Morphological Evolution of Polyaniline Nanofibers Obtained by Different Synthetic Parameters

Since 1980, considerable studies have been done on conductive polymers, due to their attractive electrical and optical properties resulting from their conjugated -electronsystems [1, 2]. However, polyaniline has unique advantages among other conductive polymers, such as easy synthesis, lowcost monomer, environmental stability and facile acid/base doping/dedoping chemistry [2]. Today, the polyaniline nanostructures have met various applications in differentfields such as corrosion inhibitors [3], anticorrosion coatings [4], electrodes, capacitors [5], rechargeable batteries [6] chemical sensors [7], semiconductors and electromagnetic shielding devices [8]. Nanostructured polyaniline (nanorods/-wires/-fibers) offersthe possibility of enhanced performance wherever a high interfacial area between polyaniline and its environment is important [9]. Different synthesis methods have been utilizedto synthesize nanofiberous polyaniline including template guided polymerization, such as Zeolites as hard template, andsurfactant, micelles, liquid crystal, polyacids as soft template [9-11]. Because all the mentioned methods are dependent on either a template or a specific complex chemical reagent, after-synthetic treatments are necessary to remove them from the products in order to recover pure nanostructuredpolyaniline. Therefore developing syntheses that do not rely on templates, structural directing molecules, or specificdopants are of great importance. Different templatelesspolymerization such as interfacial polymerization [12], oligomer-assisted polymerization[13], and seedingpolymerization[14] have been presented in the literatures. However, some synthetic parameters including monomer concentration, pH of solution, type of dopants and oxidizing agents, mechanical stirring, time of reaction and temperature are important in all of the polymerization methods. The mentioned parameters can influence the molecular weight, morphology and molecular structure. Generally, the conventional synthesis of polyaniline is achemical oxidative polymerization of aniline in presence of a suitable oxidant such as Ammonium Peroxydisulfate (APS) in the solutions [9]. However, the formation mechanism of polyaniline nanofibers is associated with the linear nature ofpolyaniline chains [9]; nanofibers appear to be the intrinsic morphology of polyaniline even in conventional synthesis method. Therefore, it is possible to produce PAni nanofiberswithout using any hard or soft templates. It is suggested that the particle morphology can be correlated with the mode of nucleation of polyaniline: homogeneous nucleation of polyaniline results in nanofibers, while heterogeneous nucleation leads to granular particulates [15].However, both homogeneous and heterogeneous nucleations occur in the conventional synthetic reaction of polyaniline [15], resulting in irregularly shaped granular particles. Therefore, in order to produce polyaniline nanofibers, the heterogeneous nucleation should be suppressed throughout thewhole polymerization process, which will allow polyaniline nanofibers to form continuously via homogeneous nucleation.For this purpose, it is recommended if polyaniline molecules are produced rapidly, then it is more likely that these embryonic nuclei will evolve to create homogeneousnucleation since it takes time to diffuse to heterosites [2, 15, and 16]. The aim of present study was to develop a templatelessmethod to produce nano fibrous polyaniline based on a simple bulk polymerization process. The nanofibers were readily obtained by rapidly mixing of aniline with oxidant in anaqueous acidic solution at 40 ºC temperature and then allowing the polymerization to proceed to completion in theabsence of mechanical agitation. The effect of concentrations of aniline monomer, APS oxidant and HCl dopant on the microstructure and morphology of PAni were investigated. To study the mentioned main factors through the least number of test runs, the experiments were designed using interrelated parameters, including acid/aniline, APS/aniline molar ratios and molarity of acid.