Syntheses And Applications Of Polyaniline Nanofibers

Both the discovery of new approaches for synthesizing polyaniline (PANI) nanofibers and the demonstration of potential applications of the nano-materials have been the research focuses in recent years. PANI is one of the most intensively investigated intrinsically conducting polymers because of its low cost, easy synthesis, tunable conductivity, environmental stability and many promising applications. PANI nanofibers exhibit the advantages of both the organic conductors and low-dimensional nano-structures, demonstrating potential applications in the fields of molecular conducting wires, actuators, energy storage and field emitting and so on. Furthermore, PANI nanofibers solved, to some extent, the problem of intractability of PANI, facilitating the wider applications of PANI. In this dissertation, a novel template free method with good scalability was put forward to prepare PANI nanofibers, i.e. by exerting ultrasonic irradiation in the chemical oxidative polymerization of aniline, PANI nanofibers with controlled structures were successfully prepared. The ultrasonic irradiation method was compared with the interfacial polymerization for preparation of PANI nanofibers, and the formation mechanism of PANI nanofibers was discussed. The applications of PANI nanofibers in the fields of transparent conductive films and electromagnetic interference (EMI) shielding were explored. All results and conclusions obtained are listed below.1. By simply replacing the mechanical stirring in the traditional PANI synthesis procedure with ultrasonic irradiation, PANI nanofibers were facilely prepared without use of any template, and a relatively higher yield of 59% was demonstrated. The effect of ultrasonic irradiation on preventing the growth and agglomeration of PANI nanofibers was confirmed by the secondary addition of the reagents. It was found that the polymerization rate of the ultrasonic irradiated system increased, but the molecular weight and conductivity of the polymer decreased. In the case of lower ammonium peroxydisulfate (APS)/aniline molar ratios (e.g.≤1.0), PANI nanofibers in diameters of 50nm with high quality were achieved, while in the case of higher APS/aniline molar ratios (e.g. 2.5), mixtures of PANI nanofibers in diameters of 100nm and micro-sized irregular PANI particles were obtained. 2. By changing the mixing manner of the solutions of APS and aniline, the effect of ultrasonic irradiation on preventing the growth and agglomeration of PANI nanofibers was confirmed further. With the conventional rapid mixing polymerization, PANI nanofibers can only be achieved at lower aniline concentrations (≤0.05 M), and aggregates of PANI nanofibers with diameters of 100nm mixed with irregular shaped PANI particles were formed at relatively higher aniline concentrations (e.g. 0.10M). However, with exertion of ultrasonic irradiation, the negative effect of higher aniline concentration on formation of PANI nanofibers was neutralized, and PANI nanofibers were also prepared easily at higher aniline concentration (e.g. 0.10M). With the solution of aniline added dropwise into that of APS, i.e. in the reverse addition manner, the primarily formed PANI nanofibers changed into mixtures of PANI nanofibers and laminas with continue of the polymerization under mechanical stirring, while under ultrasonic irradiation, PANI nanofibers were resulted for all the time.3. PANI nanofibers with higher lengths (ca. 300-1000nm) and purity were synthesized employing H2O2 instead of APS as the oxidant by the ultrasonic irradiation method. In the case of mechanical stirring, the induction period of the reaction was so long that the polymerization can be considered as a rapid mixing polymerization. Though PANI nanofibers were initially formed at early stages of polymerization, mixtures of irregular PANI particles and aggregates of PANI nanofibers with rough surfaces were resulted with progressing of the polymerization. In the case of ultrasonic irradiation, the induction period reduced greatly, indicating the increasing of the polymerization rate. The product exhibit more uniform morphology, higher aspect ratio, but slightly decreased polymer yield.4. The presence of an interface formed between two immiscible liquids was not able to depress and prevent completely the secondary growth of the primary PANI nanofibers in an interfacial polymerization, and the aniline concentration is the key factor that determines the morphology of PANI. Besides, the stirring speed had some effect on the morphology of the polymers.5. With mechanical stirring or no stirring during chemical oxidative polymerization, if the contact between aniline and the formed PANI nanofibers can be avoided (e.g., in the case of the interfacial polymerization, rapid mixing polymerization with lower aniline concentration, etc.), PANI nanofibers can be achieved in the final product. Whereas, the PANI nanofibers would catalyze further the polymerization of the aniline around them, leading to the growth of PANI nanofibers, and final product of irregular micro-sized PANI particles were obtained (e.g. the conventional prepared PANI). In the case of ultrasonic irradiation, although the PANI nanofibers co-existed with excess aniline molecules, what happened mainly were the transformation from PANI molecules to PANI nanofibers, but not the growth of PANI nanofibers, i.e., the ultrasonic irradiation prevented effectively the growth of PANI nanofibers, resulting in completely PANI nanofibers in the final product.6. The sonochemically prepared PANI nanofibers exhibited the same chemical and crystal structures as the traditionally prepared PANI irregular particles, with all the PANI molecules distributed randomly inside the PANI nanofibers. Variation of the oxidants had no influence on the Fourier transformed infrared (FTIR) spectra, X-ray diffraction (XRD) patterns, dispersibility and conductivity of the PANI nanofibers. While only head-to-tail structured PANI molecules were observed inside the PANI nanofibers prepared with H2O2 as oxidant, which is different from the product prepared with APS as oxidant. PANI nanofibers demonstrated good dispersibility and can be dispersed easily in a variety of solvents, such as water, ethanol, methyl isobutyl ketone (MIBK) and so on.7. Sulfuric acid doped PANI nanofibers were sonochemically prepared in a reaction medium of sulfuric acid. By dispersing the sulfuric acid doped PANI nanofibers in either the solution of poly (methyl methacrylate) (PMMA) or polyacrylate (PA) in MIBK with just mechanical stirring and ultrasonicating processing, transparent conductive PMMA/PANI nanofibers composite films or PA/PANI nanofibers electromagnetic interference (EMI) shielding coatings were prepared. Preparation of the PANI nanofibers based films or coatings is one of the most promising processing techniques owing to its easiness and low energy consuming characteristics, which is beneficial for practical application of PANI.