Polyaniline precipitation in aqueous medium: from bulk aggregates to nanoparticles

A postsynthetic self-assembly system was designed to investigate a construction process from suspended polyaniline (PANI) molecules to condensed aggregates. The conventionally synthesized PANI was dissolved in polar solvent and introduced into acidic medium with electrolytes similar to the aniline chemical oxidative polymerization (COP) medium. In this way, reaction interference that is usually encountered in the COP process could be avoided, and influences of medium conditions including organic electrolytes on the self-assembly behaviors of PANI were studied. It was discovered that, in a static aqueous medium with moderate pH and rich electrolytes, PANI molecules composed of bulk aggregates could self-assemble into well-dispersed nanoparticles with few structural changes. Electrostatic force is considered to dominate the self-assembly of PANI molecules as compared with other noncovalent interaction or the effect of soft templates such as ionic liquid and surfactant. The results are supposed to provide better understanding on the formation mechanism of micro/nanostructured PANI.     

Polyaniline micro-/nanostructures: morphology control and formation mechanism exploration

This article provides a brief overview of recent work by the authors’ group as well as related researches reported by others on controlling the morphology and exploring the formation mechanism of typical micro-/nanostructures of polyaniline (PANI) and aniline oligomers through template-free aniline chemical oxidation process. The contents are organised as follows: (i) tuning the morphology of aniline polymerisation products by employing ultrasonic irradiation, mass transfer, and pH profiles; (ii) exploring the formation mechanism of micro-/nanostructures during aniline chemical oxidation through examining the precipitation behaviours of aniline oligomers and polymers in a post-synthetic system; (iii) tailoring PANI micro-/nanostuctures into pre-designed morphology by introducing certain heterogeneous nucleation centres; (iv) application potential of PANI nanofibres in the areas of transparent conductive film, electromagnetic interference-shielding coating and graphene-based electrode materials. This short review concludes with our perspectives on the challenges faced in gaining the exact formation mechanism of PANI micro-/nanostructures and the future research possibility for morphologically precisely controlled PANI micro-/nanostructures.     

Influence of ethanol on the chain-ordering of carbonised polyaniline

Polyaniline (PANI) was prepared by the oxidation of aniline hydrochloride with ammonium peroxydisulphate in water or in a water-ethanol mixture. In the presence of ethanol, PANI nanotubes and nanorods were observed. Both products were carbonised in a nitrogen atmosphere at 650°C. Initial and carbonised products were characterised by scanning and transmission electron microscopies, thermogravimetric analysis and wide-angle X-ray scattering. Their molecular structure was studied by UV-VIS, infrared, and Raman spectroscopies. Carbonised sample obtained from the PANI salt prepared in the presence of ethanol exhibits Raman spectrum which corresponds to a more ordered carbon-like material than carbonised samples obtained from the PANI base and the PANI salt prepared in pure water. The influence of ethanol present in the reaction mixture on the molecular and supra-molecular structure of PANI and, consequently, on the enhancement of chainordering of carbonised PANI is discussed.     

The kinetics and spectral studies of the in situ polyaniline film formation

The chemical oxidation of aniline with ammonium persulfate (APS) in an aqueous acidic solution to form polyaniline (PANI) films has been studied using the quartz crystal microbalance (QCM) technique. The kinetics of the film formation was investigated. The reaction exhibited half-order with respect to APS and first-order to aniline. The effect of temperature on the growth rate of PANI films was studied. The activation energy is 39.79 kJ/mol. This is in agreement with the corresponding one determined for the chemical polymerization of PANI in the bulk. The UV-visible spectra of the PANI films grown onto a glass support immersed into the bulk solution were measured. The absorption of the PANI film with time was also studied and compared to the growth of the PANI film thickness using the QCM technique.     

On the origin of colloidal particles in the dispersion polymerization of aniline

When aniline is oxidized in an aqueous medium in the presence of a steric stabilizer, colloidal polyaniline (PANI) dispersions are obtained. The generally accepted model of the stabilization assumes that the macromolecules of the water-soluble steric stabilizer are adsorbed at the polymer, precipitating during the dispersion polymerization, and provide steric protection against further aggregation. An alternative mechanism of conducting-polymer particle formation is proposed in the present study. We suggest that the steric stabilizer provides a site for adsorption of oligoaniline initiation centers; subsequent polymerization from anchored centers yields particle nuclei that grow to produce colloidal PANI particles. This hypothesis is based on the observation that the colloidal particles are obtained only in the case where the steric stabilizer is introduced in the early stages of polymerization when aniline oligomers are present in the reaction mixture. If the stabilizer had been added during the growth of PANI chains, colloidal dispersions would not have been produced. The process of particle growth is completely analogous to the formation of conducting PANI films on the surface of microparticles and various materials. There, the polymerization of aniline at the surfaces is preferred to the same process proceeding in the bulk of the reaction mixture. While the films grow at the interfaces with the reaction mixture, the dispersion particles similarly emanate from the stabilizer chains. The particle size, the formation of nonspherical morphologies, the importance of the chemical nature of the stabilizer chains, and the general relation between the conducting-polymer film and particle growth are discussed in the light of the proposed model.     

Formation of polyaniline nanofibers: a morphological study

Polyaniline (PANI) powders were prepared by solution precipitation, rapid mixing polymerization, and interfacial polymerization to find the key factors that influence the formation and growth of PANI nanofibers. In chemical oxidative polymerization of aniline, the morphology of the product is mainly determined by aniline concentration. In the case of lower aniline concentration, PANI nanofibers were formed and can be preserved and collected as final product, while in the case of higher aniline concentration, larger sized PANI particles or agglomerates were obtained owing to the growth of the nanofibers. Without participation of the oxidizing step, solid PANI samples with compact structures and dissimilar morphologies were achieved by random accumulation of PANI molecules.     

Self-assembly of aniline oligomers in aqueous medium

By dissolving branched or linear aniline oligomers in polar solvent and introducing their stock solution into an aqueous acidic medium, sheet-like as well as wire-like supramolecular structures with well-defined morphology were obtained, respectively. These oligomeric supramolecular structures were constructed via a post-synthetic precipitation process, indicating that aniline oligomers are capable of self-assembling in an aqueous medium, which is similar to the reaction medium of aniline chemical polymerization. Possible formation mechanisms of these supramolecular structures were proposed, i.e., sheet-like products were probably constructed by collapsed molecular chains of aniline oligomers with branched units through π–π stacking and hydrogen bonding, whereas formation of the wire-like products was attributed to “oriented-attachment” of collapsed molecular chains of linear aniline oligomers. The findings obtained in this study are supposed to provide useful clues for uncovering the formation mechanism of polyaniline micro-/nanostructures.     

聚苯胺内方形微米管和方形微米棒的可控制备和结构

以柠檬酸为掺杂剂、过硫酸铵为氧化剂,通过改变苯胺单体的浓度实现了聚苯胺微/纳米结构的可控的自组装制备.在较高苯胺单体浓度时,自组装得到具有内方形的导电态聚苯胺微米管,其直径约为580~300nm,管壁厚约为80 nm.而在极稀的苯胺单体浓度时,自组装得到正方形横截面的方形微米棒,其横截面的边长约为690~290 nm,长度约为1~40μm.结构表征结果证明,所得的微米方形棒是N—N单键结合的聚氮烷;而延长聚合反应时间,则可得到本征态聚苯胺微米管.     

Voltammetric and waveguide spectroelectrochemical characterization of ultrathin poly(aniline)/poly(acrylic acid) films self-assembled on indium-tin oxide

We report on the spectroelectrochemical characterization of conducting polymer (CP) films, composed of alternating layers of poly(aniline) (PANI) and poly(acrylic acid) (PAA), deposited on ITO-coated, planar glass substrates using layer-by-layer self-assembly. Absorbance changes associated with voltammetrically induced redox changes in ultrathin films composed of only two bilayers (ITO/PANI/PAA/PANI/PAA) were monitored in real time using a unique multiple reflection, broadband attenuated total reflection (ATR) spectrometer. CP films in contact with pH 7 buffer undergo a single oxidation/reduction process, with ca. 12.5% of the aniline centers in the film being oxidized and reduced. The ATR spectra indicate that during an anodic sweep, the leucoemeraldine form of PANI in these films is oxidized to generate both the emeraldine and pernigraniline forms simultaneously. A comparison of the behavior observed during anodic and cathodic sweeps suggests that the rate of oxidation is limited by structural changes in the polymer film originating in electrostatic repulsion between positively charged PANI chains.     

Fabrication of polyaniline with hierarchical structures in alkaline solution

Nanosheet- or nanorod-based microspheres and nanorod-based microrods of polyaniline (PANI) with hierarchical structures were successfully prepared by oxidation polymerization of aniline in alkaline solution. Temperature was found to have important influence on the morphology of PANI hierarchical structures and their building blocks. The concentration of alkali (NaOH) could be used to guide the morphological evolution of PANI, from leaf-like structures to nanosheet-based particles, and to nanorod-based microspheres and nanorod-based microrods with increasing concentration of NaOH in synthesis. The chemical structures of product were characterized by FTIR, UV-vis spectra and XRD, and its solubility was also studied in this report.     

Preparation of polyaniline in the presence of polymeric sulfonic acids mixtures: the role of intermolecular interactions between polyacids

Polyaniline (PANI) was synthesized by chemical oxidation of aniline in the presence of mixtures of water-soluble poly(sulfonic acids) of different nature. Under these conditions, the use of polyacid templates leads to the formation of interpolymer complexes of PANI and polyacid mixtures. The obtained PANI complexes were characterized by UV, visible, near IR, and Fourier transform infrared spectroscopy. It was shown that the rigidity of the polyacid backbone and the composition of a polyacid mixture affect the electronic structure of PANI complexes and the duration of the induction period of aniline oxidation. Domination of the more rigid-backbone template in the synthesis of PANI complexes with mixtures of the rigid- and flexible-backbone polyacids was observed. According to the viscometry and FTIR spectroscopic data, the reason of the domination is the existence of the intermolecular interaction between the polyacids in the mixture. In this case, duration of the induction period of aniline oxidation was between these values for pure polyacids.     

Electrochemical oxidation of aniline in a silica sol–gel matrix

Electrochemical oxidation of aniline encapsulated in a silica solid electrolyte prepared by a sol–gel process yielded products that were dependent on the pore size. An acid-catalyzed process that used tetramethyl orthosilicate as the precursor and aniline as a dopant yielded the silica. When the aging time was limited to one day so that a mesoporous solid was obtained, the potentiodynamic oxidation of aniline at a carbon fiber electrode resulted in the formation of polyaniline. With aging times of 3–5 days, microporous silica was obtained. In this electrolyte, the formation of dimers and other oligomers was observed by cyclic voltammetry. Evidence for these products was the presence of a quasi-reversible redox couple at 0.2 V vs Ag/AgCl that was previously related to oligomeric aniline by Raman spectroscopy. The results supported the hypothesis that the pore structure of sol–gel electrolytes can influence the pathways of electrode reactions therein.     

DNA-directed assembly of polyanilines: modified cytosine nucleotides transfer sequence programmability to a conjoined polymer

A series of polyaniline (PANI) oligomers was constructed from monomer units covalently linked to duplex DNA through N-(2-aminoethyl) groups bonded through cytosines. DNA oligomers containing the aniline monomers were treated with horseradish peroxidase (HRP) and H2O2 under conditions known to cause polymerization of aniline. No change in the absorption spectrum of the DNA was observed for samples containing fewer than four contiguous aniline groups. However, for oligomers containing four, five, or six aniline units, treatment with HRP and H2O2 led to the appearance of absorption features characteristic of the conducting "proton doped" emeraldine oxidation state of PANI. Molecular modeling shows that the DNA is distorted in the region of the PANI, but flanking regions of the DNA maintain their B-form structure. These findings provide a method to exploit the self-recognition, self-assembly, and sequence programmability of DNA for the formation of conducting polymers.     

层层自组装原位聚合聚苯胺复合膜成膜机理研究

从苯胺单体出发, 通过原位聚合、现场掺杂以及基于静电力的层层自组装制备了聚苯胺复合膜. 通过苯胺活性溶液的温度及颜色变化跟踪聚合反应进程, 同时考察不同聚合反应阶段所得聚苯胺复合膜的紫外-可见吸收, 并进一步探讨聚苯胺复合膜的成膜机理. 研究表明, 成膜机制是由聚合反应初始阶段的苯胺阳离子或苯胺阳离子自由基通过静电作用快速吸附到负电性的基片表面, 形成均匀的聚合中心, 链增长生成聚苯胺; 该聚苯胺在酸性条件下经现场掺杂显电正性, 可吸附电负性的聚苯乙烯磺酸钠(PSS), 以此循环层层组装得到多层聚苯胺复合膜.     

Cerium dioxide/polyaniline core-shell nanocomposites

The preparation of CeO2/polyaniline (CeO2/PANI) core-shell nanocomposites via chemical oxidation of aniline using CeO2 as an oxidant is reported. TEM, TGA, FT-IR, XPS, and conductivity measurement are used to characterize the resulting composites. TEM measurements reveal that the shape of PANI/CeO2 nanocomposites is different from CeO2 nanoparticles and fibular PANI oxidized with soluble oxidant. Electron diffraction (ED) patterns of CeO2/PANI nanocomposites reveal single crystal of CeO2. FT-IR spectra confirmed the formation of PANI; the amount of PANI in the nanocomposites is estimated by TGA results. The conductivities increase with the increasing ratio of PANI/CeO2. XPS results reveal that in the nanocomposites Ce4+ of CeO2 is reduced to Ce3+. In addition, the degree of protonation of polyaniline obtained from N 1s XPS results in cerium dioxide/polyaniline composites is about 48.52%.     

Polyaniline film deposition from the oxidative polymerization of aniline using K2Cr2O7

The deposition of the polyaniline (PANI) films was monitored using the quartz crystal microbalance (QCM) technique. The films were grown from an aqueous dilute hydrochloric acid solution by the chemical oxidation of aniline using potassium dichromate (KDC). The effect of the initial molar ratio of the KDC/aniline on the yield and the growth rate of the PANI films were studied. There is no optimum initial molar ratio of KDC/aniline of PANI film deposition. Also there was a small depletion period and no degradation to the deposited PANI films. The order of the polymerization kinetics was studied with respect to KDC. The UV-visible spectra of the PANI films grown onto a glass support immersed into the bulk solution were measured. The absorption of the PANI film with the time of polymerization was compared to the growth of the PANI film thickness with time determined from the QCM technique. The characteristics of the PANI film deposition were compared to the corresponding ones that were observed during the oxidative polymerization of aniline with ammonium persulphate (APS).     

Dendritic polyaniline nanoparticles synthesized by carboxymethyl chitin templating

Dendritic polyaniline (PANI) nanoparticles were synthesized via oxidative polymerization of aniline, using ammoniumperoxodisulfate as an oxidant, and CM-chitin as a template. The reaction was performed under acidic conditions and the template was removed after the polymerization was completed. Molecular characterization (including UV-vis, FTIR, TGA, and XRD) suggests that the structure of the synthesized dendritic PANI nanoparticles is identical to that of the emeraldine form of PANI, synthesized by the conventional route (without the addition of the CM-chitin template). SEM images reveal that the dendritic PANI nanoparticles have an average diameter in the nanometer range, and are globular in shape, with radially oriented PANI dendrites; in contrast, irregularly-shaped aggregates of PANI are obtained using the conventional synthesis. It was further found that the size of the dendritic PANI nanoparticles is dependent on the CM-chitin content. The higher the CM-chitin concentration, the smaller is the size of the dendritic PANI nanoparticles obtained. An interpretation of these observations and a possible formation mechanism are proposed based on self-assembly between the CM-chitin chains and the aniline monomer.     

BrΦnsted酸性离子液体掺杂聚苯胺的无溶剂制备及其表征

首先合成了两种带—SO3H官能团的Brφnsted酸离子液体([MIMPS][HSO4]和[PYPS][HSO4]),然后分别以这两种带—SO3H官能团的Brφnsted酸性离子液体做为掺杂剂,在无溶剂条件下通过机械化学聚合反应制备了掺杂导电态聚苯胺.由于带—SO3H官能团的Brφnsted酸性离子液体中H+可以单独以离子形式存在,因此可形成质子化导电态的翠绿亚胺盐,并以红外光谱、紫外可见-近红外、X-射线衍射、循环伏安和四探针技术等测试方法对聚苯胺进行了结构和性能表征.PANI-[MIMPS][HSO4]的结晶性、电导率和电化学活性要优于PANI-[PYPS][HSO4].     

Micelle-assisted synthesis of polyaniline/magnetite nanorods by in situ self-assembly process

Polyaniline/magnetite nanocomposites consisting of polyaniline (PANI) nanorods surrounded by magnetite nanoparticles were prepared via an in situ self-assembly process in the presence of PANI nanorods. The synthesis is based on the well-known chemical oxidative polymerization of aniline in an acidic environment, with ammonium persulfate (APS) as the oxidant. An organic acid (dodecylbenzenesulfonic acid, DBSA) was used to replace the conventional strong acidic (1 M HCl) environment. Here, dodecylbenzenesulfonic acid is used not only as dopant, but also as surfactant in our reaction system. So, DBSA can excellently control the morphology and size of PANI nanorods and magnetite particles. Magnetite particles were formed simultaneously during sedimentation, and the formed nanorods were also decorated by the particles. The resulting PANI/magnetite composites were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). It is found that PANI/magnetite nanorod composites have uniform size, superparamagnetism and a small mass fraction of magnetite, thermal stabilization even at a higher temperature.     

微/纳米结构的聚苯胺及其浸润性研究

借助沉积聚合辅助的“无模板”法在玻璃基片上制备出水杨酸掺杂的微/纳米结构的聚苯胺.实验发现,微/纳米结构的形貌及其浸润性依赖于掺杂剂与单体的摩尔比和沉积时间.当低分子量的聚苯胺微米球和纳米球共存时,其沉积的表面呈现出高的疏水性(接触角θ=148.0°),这主要来源于微/纳米共存的结构导致高的表面粗糙度,能捕获更多的空气所致.FTIR,紫外-可见光谱和X光射线衍射表征了微/纳米球的分子结构及其结晶性.