The structure change-induced morphology transition of polyaniline in 1.6-hexanediol aqueous and acid-free solutions: From submicron-spheres to nanofibers

This study shows that the polymerization of aniline monomers in 1.6-hexanediol aqueous and acid-free solutions can produce three-dimensional (3D) polyaniline submicron-spheres and one-dimensional (1D) nanofibers at different reaction stages through a morphology transition process. Fourier transform infrared spectra indicate that the aniline monomers form phenazine-like units in the initial reaction stage, producing polyaniline submicron-spheres with a diameter of ～400 nm. The hydrogen bonds between 1.6-hexanediol molecules and polyaniline chains serve as the driving force for the polyaniline chains to construct submicron-spheres. However, as the reaction proceeds, the solution acidity increases and the initially formed phenazine-like segments function as the nucleate for the free aniline monomers to polymerize through para-coupling linkage. This process forms polyaniline samples with a structure consisting of a core of phenazine-like units and a shell of para-linked units. In this stage, the newly formed para-linked aniline units exhibit an intrinsic tendency to transit the polyaniline morphology from submicron-spheres into nanofibers. These results indicate that a change in polyaniline structure during the polymerization process can produce different micro/nanostructures at different reaction stages. This finding provides a practical route for the further design and synthesis of different polyaniline micro/nanostructures.     

Polyaniline nanofibers synthesized by rapid mixing polymerization

Polyaniline (PANI) nanofibers were chemically synthesized by a rapid mixing polymerization with aniline concentration of 0.5 M. The time needed for the formation of PANI in the reaction medium decreased with increase of the molar ratio of ammonium peroxydisulphate (APS)/aniline and temperature. Morphological study showed at the end of polymerization, only the ones prepared with low molar ratio of APS/aniline (e.g. 0.25 and 0.50) and temperature (e.g. 0 and 20 °C) are nanofibers with diameters of ∼50 nm, though the initially formed products are all nanofibers, while with increasing of molar ratio of APS/aniline to 1 and temperature to 20 °C or higher, agglomerates of PANI nanofibers with diameters of ∼100 nm and larger sized irregular particles were formed. The yield of PANI nanofibers was in the range of 13.4–42.3%, which is favorable for mass production of PANI nanofibers. Conductivity measurement, UV–vis and FTIR spectra were performed to characterize the products. The conductivity of the PANI nanofibers increased with molar ratio of APS/aniline at low temperature, while decreased at higher temperature, which might be resulted from the degradation of PANI molecules in the presence of more APS molecules at higher temperature.     

In situ polymerized polyaniline films: The top and the bottom

Polyaniline (PANI) films of submicrometer thickness were deposited in situ during the polymerization of aniline on polystyrene support. Aqueous poly(N-vinylpyrrolidone) solution was subsequently evaporated on the top of PANI films. During peeling off, the PANI film was transferred to a poly(N-vinylpyrrolidone) film. Both sides of the PANI film, the top surface of the film deposited on polystyrene, and the bottom surface originally in contact with a polystyrene dish, became subsequently available for the analysis by Raman and X-ray photoelectron spectroscopies. The molecular structures of both surfaces in protonated state were different, the top side corresponded to protonated emeraldine, while the bottom side to partly deprotonated cross-linked structure containing phenazine-like constitutional units. This result supports the concept of hydrophobic phenazine-like oligomers that are adsorbed at immersed surfaces during the oxidation of aniline, and PANI chains that grow from them, thus creating a brush-like PANI morphology.     

SERS,FT-IR and photoluminescence studies on single-walled carbon nanotubes/conducting polymers composites

Surface-enhanced Raman scattering (SERS), Fourier transform infrared (FT-IR) and photoluminescence (PL) spectroscopies were used to investigate composites based on single-walled carbon nanotubes (SWNTs) and different conducting polymers like polyaniline (PANI), poly-paraphenylene vinylene (PPV) and poly 3-hexylthiophene (3-PHT). In the case of SWNTs/PANI, different composites are obtained by adding dispersed SWNTs powder to the polymer solutions and by chemical polymerization of aniline in presence of SWNTs. The difference originates in the irreversible chemical transformation of SWNTs in the polymerization medium. The synthesis medium used for the preparation of PANI transforms SWNTs in fragments of shorter length like closed-shell fullerenes. This explains the similarity of SERS and FT-IR spectra of the composites PANI/SWNTs and PANI/C₆₀, chemically synthesized. Electrochemical polymerization of aniline in an HCl solution on a SWNT film leads to a covalent functionalization of SWNTs with PANI. In this case, Raman scattering data suggest an additional nanotubes roping with PANI as a binding agent. A post treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. In the case of PPV and 3-PHT, the effect of the concentration of SWNTs on the photoluminescence properties will be described and compared, as probes of interaction.     

Preparation and characterization of polyaniline micro/nanotubes with dopant acid mordant dark yellow GG

Polyaniline(PANI) micro/nanotubes doped with novel dopant acid mordant dark yellow GG (AMY GG) were prepared by soft template method in the presence of ammonium persulfate (APS) as an oxidant. It was found that the molar ratio of HCl to aniline and washing method of the products played key roles in the formation of PANI micro/nanotubes. Changing the molar ratio of HCl to aniline, the typical morphology of PANI could be changed from nanotubes to microtubes. In order to get the final product, different solvents were tried to wash away the by-products. After the by-products were removed by water/methonal/ether, the PANI micro/nanotubes appeared. The morphology of PANI micro/nanotubes was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical structure and thermal stability of PANI micro/nanotubes were examined by Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) and the thermogravimetric analysis (TGA). The formation mechanism of PANI micro/nanotubes was also discussed.     

Synthesis of polyaniline nanotubes using Mn2O3 nanofibers as oxidant and their ammonia sensing properties

In this work, a new method for the synthesis of polyaniline (PANI) nanotubes was presented. Experimentally, Mn₂O₃ nanofibers prepared by electrospinning technique were used as the oxidant template to initiate the polymerization of aniline in acid solution. After reaction, polyaniline shells were formed on the Mn₂O₃ nanofiber surface, and the Mn₂O₃ nanofibers were spontaneously removed. As a result, PANI nanotubes were obtained. As-prepared PANI nanotubes show an average diameter of 80 nm and inner diameter of 38 nm. The final PANI nanotubes were characterized by SEM, EDX, TEM, FTIR and XRD. The gas sensing of as-obtained PANI nanotubes was also investigated. It was found that the PANI nanotube sensing device could detect as low as 25 ppb NH₃ in air at room temperature with good reversibility.     

Conducting polyaniline–montmorillonite composites

Polyaniline (PANI)–montmorillonite (MMT) composites were prepared in two ways: (a) by the polymerization of aniline hydrochloride with ammonium peroxydisulfate (APS) in aqueous suspensions of MMT, (b) by the intercalation of aniline hydrochloride into MMT in aqueous suspension followed by the oxidation with APS, i.e. by the surface and intercalative polymerizations of aniline. The products were analyzed by SEM, XRD, TGA, FTIR and Raman spectroscopies. The formation of red coloration after interaction of MMT with aniline is discussed. The conductivity of PANI–MMT composites increased to units S cm^?1 as the content of PANI reached 50–60 wt.%. The intercalation of aniline into MMT before the polymerization had no marked effect on the conductivity of resulting composites, which was determined mainly by the PANI present at the MMT particles surfaces.     

Polyaniline nanofibers prepared with hydrogen peroxide as oxidant

Polyaniline (PANI) nanofibers were successfully prepared by a sonochemical way with hydrogen peroxide as the oxidant. In comparison with the polymerization performed with mechanical stirring, the polymerization rate of aniline was greatly enhanced and PANI nanofibers were achieved instead of the particulate PANI, though the yield was decreased relatively, indicating the positive effect of ultrasound in producing PANI nanostructures. The uniformity and lengths of the PANI nanofibers were greatly improved as compared with the PANI nanofibers synthesized with ammonium peroxydisulfate (APS) as oxidant under the same conditions, rendering hydrogen peroxide a better oxidant in producing high-quality PANI nanofibers. The PANI nanofibers exhibited similar FTIR spectra, XRD patterns, and dispersibility, but different UV–vis spectra, to their counterpart synthesized with APS as oxidant. UV–vis spectra showed only the head-to-tail structured PANI molecules were produced in the high-quality PANI nanofibers. The findings will be of some help to elucidation of the formation mechanism of PANI nanofibers.     

Self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

Self-assembled polyaniline (PANI) nanotubes, accompanied with nanoribbons, were synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous medium, in the presence of colloidal titanium dioxide (TiO₂) nanoparticles of 4.5 nm size, without added acid. The morphology, structure, and physicochemical properties of the PANI/TiO₂ nanocomposites, prepared at various initial aniline/TiO₂ mole ratios, were studied by scanning (SEM) and transmission (TEM) electron microscopies, FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies, elemental analysis, X-ray powder diffraction (XRPD), conductivity measurements, and thermogravimetric analysis (TGA). The electrical conductivity of PANI/TiO₂ nanocomposites increases in the range 3.8 × 10^?4 to 1.1 × 10^?3 S cm^?1 by increasing aniline/TiO₂ mole ratio from 1 to 10. The morphology of PANI/TiO₂ nanocomposites significantly depends on the initial aniline/TiO₂ mole ratio. In the morphology of the nanocomposite synthesized using aniline/TiO₂ mole ratio 10, nanotubes accompanied with nanosheets prevail. The nanocomposite synthesized at aniline/TiO₂ mole ratio 5 consists of the network of nanotubes (an outer diameter 30–40 nm, an inner diameter 4–7 nm) and nanorods (diameter 50–90 nm), accompanied with nanoribbons (a thickness, width, and length in the range of 50–70 nm, 160–350 nm, and ～1–3 μm, respectively). The PANI/TiO₂ nanocomposite synthesized at aniline/TiO₂ mole ratio 2 contains polyhedral submicrometre particles accompanied with nanotubes, while the nanocomposite prepared at aniline/TiO₂ mole ratio 1 consists of agglomerated nanofibers, submicrometre and nanoparticles. The presence of emeraldine salt form of PANI, linear and branched PANI chains, and phenazine units in PANI/TiO₂ nanocomposites was proved by FTIR and Raman spectroscopies. The improved thermal stability of PANI matrix in all PANI/TiO₂ nanocomposites was observed.     

In situ synthesis and characterization of polyaniline–CaCu3Ti4O12 nanocrystal composites

High dielectric constant (ca. 2.4 × 10⁶ at 1 kHz) nanocomposite of polyaniline (PANI)/CaCu₃Ti₄O₁₂ (CCTO) was synthesized using a simple procedure involving in situ polymerization of aniline in dil. HCl. The PANI and the composite were subjected to X-ray diffraction, Fourier transform infrared, thermo gravimetric, scanning electron microscopy and transmission electron microscopy analyses. The presence of the nanocrystallites of CCTO embedded in the nanofibers of PANI matrix was established by TEM. Frequency dependent characteristics of the dielectric constant, dielectric loss and AC conductivity were studied for the PANI and the composites. The dielectric constant increased as the CCTO content increased in PANI but decreased with increasing frequency (100 Hz–1 MHz) of measurement. The dielectric loss was two times less than the value obtained for pure PANI around 100 Hz. The AC conductivity increased slightly up to 2 kHz as the CCTO content increased in the PANI which was attributed to the polarization of the charge carriers.     

NH3 and HCl sensing characteristics of polyaniline nanofibers deposited on commercial ceramic substrates using interfacial polymerization

This paper reports the use of the interfacial polymerization method in a simple route for fabricating low-cost, highly sensitive NH₃ and HCl gas sensors made of polyaniline (PANI) nanofiber coatings on commercially available ceramic substrates. The PANI coatings consisted of uniform nanofibers and formed emeraldine salt. NH₃ gas-sensing properties of as-synthesized PANI nanofibers were also investigated in detail. The influence of PANI nanofiber acidification treatment at different HCl solutions (pH = 0, 1, 2, and 3) on NH₃-sensing properties was discussed as well. As-synthesized PANI nanofibers exhibited very low detection limit (1 ppm) and high sensing response (response S) was above 2 when the sensor was exposed to 50 ppm NH₃ gas, though a limited drift of the response appeared in the reproducibility test. For acid-treated PANI sensors, the response time increased and response S decreased with increasing pH value of HCl solutions. Response S, especially after acid-treatment, increased as testing proceeded. This is possibly due to the creation of reaction sites on the surface and/or the bulk of the sensors. When PANI was used to detect gaseous HCl, an irreversible process occurred and response S became saturated when the concentration of gaseous HCl reached 200 ppm.     

Electrodeposition of polyaniline nanostructures: A lamellar structure

The growth process of polyaniline (PANI) nanofibers during the electrochemical polymerization was investigated in detail. The nano-fibrillar morphology appears to be intrinsic to PANI, and the unique character is attributable to a combined effect of electrophilic substitution reaction mainly taking place at the para-position of aniline or its oligomers and aniline oligomers with one-dimensional (1D) structure. Interestingly, the PANI film formed on the electrode exhibits a lamellar structure with compact two-dimensional (2D), micro-granular, nanorod-shaped, and nano-fibrillar PANI layers from bottom to top. In addition, the possible formation mechanism of the lamellar structure of PANI film is discussed.     

Preparation and characterization of polyaniline nanoparticles synthesized from DBSA micellar solution

Chemical oxidative polymerization of aniline was performed in a micellar solution of dodecylbenzene sulfonic acid (DBSA, anionic surfactant) to obtain conductive nanoparticles with enhanced thermal stability and processability. DBSA was used to play both the roles of surfactant and dopant. The polymerization kinetics and optimum polymerization conditions were determined by UV–VIS spectra. The optimum molar ratio of oxidant to aniline was 0.5 and DBSA content was the most important factor in the formation of polyaniline (PANI) salt. The polymerization rate was increased with increasing DBSA concentration. The reaction model was proposed on the basis of the roles of DBSA. The electrical conductivity varied with the molar ratio of DBSA to aniline and the highest conductivity of particles was 24 S/cm. The layered structure due to PANI backbone separated by long alkyl chains of DBSA was observed and it seems to facilitate the electrical conduction. The doping level of particle was fairly high and was dependent on the preparative conditions. The average size of the PANI particles determined by Guinier plot of small-angle X-ray scattering (SAXS) measurement was 20–30 nm, which was well coincidence with scanning electron microscopy (SEM) results     

Properties and structural investigation of one-dimensional SAM-ATP/PANI nanofibers and nanotubes

Core–shell one-dimensional (1D) conducting nanofibers were prepared by the encapsulation of attapulgite (ATP) template with polyaniline (PANI) after the surface modification with APTES to form a SAM on the surface of ATP needle-shaped particle (SAM-ATP), and the 1D PANI nanotubes were obtained by etching the SAM-ATP template with HCl/HF solution after the PANI layers were coated onto the SAM-ATP template. The room temperature conductivity and average specific capacitance of 1D 18.7 wt% SAM-ATP/PANI nanofibers (2.21 S/cm and 418 F/g) were increased compared with pure PANI (0.47 S/cm and 342 F/g) and PANI nanotubes (0.01 S/cm and 282 F/g). The conductivity stability and thermal stability of 1D 18.7 wt% SAM-ATP/PANI nanofibers were clearly improved. These trends were accompanied by significant structural changes as evidenced by TEM, FTIR, and XRD studies.     

Solid-state NMR of polyaniline nanofibers

The structure of nanofibers of polyaniline (PANI) formed by oxidation of aniline with ammonium persulfate in the presence of HCl have been determined by solid-state ¹³C and ¹⁵N NMR experiments. The nanofibers synthesized in this way by rapid mixing and in the presence of strong acid resemble standard PANI in structure. Due to rapid mixing the reactants are consumed very quickly at the very beginning of the reaction, preventing secondary growth. Solid-state ¹⁵N and ¹³C CP MAS NMR results suggest that the emeraldine base form of the nanofibers exists mainly as an alternating copolymer of reduced and oxidized repeat units. A broad shoulder between 80 and 150 ppm in the ¹⁵N spectra is attributed to the presence of positively charged radical centers distributed along the polymer backbone, due to protonation of the imine nitrogens. Removal of the shoulder by dedoping with LiOH confirms that it is due to the positively charged imine nitrogens. Non-quaternary suppression ¹⁵N NMR experiment indicates the cross-linking and the presence of tertiary nitrogens in the nanofibers. The imine to amine ratio obtained from a variable contact time experiment is 0.8.     

Synthesis and characterization of novel conducting composites of polyaniline prepared in the presence of sodium dodecylsulfonate and several water soluble polymers

Various conductive composites were prepared by in situ chemical polymerization of aniline in the presence of several water soluble polymers [alginic acid (2a, AA), poly(acrylic acid) (2b, PAA), and poly(vinyl alchol) (2c, PVA)] and/or anionic surfactants [dodecylbenzenesulfonic acid (1a, DBSA) and sodium dodecylsulfate (1b, SDS)] under various polymerization conditions. As a result, the corresponding composites having good film forming property were readily obtained even in the cases with SDS, although PANI prepared in the presence of SDS (PANI/SDS) generally shows extremely poor film forming property due to its low solubility/miscibility and processability in the similar manner as PANI doped with HCl (PANI/HCl). Among the resulting composites, the conductivities of the composites synthesized with SDS tended to be higher than those of the similar composites prepared with DBSA or without anionic surfactants. In particular, the composite prepared by using PVA bearing high molecule weight (PVA-H) and 20 mmol of SDS to aniline monomer was found to show the highest conductivity among the present investigations (32 S/cm), although the conductivity of typical conductive polyaniline doped with HCl, which was synthesized under the similar polymerization conditions, was ca. 3 S/cm at the best. The present PANI composites were characterized by spectroscopic and thermal analysis. Formation of oxidation states of PANIs in these composites was confirmed by the spectroscopic (UV–vis and FT-IR) analysis. The thermal stability of the resulting composite was somewhat lower than those of PANI/SDS itself and PANI/HCl.     

Electrochemical polymerization of aniline on aluminum electrode from an oxalic acid and H2SO4 solutions

In this study, the polymerization of aniline on Al and Pt electrode was examined by cyclic voltammetry There had been reversible reaction on Al electrode. But on the other hand there had been irreversible reaction on Pt electrode. The addition of aniline into the solution led the decrease of current values. The current decreased by adsorption of anodic products on polymer surface. The fact that the anodic peak potential shifted to positive value shows that polyaniline (PANI) catalyzed the formation of polymer. This case shows that the aniline shifted the electrode potential to positive side by the adsorption on the surface. When Al electrode covered with polymer (in 50 mV s⁻¹ potential scanning rate after 20 cycles) was immersed into 1 N HCl solution, the inorganic layer decomposed on the metal surface. This led to decrease the polarization resistance of the metal. SEM microphotographs and EDX fingerprints also confirmed these results.     

Synthesis of PANI nanostructures with various morphologies from fibers to micromats to disks doped with salicylic acid

Shape-controllable polyaniline (PANI) nanostructures varying from fibers to micromats and disks were synthesized via a self-assembly process with salicylic acid (SA) as dopant. It has been achieved just by tuning the concentration of aniline, the mole ratio of SA to aniline, and the mole ratio of APS to aniline in the same reaction. The diameters of the fibers could be controlled from 30 to 400 nm by adjusting the concentration of aniline. Micromats of fibers would be formed by changing the mole ratio of SA to aniline. Disk-like PANI nanostructures were synthesized when decreasing the mole ratio of APS to aniline. Scanning electron microscopy and transmission electron microscopy were applied to investigate various kinds of morphologies. The mechanism of forming these morphologies was proposed to be adjusted by the pH value during polymerization. Ultraviolet–visible (UV–vis) absorption spectra and Raman spectra suggested that these as-prepared PANI were in conductive emeraldine state and featured obviously different molecular structures, which aroused from different reaction conditions.     

Structural characterization of poly-para-phenylenediamine–montmorillonite clay nanocomposites

The structure of the poly-(para-phenylenediamine) (PpPD) formed by oxidative polymerization of 1,4-phenylenediamine (p-PD) into the galleries of the Montmorillonite (MMT) clay was determined by different spectroscopic techniques. According to X-ray diffraction (XRD) pattern the polymer is formed between the clay layers. In addition, the composite morphology, visualized by electron microscopy (SEM), is similar to the pristine clay. The UV–vis–NIR data of PpPD–MMT sample shows strong absorption bands at 620 and 670 nm, being related to groups with protonated phenazine-like rings. The resonance Raman spectrum of PpPD–MMT displays bands at 1179, 1201, 1347, 1412, and 1630 cm^?1, which are related to phenazine-like rings modes. X-ray absorption (XANES) at Nitrogen K edge of PpPD–MMT confirms the presence of azo, hydrazo and phenazinic nitrogens. The intercalated material is EPR silent, confirming that the charge carriers, responsible for the conductivity of ca. 10^?4 S cm^?1, have diamagnetic behavior. Hence, the results confirm that when aniline or its derivates are polymerized in confined environments, it causes preferentially the formation of diamagnetic segments having protonated phenazine and azo segments.     

Synthesis and electromagnetic characterization of polyaniline nanorods using Schiff base through ‘seeding’ polymerization

Two kinds of Schiff base, m-phenylenediamine-glyoxal (Schiff base A) and p-phenylenediamine-glyoxal (Schiff base B), were used as ‘seed’ to induce the polymerization of aniline and hence prepare polyaniline (PANI) nanorods. The different preparation conditions including the Schiff base structure, dosage and acidity of the reaction medium, were investigated to discuss the influence of these conditions on the conductivity of the resulting samples through two-probe method at room temperature. The products were also characterized by Fourier transform infrared (FTIR), ultraviolet–visible (UV–vis), scanning electro microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) techniques. The results implied that these conditions play an important role in the formation of PANI nanorods. Moreover, the resulting PANI nanorods exhibited an unusual electromagnetic loss at the microwave frequency (f = 8.2–12.4 GHz) arose from order arrangement of polaron as charge carrier caused by a nanorods morphology and can be used for the potential application as microwave absorbing materials.