Synthesis and structural characterization of polyaniline thin films
Name and Surname:Maria Skosana
Subject name and code:Polymer Science Practical IV PWP410T
Month and year:January 2018
Polyaniline (PANI), Films, Spin coating, Scanning electron microscopy (SEM), X-ray Diffraction (XRD)
TOC o “1-3” h z u 1.INTRODUCTION PAGEREF _Toc516482623 h 42. LITERATURE REVIEW…………………………………………………………………………5
3.EXPERIMENTAL PAGEREF _Toc516482624 h 63.1MATERIALS PAGEREF _Toc516482625 h 63.2PROCESSING PAGEREF _Toc516482626 h 63.3EVALUATION PAGEREF _Toc516482627 h 74.RESULTS AND DISCUSSION PAGEREF _Toc516482628 h 85.CONCLUSION PAGEREF _Toc516482629 h 86.REFERENCES PAGEREF _Toc516482630 h 87.APPENDIX PAGEREF _Toc516482631 h 8
INTRODUCTIONIntrinsically conducting polymers have been studied by many researchers because of their electrical properties CITATION Ser13 l 1033 (Sergeyev & Boeva, 2013). Examples of these polymers include polyacetylene, polyphenylene sulfide, polythiophene, polypyrrole and polyaniline. Polyaniline (PANI) is one of the most studied and exhibits interesting properties compared to other conducting polymersCITATION Mac01 l 1033 (MacDiarmid, 2001). The properties include oxidation states that can be altered for a suitable application and the conductivity. The conductivity arises from doping of the PANI either by protonation or oxidation. PANI can be easily fabricated and coated onto different substrates. It is chemically stable under different environmental conditions CITATION Rek18 l 1033 (Reka , Devi, Saranya, & Dharmaraja, 2018). It is prepared by chemical oxidative polymerization method in the presence of dopants.
PANI exists in various oxidation states CITATION Fea l 7177 (Feast, Tsibouklis, Pouwer, & Groenendaal, 1996). Leucoemeraldine (C6H4NH)n is clear and colourless, emeraldine (C6H4NH2 C6H4NHn) is blue for the emeraldine salt and green for emeraldine base, (per)nigranile (C6H4NH)n is blue/violet. The emeraldine base is neutral, if doped, it is called emeraldine salt. Leucoemeraldine and pernigraniline are poor conductors notwithstanding when doped with an acidCITATION Mac01 l 1033 (MacDiarmid, 2001). Doping is the addition of organic or inorganic acids/bases to change the chemical state of a material.
PANI can easily be switched from an insulating form to a conducting form by either protonic acid doping or base de-doping processes. It is the emeraldine form which can be switched from conducting to insulating CITATION Ver07 l 1033 (Verma & Dutta, 2007). This allows it to be used in different applications. Examples of applications include organic electrodes, sensors, and actuatorsCITATION Pol02 l 1033 (Polowinski, 2002).
The overall aim is to develop inexpensive nuclear radiation sensors; and the objective in this project is to study structural properties of PANI films cast from the insulating PANI emeraldine base (EB). Thereafter, the EB form will be obtained through de-protonation of the emeraldine salt using sodium hydroxide solution. The emeraldine base precipitate will be separated by filtration and drying CITATION Msi16 l 1033 (Msimanga, 2018). Polyaniline EB is known to be very stable at room temperature. EB is also has a few N-H groups in its main chain and it is partially oxidised and insulative as shown in Figure 1 CITATION Mac01 l 1033 (MacDiarmid, 2001).
Figure 1. Polyaniline (emeraldine) salt is deprotonated in the alkaline medium to polyaniline (emeraldine) base. A is an arbitrary anion chloride (Cl) CITATION CHE02 l 7177 (Stejkal, 2002).
In this work PANI will be synthesized by the oxidative polymerization of aniline CITATION Hig04 l 1033 (Hig04). Aniline Hydrochloride (ANI-HCl) will be used as the source of monomer aniline. The synthesis will be done by mixing aqueous solutions of aniline hydrochloride and ammonium peroxydisulfate (APS) at room temperature to form polyaniline hydrochloride. This is the emeraldine salt form. Aniline hydrochloride will be used as a monomer, because the handling of solid aniline salt is preferred to liquid aniline from the point of view of toxic hazards CITATION CHE02 l 1033 (Stejkal, 2002).
PANI-EB films are going to be prepared by spin coating. Spin coating is a process used to deposit uniform thin films on to flat substrates. To spin coat the emeraldine base (EB), it will be dispersed in dimethyl sulfoxide via sonication and spun on to silicon substrates. The amount of the EB for coating is applied on the centre of the silicon substrate, which may be spinning at low speeds or not spinning at all. Rotation continues while the fluid spins off to the edges of the substrate until the desired thickness of the film is reached CITATION LES88 l 1033 (Scriven, 1988).
The spin coated EB films should be semi-crystalline. The crystallinity will arise from the cross-linking in the PANI chains as they undergo polymerization. The c=c stretching and N-H stretching will result from the addition of the APS and the presence of –SO3- confirms successful polymerization. The degree of crystallinity has an effect on the mobility of charge carriers of the finished sensor device. Therefore, a completely amorphous structure would be undesirable CITATION Msi16 l 1033 (Msimanga, 2018).
This work will be looking at the synthesis and structural characterisation of the polyaniline thin films. To confirm successful synthesis, FTIR will be used to determine the functional groups present in the PANI-EB films. The functional groups must show N-H groups, -SO3- and C=C stretching to confirm successful polymerization. The XRD will be used to determine the crystallinity of the films. SEM evaluation methods will be used to determine the morphology of the cast films. A homogeneous morphology is ideal. These structural properties have an effect on the electrical properties of the films. The desired properties include a homogeneous morphology and semi-crystalline structure. The films have to be responsive to the SO2 arising from the APS for them to be used in sensors. The films produced here will be processed further as part of a broader study to develop inexpensive nuclear radiation sensorsCITATION Msi16 l 1033 (Msimanga, 2018).
Characterisation of the polymer films will be done through the three evaluation methods. Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). FTIR is an infrared spectroscopy method used to identify organic, polymeric, and in some cases, inorganic materials. The FTIR test uses infrared light to scan samples and observe chemical properties. XRD is an evaluation technique used for phase identification of crystalline material and it provides unit cell dimensions of the material. X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline sample CITATION Klu74 l 1033 (Klug & Alexander, 1974). SEM is a type of electron microscope that results in images of a sample by scanning the surface of the sample with a focused beam of electrons. The electrons interact with the atoms in the sample, resulting in various signals that have information of the sample’s surface topography and composition. CITATION Smi55 l 1033 (Smith ; Oatley, 1955).
EXPERIMENTALMATERIALSSample Type Supplier
Aniline hydrochloride (ANI-HCL) 97% Sigma Aldrich
Ammonium peroxydisulfate (APS) 98 % APS reagent
Sodium hydroxide (NaOH) 99% reagent Sigma Aldrich
Dimethyl sulfoxide 99% reagent Sigma Aldrich
Silicon wafers- substrates ;100;, N-type Sigma Aldrich
PROCESSING Synthesis of Polyaniline by Oxidation Polymerization
PANI will be synthesized by mixing 0.2 M Aniline hydrochloride with 0.25 M ammonium peroxydisulfate (APS) in aqueous medium and at room temperature to produce an emeraldine hydrochloride form. To minimize the presence of residual aniline and to get the best yield of PANI, the stoichiometric peroxydisulfate/aniline ratio 1.25, recommended by CITATION CHE02 l 7177 (Stejkal, 2002) will be used. 20 mmol Aniline hydrochloride and 25mmol ammonium peroxydisulfate will be separately dissolved in distilled water in a volumetric flask of 50ml CITATION Ver07 l 1033 (Verma ; Dutta, 2007). The solutions will be kept at 0-2° for 1 hour, and then mixed in a beaker then stirred then will be left to polymerize overnight.
The polyaniline hydrochloride precipitate formed in the reaction will be removed by filtration.Thereafter the emeraldine base (EB) form will be obtained through de-protonation of the salt using 0.1 M sodium hydroxide solution CITATION Ver07 l 1033 (Verma ; Dutta, 2007). The base will then be filtered, washed and dried in vacuum at 60°C for 48 hours. The oxidation of aniline is exothermic, that is why the reaction of usually conducted in an ice bath CITATION CHE02 l 7177 (Stejkal, 2002).
3.2.2. Synthesis Polyaniline emeraldine base (PANI-EB) films by spin coating
The dried emeraldine base powder will be collected from the filter paper and stored in a closed vial which will be placed in a desiccator CITATION Ser13 l 7177 (Sergeyev ; Boeva, 2013). From the EB powder, PANI films are going to be prepared by the process of spin coating. For spin coating, the emeraldine base (EB) will be dissolved in Dimethyl sulfoxide and spun on silicon substrates. The amount of the EB for coating is applied on the centre of the silicon substrate, which may be spinning at low speeds or not spinning at all.
EVALUATION3.3.1. Fourier transformed infrared spectroscopy (FTIR)
FTIR will be performed on powder samples of emeraldine base polyaniline to check whether the base has properties of polyaniline. The chemical composition of the EB will be investigated. For the EB, a few N-H groups are expected to appear as a sign of chain stretching. This technique is used to assess the purity of a sample and is highly reliable for identifying base polymer composition, additives, and organic contaminants. Before FTIR analysis begins, the sample is prepared for testing using thin film technique. Enough sample is required to obtain an absorption spectrum CITATION MAT18 l 7177 (Parker, 2018).
3.3.2. X-ray diffraction (XRD)
The structure of emeraldine base polyaniline (EB-PANI) films will be analysed by x-ray diffraction. XRD will be performed on (EB-PANI) films to check for the crystallinity of the films. To differentiate whether the EB-PANI is amorphous or crystalline. This will provide quantification of the degree of crystallinity of the samples. The PANI is expected to be highly crystalline. The additions of NaOH during de-doping of the emeraldine salt influences the crystallinity of the PANI. It is not good for the films to be highly crystalline as they will affect the mobility of charge carriers in the film.
3.3.3. Scanning Electron Microscopy (SEM)
SEM will be performed on the emeraldine base polyaniline (EB-PANI) films to analyse the surface morphology of specimens at very high magnifications using the scanning electron microscope. For sample preparation, the films will be coated with a very thin layer of carbon prior to SEM imaging.
RESULTS AND DISCUSSIONCONCLUSIONREFERENCES BIBLIOGRAPHY
Feast, W. J., Tsibouklis, J., Pouwer, K. L., ; Groenendaal, L. (1996). Synthesis, processing and material properties of conjugated polymers. Polymer, 37(22), 5017-5047. doi:https://doi.org/10.1016/0032-3861(96)00439-9
Klug, H. P., ; Alexander, L. E. (1974). X-Ray Diffraction procedures for polycrystalline and amorphous materials. Wiley, New York: Oxford University Press.
MacDiarmid, A. G. (2001). Synthetic Metals. A Novel Role for Organic Polymers (Nobel Lecture), 40(14), 40. Retrieved March 14, 2018
Msimanga, M. (2018). Electrical properties of ion-implanted semiconducting polymer-metal nanocomposites for radiation sensing applications. Pretoria: Private comminication.
Parker, S. (2018). Polymer testing FTIR analysis. Materials testing, non-destructive testing ; calibration services. Hatfiled PA: Laboratory Testing Inc.
Perkins, D. I., ; Sorensen, P. (2007). Mineral Synthesis and X-ray Diffraction Experiments. London: Geoscience Education.
Polowinski, S. (2002). In Polymer Science (pp. 27,573).
Reka , M., Devi, A., Saranya, J., ; Dharmaraja, J. (2018). Fabrication, Spectral characterisation, XRD and SEM studies on some organic acids doped polyaniline thin films on glass subtrates. Journal of King Sand University – Science, 10.
Scriven, L. E. (1988). Physics and applications of dip coating and spin coating. MRS proceedings.
Sergeyev, V. G., ; Boeva, Z. A. (2013). Polyaniline: Synthesis, Properties, and Application. Russia: Moscow State University.
Smith, K., ; Oatley, C. (1955). The scanning electron microscope and its fiels of application. British Journal of Applied Physics, 391-399.
Stejkal, J. (2002). Polyaniline. Preparation of a Conducting Polymer . Czech Republic: International Union of Pure and Applied Chemistry.
Verma, D., ; Dutta, V. (2007). Novel Microstructure in spin coated polyaniline thin films. Journal of Physics: Condensed Matter, 19, 1-7.