Electrospun polyacrylonitrile based composite nanofibers containing polyindole and graphene oxide

Abstract

Studies on the conductive polymers has gained great interests when the Nobel Prize in Chemistry was awarded by discovery and development of the conductivity of polyacetylene in 2000. Conductive polymers are also called organic metals. They conduct electricity thanks to the conjugated chain structure consisting of consecutive single and double bonds in their structures. Conductive polymers, which are insulating in neutral state, gain conductivity by doping. Polyacetylene, polypyrrole, polythiophene, poly(3,4-ethylenedioxythiophene) (PEDOT) are some of the conductive polymers that have been studied extensively. These polymers can be used in solar cells, super capacitors, chemical and biosensor application areas. Polyindole (PIN) is one of the conductive polymers which can show electrochromic properties with high redox activity, good thermal stability, slow degradation rate and good air stability. Polyindole containing studies have been increasing in recent years and this polymer can be used in the pharmaceutical field, anticorrosion coatings, photovoltaic batteries, supercapacitor applications or anode material in batteries. On the other hand, with the advancement of nanotechnology, it has been found that materials in nano scale show physical and chemical property differences compare to the bulk form. Nanoscale generally includes the range of 1-100 nm. When the size of the particle in the material becomes too small, the electronic structure of the material can change. For example; gold normally does not react, but can be active at the nano level. Nanofibers are fibers with a high length/volume ratio with average diameters in the order of nanometers. In addition to their chemical properties also depending on the surface properties such as morphology and topography, materials can improve and can be used various areas. Due to their low densities, large surface areas with porous structures, it has a wide range of research and application areas of nanofibers such as filtration, tissue engineering, drug release systems, biomedical, textile, energy storage and sensor. Especially in recent years, electrospinning technique has attracted interests by scientists to generate nanofibers because of it is extremely simple, cheap and practical usage. On the other hand, scientists have great expectations since discovering a few atoms thick materials. Graphene oxide (GO) is a two dimensional material with high surface area. It can be semiconductor or insulating material which depends on the degree of oxidation, sheet size, microstructure and among many other factors. Moreover, graphene oxide contains some functional groups (epoxy, hydroxyl, carbonyl, etc.) on the structure which makes the dispersive ability in the solvent. These oxygen containing functional groups enable the development of GO-based composites, especially due to their ability to disperse in the solvent. Unfortunately, nanofiber production from conductive polymers and GO like materials can be limited or not possible by electrospinning method. For this reason, nanofibers of conductive polymers and GO are produced by making blend or composite with a different polymer called as carrier polymer, whose nanofibers can be easily obtained by electrospinning. Polyacrylonitrile (PAN) is one of the carrier polymer which is a very common usage area especially in the textile manufactory and carbon fiber production. Also, PAN fibers are the precursor of high quality of carbon fibers. PAN is choosen as a carrier polymer and PAN based composites are studied in this thesis. In the first part of the study; the oxidative chemical reaction of polyindole has taken place in the presence of FeCl3. Nanofibers were produced by mixing polyindole with polyacrylonitrile in N, N-Dimethylformamide (DMF) solvent at different weight / volume ratios. Polyacrylonitrile and polyindole blends were generated in different proportions of polyindole. Composite fibers were produced from the solutions by adjusting the optimum conditions using electrospinning method. Morphological, thermal properties, spectral analysis of these fibers were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (FTIR-ATR) and Differential Scanning Calorimetry (DSC). Electrochemical characterization of fibers has been studied by Electrochemical Impedance Spectroscopy (EIS). The experimantal data was used to fit the equivalent circuit with Zsimpwin Software. In addition, it was found that the electrochemical properties (such as double layer capacitance, solution resistance and charge transfer resistance) of composite fibers were effected by surface tension and conductivity of solution. Iron, is an important element in the industry, environment, medical applications areas, biological studies and human health. Different methods such as electron spin coulometry and ion selective electrodes are used in the determination of Fe(II). Differently, in this study, Electrochemical Impedance Spectroscopy is presented as an alternative technique to determine Fe(II) concentration. Electroactive behavior of the fiber electrode was investigated by Cyclic Voltammetry (CV). Electroactivity of the nanofiber selected depending on the impedance and morphological properties of the nanofibers was measured with the help of K3Fe(CN)6/K4Fe(CN)6 electrolyte. It was discussed that the presence of Polyindole (PIN) content showed an electrocatalytic activity against K3Fe(CN)6/K4Fe(CN)6. The lowest Fe(II) ion analyte concentration detection limit for the selected electrode was calculated as 1x10-4 mol.l-1. In the second part of the study which is different from the first part, graphene oxide (GO) is choosen as a material to improve the capacitive property of Polyacrylonitrile. Polyacrylonitrile / Graphene oxide (GO) nanofibers were produced by using a rotary collector instead of a fixed collector in the electrospinning device. Thus, thinner, more aligned nanofibers with higher young modulus were acquired. Oxidative stabilization and carbonization applied to composite nanofibers through the thermal process. In particular, the stretching applied to the nanofiber during oxidation determines the mechanical strength and structure of the final product carbon nanofiber to be formed. Therefore, understanding of the oxidation mechanism is an essential part of the production of carbon nanofibers (CNFs). The stress, temperature and application time utilized to the material in oxidation affect the structure of the carbon nanofiber. Oxidation step of electrospun polyacrylonitrile based composite nanofibers was studied and morphological, spectral and electrochemical properties of composite nanofibers were investigated. Morphological and spectral characterizations of composite nanofibers were performed by FTIR-ATR and Raman Spectroscopy, SEM, AFM and Transmission Electron Microscopy (TEM). Mechanical tests were performed with Dynamic Mechanical Analysis (DMA). Thermal behaviours of composite nanofibers were investigated by Thermal Gravimetric Analysis (TGA). Capacitive behavior of nanofibers were performed by EIS and CV. When there is GO in the structure, the ions in the solution can penetrate into the pores which cause the double layer capacitance (Cdl) value increasement. Average pore diameters have been measured with the ImageJ program to be around 38.5 nm and it has been found that the double layer capacitance (Cdl) of PAN nanofibers containing GO is 0.600 µF which is the highest value. Also, it was observed that the capacitive behaviour of carbon nanofiber formed in the presence of graphene oxide improved. PAN/GO carbon nanofibers exhibit potential for capacitive applications in the light of these results. In the third part of the study, X-ray Photoelectron Spectroscopy (XPS) and FTIR analysis methods were used to understand the oxidative stabilization deeply. The thermal oxidative stabilization of polyacrylonitrile has a complex mechanism with the cyclization and dehydrogenation steps. Polyacrylonitrile (PAN) composite nanofibers with GO were fabricated, and thermal oxidation were performed to these nanofibers. The oxidation process were applied at various temperatures (250 0C, 280 0C, and 300 0C) during 1h and 3h. Nanofibers were significantly effected by high temperature with during long duration time. The effect of GO addition into the nanofibers were analyzed by XPS, FTIR-ATR and, EIS. After heat treatment, change in C1s spectra and development of sp2 carbon was detected by XPS. It was concluded that the presence of GO accelerated the oxidation mechanism and developed the final structure.