ageing. Next, we carried chemical polymerization at the best electric field at different times. Finally, the best time and amount of electric field were determined. Polymers synthesized under an electric field probably have better physical properties regarding the existence of less branching and high electric conductivity.

Studies on the synthesis of low molecular weight, one‐dimensional polyanilines prepared by fast emulsion polymerization using (n‐dodecylbenzenesulfonic acid)/HCl emulsifiers

AbstractLow molecular weight polyaniline nanotubes were prepared by fast emulsion polymerization in the presence of both n‐dodecylbenzenesulfonic acid (DBSA) and HCl(aq). The anilinium monomers associated with DBSA emulsifiers were found to self‐arrange into strands of associated cylindrical micelles before polymerization, as monitored by their optical activity and optical image. In the presence of some HCl(aq), the monomer‐associated cylindrical micelles expanded and the polymerization rate could be speeded up. It was found that the low molecular weight (viscosity‐averaged molecular weight) polyaniline obtained can easily lead to the formation of highly conductive, one‐dimensional nanotubes or nanofibers monitored by the variation of optical activities and λmax of the UV–visible–near IR spectra during polymerization. The DBSA/HCl ratio played an important role in the eventual properties and morphologies of the one‐dimensional polyanilines, which can be illustrated by conductivity, SEM and transmission electron microscopy measurements. The resultant one‐dimensional polyaniline nanotubes can be arranged into a layered structure by orientation, illustrated by AFM and wide‐angle X‐ray diffraction. © 2012 Society of Chemical Industry

Synthesis and characterization of high molecular weight polyaniline for organic electronic applications

AbstractHigh molecular weight polyaniline (PANI) was synthesized by a combined procedure incorporating various synthesis methods. Temperature and open circuit potential of the reaction mixture were collected to monitor the reaction progress. The polymer is characterized by various techniques including gel permeation chromatography, dynamic light scattering, infrared spectroscopy, solid‐state nuclear magnetic resonance, and differential scanning calorimetry for elucidating the molecular architecture obtained by this method. As‐synthesized PANI was found to possess high molecular weight, reduced branching, reduced cross‐linking, and to predominantly consist of linear polymer chains. This polymer was also found to be more stable in solution form. J–V characteristics of as‐synthesized PANI films indicate a high current density which is due to increased free pathways and less traps for the charge transport to occur in PANI films. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

The influence of polymerization temperature on structure and properties of polyaniline

AbstractThe influence of polymerization temperature (from -25 to +25 oC) on molecular weight, crystallinity, electrical conductivity, thermal and morphological properties of polyaniline has been investigated. Aniline was oxidized in an aqueous medium with ammonium persulfate and 1.0 M hydrochloric acid. The reaction mixture freezes below -10 oC and hence lithium chloride was used in sufficient amount to prevent freezing. As the reaction temperature decreases, both the molecular weight of polyaniline and its crystallinity increase. Thermal studies show a common three step decomposition pattern irrespective of the temperature used during synthesis. The morphology of polyaniline changes from granular (reaction in liquid medium) to tubular (polymerization in the frozen state). Thermal analysis indicated that the polyaniline stability increased with decreasing synthesis temperatures. Electrical conductivity of polyaniline was found to be independent of the polymerization temperature and, consequently, of the molecular weight.

Catalytic activity of polyaniline/MnO2 composites towards the oxidative decolorization of organic dyes

The kinetics of the oxidative reaction of the textile dyes: direct red 81 (DR-81), indigo carmine (IC), and acid blue 92 (AB-92) with H2O2 in the presence of polyaniline (PANI)/MnO2 composites has been investigated. Experiments were carried out in batch reactor under pseudo-first order conditions with respect to the [dye]. Effects of the preparation conditions of the composites on the reaction rate have been investigated. Among these conditions are [aniline], [acid], type of acid, mass of β-MnO2, polymerization time, and annealing temperatures. At fixed [dye], [H2O2], and mass of composite, the rate constants increased upon increasing (i) the doping ratio of PANI, (ii) the percentage of PANI content, (iii) the degree of crystallinity of PANI, and (iv) the molecular weight of PANI. The presence of MnO2 counterpart kept the medium at pH range (3–4) via adsorption of H+. The oxidation rate was enhanced with the increase in the initial concentration of H2O2, attained a maximum at 0.01M and thereafter decreased. In contrast, the rate was inhibited with the increase in the initial dye concentration, and the pH. The activation parameters were determined. From the isokinetic relationship it was concluded that the catalytic reactions are entropy controlled. A reaction mechanism involving electron transfer from H2O2 to the PANI counterpart has been proposed. These results demonstrate that the PANI/MnO2–H2O2 catalytic system may be envisaged as a method for the treatment of colored wastewaters.

Conductive Fibre Prepared From Ultra-High Molecular Weight Polyaniline for Smart Fabric and Interactive Textile Applications

Panion™ fibre is prepared from a high molecular weight version of chlorine and defect free polyaniline (PANI). It is processed into textile fibre by continuous wet spinning techniques. The room temperature DC conductivity for unstretched and stretched (5x at 373 K) high molecular weight samples was 72 and 725 Ω −1 cm −1, respectively. Resistivity measurements of Panion™, from 4 to 350 K, indicate that the fibre manufacturing process and the polymer synthetic route both serve to minimize structural disorder in the solid-state. The transport resides on the metallic side of the metal-insulator (M-I) phase boundary for unstretched and stretched fibre samples prepared from high M w PANI emeraldine base. The monofilament fibre can be formed into yarns for weaving fabric, braided into 2- and 3- dimensional structures, or used to form knitted fabrics, all with tunable electronic functionality.

SEM study of the morphology of high molecular weight polyaniline

High molecular weight polyaniline (PANI) was synthesized by adding neutral salt technique with MacDiarmid's method and the morphology of the obtained PANI was examined by the scanning electron microscope (SEM). A remarkable and special phenomenon has been found that the SEM images of resulting PANI show a mass of clews, the average diameter of which is about one micrometer. Furthermore, these honeycombed clews are tenuous, interconnected fibrillar network with characteristic cross-sectional fibril dimensions of a few tens of nanometers. In addition, various preparation parameters which will probably influence the morphology of the high molecular weight PANI in our investigation are: (a) the concentration of lithium chloride ([LiCl]); (b) reaction temperature ( t); (c) molar ratio of aniline:oxidant ([An]/[APS]); (d) the concentration of aniline ([An]); (e) the concentration of hydrochloric acid ([HCl]). Some correlations between the sample preparation process and its morphology properties have been obtained.

Simple one-pot conversion of organic compounds by hydrogen peroxide activated by ruthenium(III) chloride: organic conversions by hydrogen peroxide in the presence of ruthenium(III)

The aromatic compounds p-nitrobenzaldehyde, p-hydroxybenzaldehyde, naphthalene, toluene, catechol, quinol, aniline and toluidine dissolved in aqueous acetic acid or aqueous medium were oxidized in quantitative to good yields by 50% H2O2 in the presence of traces of RuCl3 (∼10−8 mol; substrate/catalyst ratio 1488:1 to 341 250:1). Conditions for highest yields, in the most economical way, were obtained. Higher catalyst concentrations decrease the yield. Oxidation in aromatic aldehydes is selective at the aldehydic group only. In the case of hydrocarbons, oxidation results in the introduction of a hydroxyl group with >85% (in the case of toluene) selectivity for the ortho position. Formation of low-molecular-weight polyaniline was reduced to 10%, along with 90% formation of higher molecular weight polyaniline. In this new, simple and economical method, which is environmentally safe and requires less time, oxo-centered carboxylate species of ruthenium(III) in acetic acid medium and hydrated ruthenium(III) chloride in aqueous medium probably catalyze the oxidation. Copyright © 2005 John Wiley & Sons, Ltd.

Polyaniline/single-walled carbon nanotube composite electronic devices

Composites of high molecular weight polyaniline (PANI) and various weight percentages of single-walled carbon nanotubes (SWNT) were fabricated using solution processing. Electrical characteristics of metal–semiconductor (MS) devices fabricated from the PANI/SWNT composites were studied. Current–voltage ( I– V) characteristics of these devices indicate a significant increase in current with an increase in carbon nanotube concentration in the composite. The dominant transport mechanisms operating in these devices were investigated by plotting the forward I– V data on a log–log scale, which revealed two power-law regions with different exponents. In the lower voltage range, the exponent is approximately 1, implying that the charge transport mechanism is governed by Ohm's law. The charge transport mechanism in the higher voltage range, where the exponent varies between 1.1 and 1.7, is consistent with space-charge-limited (SCL) emission in the presence of shallow traps. The critical voltage ( V c), which characterizes the onset of SCL conduction, decreases with increasing SWNT concentration. In addition, V c was observed to increase with temperature. These initial results indicate that with further improvements in material consistency and reduction in defect densities, the polyaniline/single-walled carbon nanotube composite material can be used to fabricate organic electronic devices leading to many useful applications in microelectronics.

Polyaniline/carbon nanotube composite Schottky contacts

AbstractSchottky contacts were fabricated on composites of high molecular weight polyaniline and pristine multiwalled carbon nanotubes. Physical and electrical characteristics of these organic composite materials were studied by using atomic force microscopy (AFM), scanning electron microscopy (SEM), and electrical measurements. The RMS surface roughness of the composite films was found to be 4 nm. From the IV characteristics of these composite devices, it appears that the current follows Ohm's law at lower voltages and Child's law at higher voltages, indicating a space‐charge‐limited emission mechanism in the presence of a distribution of shallow traps.

Electronic Properties of Polyaniline/Carbon Nanotube Composites

Composites of high molecular weight polyaniline and carbon nanotubes are investigated for electronic device applications. Physical characterization by Thermogravimetric analysis and atomic force microscopy indicates that polyaniline containing 1% Carbon Nanotubes is suitable for organic devices. Measured electrical characteristics of Schottky diodes fabricated using these materials exhibit current levels in the polyaniline/carbon nanotube composite devices nearly an order of magnitude higher than in the polyaniline devices.

Mechanical Properties Of Polyaniline / Multi-walled Carbon Nanotube Composite Films

ABSTRACTHigh molecular weight polyaniline / multi-walled carbon nanotube composite films were fabricated using solution processing. Composite films with various weight percentages of multiwalled carbon nanotubes were fabricated. Physical properties of these composites were analyzed by thermogravimetric analysis, tensile testing, and scanning electron microscopy. These results indicate that the addition of multiwalled nanotubes to polyaniline significantly enhances the mechanical properties of the films. In addition, metal–semiconductor (composite) (MS) contact devices were fabricated, and it was observed that the current level in the films increased with increasing multiwalled nanotube content. Furthermore, it was observed that polyaniline containing one weight percent of carbon nanotubes appears to be the most promising composition for applications in organic electronic devices.

Electronic Device Fabricated From Polyaniline / Single walled Carbon Nanotubes Composite

AbstractComposites of high molecular weight polyaniline and single walled carbon nanotubes are investigated for applications to electronic devices. Electrical characteristics of devices fabricated using this composite show an order magnitude higher current level than those measured in the neat polyaniline devices. Composite materials and devices with various weight percentages of single walled carbon nanotubes were also fabricated. Current-voltage (I-V) characteristics of these devices indicate a significant increase in current with an increase in carbon nanotube concentration in the composite. Analysis of the forward I-V characteristics of these composite devices on a log-log scale shows two power law regions. In the lower voltage range, the exponent is 1, which is consistent with ohmic conduction, while in the higher voltage range, the exponent is between 1.1 and 1.5, which is consistent with space-charge-limited conduction.

The influence of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) additive concentration and stretch orientation on electronic transport in AMPSA-modified polyaniline films prepared from an acid solvent mixture

We examine the effects of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA)-additive concentration, molecular weight, and specimen elongation on the temperature-dependent electronic transport properties of polyaniline films prepared with AMPSA in the presence of a great excess of a formic and dichloroacetic acid (DCAA) solvent mixture. The AMPSA-additive dependent resistivity and thermopower are reported for free-standing PANI:AMPSA x ( M w=200,000 g mol −1) films ( x=0.1–0.5) in the temperature range from 2 to 325 K. The low-temperature data indicate that these samples are just on the insulating side of a disorder-induced metal–insulator (M–I) transition (d ρ/d T<0), and that thermal motion at elevated temperatures is sufficient to produce a metallic state (d ρ/d T>0) at room temperature. Transport occurs via variable-range hopping (VRH) for temperatures below 200 K, with hopping parameters that are a strong function of x; increased AMPSA concentration decreases the resistivity and moves the samples towards the M–I phase boundary. There is a minimum in the resistivity for all samples at a temperature T min that is also doping dependent. Stretch orientation of PANI:AMPSA 0.5 films, prepared with higher molecular weight PANI ( M w=300,000 g mol −1), along the resistivity measurement direction decreases the room-temperature resistivity at the expense of a more insulating low-temperature state. The T min values of both stretched and unstretched PANI:AMPSA 0.5 films are pushed below 200 K, which reflects reduced disorder in film processed with higher molecular weight polyaniline. These results reflect an evolution in the underlying inhomogeneous mesoscopic disorder present in doped conducting polymers.

The effect of polymerization temperature on molecular weight, crystallinity, and electrical conductivity of polyaniline

The influence of the polymerization temperature (from −50 to +50°C) on molecular weight, crystallinity, and electrical conductivity of polyaniline has been investigated. Aniline was oxidized in aqueous medium with ammonium peroxodisulfate at equimolar and excess concentrations of hydrochloric acid. The reaction mixture freezes below −10°C and the polymerization of aniline then proceeds in the solid state. As the reaction temperature decreases, both the molecular weight of polyaniline (determined by gel permeation chromatography) and its crystallinity (observed by X-ray diffraction) increase. The morphology of polyaniline changes from granular (reaction in a liquid medium) to macroporous (polymerization in the frozen state). Electrical conductivity of polyaniline is higher for samples prepared under more acidic conditions. It was found to be independent of the polymerization temperature and, consequently, of the molecular weight.

Characterization of high molecular weight polyaniline synthesized at −40 °C using a 0.25:1 mole ratio of persulfate oxidant to aniline

Aniline dissolved in hydrochloric acid solution containing sufficient lithium chloride to ensure that the reaction mixture remained mobile was polymerized at temperatures of 0 and −40 °C using a 0.25:1 mole ratio of ammonium persulfate oxidant to aniline. Gel permeation chromatography (GPC), using both polystyrene and polyvinylpyridine molecular weight standards, was used to determine molecular weight values, and the structural quality of the polymer was determined by solution state 13C NMR spectroscopy. The results were compared to previous values obtained using different conditions for GPC, and structural data together with visual observations were used to propose different reaction mechanisms at the start of the reaction dependent upon the rate of oxidant addition.

Low temperature synthesis of high molecular weight polyaniline using dichromate oxidant.

Aniline was polymerised at temperatures of between 0 and −35°C in acid solution containing lithium chloride and using dichromate oxidant. The optimum temperature to give high molecular weight polyaniline in the emeraldine oxidation state in good yield was found to be −22 ± 3°C. A solvent cast polyaniline camphorsulfonate film was uniaxially stressed to give a conductivity of 848 ± 70 Scm −1 along the stretch direction and 118 ± 10 Scm −1 perpendicular to this.

Low temperature synthesis of high molecular weight polyaniline

Abstract Oxidative chemical polymerization of aniline in hydrochloric acid solution at sub-zero temperatures leads to polyaniline with a molecular weight five to ten times higher than that prepared at room temperature. The optimum polymerization temperature to give a polymer with the highest yield and molecular weight has been determined, and solution state 13C n.m.r. spectroscopy shows that the improved polyaniline has fewer defect sites compared to the standard polymer. Despite these intrinsic improvements, the conductivity is of the same order of magnitude as standard polyaniline. A solvent-cast film of polyaniline camphorsulfonate had a conductivity of 295 S cm−1, although this rose to 820 S cm−1 along the stretch direction after the film had been uniaxially oriented. An oriented film cast from N-methyl-2-pyrrolidone solution and protonated with methanesulfonic acid showed a conductivity of 260 S cm−1 along the stretch direction and 31 S cm−1 perpendicular to this.