Development of multi-layer conductive polymer nanocomposites for electromagnetic shielding application

Nowadays, due to requirement of humanity and technology, development of new multifunctional material researches have been receiving a revitalized interest. Academical, commercial, economical and ecological participants of nano technology, surface science, polymer technology and material science have been congregated for developing multifunctional materials for next generation applications. Multi layer nano-particle polymer composites exhibits several benefits like widely exchangeable properties due to polymer or particle type and obtaining multifunction on unique material such as conductivity, thermal resistivity, electrochemical and electromagnetic shielding efficiency. Therefore, multi layer nanoparticle-polymer composites and their coating on substrates researches have mostly interest for many applications such as conductive nanocomposite based materials, conductive textiles, electromagnetic shielding materials, paints, dyes, medical and biomaterials, sensors multilayer ceramic capacitors, and holographic memory materials. Electromagnetic shielding problem has emerged as a result of enormous and sophisticated increase in the application of electronic devices and equipment. Therefore electromagnetic waves have been one of the most important research areas for many multidisciplinary researches during the past decades because of their negative effects on the living tissues and electronic systems. For decades, many studies have been designed with a variety of ferroelectric/magnetic core particles such as gold, silver, titanium dioxide, ferrite, etc. for improving electrical, magnetic, thermal and mechanical properties of produced materials. Ferroelectric materials could emit electromagnetic radiations due to polarization process. Lead-based ferroelectric materials have excellent properties, however they are restricted for health and environmental issues due to their toxicity. Due to ferroelectric properties and high dielectric constant barium titanate (BaTiO3) should be used for developing electromagnetic shielding materials. Nano sized BaTiO3 is a ferroelectric material with high dielectric constant, high permittivity and a large propagation constant leading to a high value of attenuation constant due to the exploitation of the pronounced surface area to volume ratio. On the other hand , due to the bandwidth of the barium titanate, the value of the reflection loss is no more satisfactory. If BaTiO3 can be coated with conductive polymers, both the physiochemical properties and the electromagnetic shielding performance are expected to be improved. Conducting polymers are widely used in new material applications, since discovery of their inherent electrical conductivity due to conjugated system, which is closely connected to the charge transfer rate and electrochemical redox efficiency. Pyrrole derivatives were chosen in this study because, it can be dissolved in water and has high reaction efficiency, electrical conductivity (to 100 S/cm in doped form) and low oxidation potential. The addition of inorganic fillers to conductive polymers modifies desirable properties but this procedure often leads to poor processability and deteriorated mechanical properties. The conductivity and other desirable properties can be enhanced by adding another layer which has large manufacturing are, flexibility, and cost effectiveness. In this thesis BaTiO3 nanoparticles have been used as core to obtain high absorbing shielding material by combination with polypyrole derivatives, in order to enhance conductivity, thermal and electrochemical properties of conductive polymers. Also, Acrylonitrile (AN) copolymer has been chosen as shell material in this structure to improve processability and flexibility, so these structures can be used in textile industry. Copolymers of (AN) have found great interest for the different high-performance products applications as fibres, thin films and composites in conventional and technical textiles, polymer electrolytes, coating, adhesive and plastic industries. There are several methods for generating multi layer nanoparticle-polymer composites. Core-shell nanostructures have widely received attention because the material properties can be enhanced independently from core or shell contents properties; like electrical, mechanical, thermal and electrochemical properties. Core shell structures can be synthesized with different methods, but in this system emulsion polymerization technique has been chosen due to such desired properties; easy process control, water usage as solvent which make process environmentally friendly, additives can easily be used and there is a broad range of design possibilities. Emulsion polymerization is propagated in micelles and micro emulsion technique allows particles to transfer into spherical aggregates through the surfactant template. So firstly ceramic nanoparticles dispersed in the aqueous surfactant solution by stirring vigorously which is important for micro emulsion polymerisation. Also surfactant was used to improve the physical properties of polymers such as conductivity, stability, solubility in solvents and processability. In this thesis, three different publications are presented, which are respectively based on synthesis and characterization of polypyrrole derivatives –BaTiO3- polyacrylonitrile-co-methacrylate [P(AN-co-MA)] core shell nanocomposites by in-situ emulsion polymerization in the presence of dodecylbenzene sulfonic acid (DBSA) surfactant and then investigation of their electrical conductivity and electrochemical behaviour. In the last part of the thesis electromagnetic shielding efficiency of the coated fabrics were investigated due to obtained conductivity and capacitive behaviour of these nanocomposites. The first section of the thesis is about the preparation and characterization of core shell nanocomposites. Preliminary studies were performed for finding optimum conditions, derivatives, ratios to obtain best yield and conductivity. As a result of this step Pyrrole (Py) and Tert- butyl 1- pyrrole- carboxylate (TBPy) monomers were chosen and the best conditions were obtained. Then emulsion polymerization method was used to synthesize [P(AN-co-MA)] by using ammonium per sulphate (APS) in the aqueous medium. Lastly [P(AN-co-MA)]/ PPy derivatives core–shell nanoparticles were obtained by in situ emulsion polymerization process. At the last step of this section BaTiO3 -[P(AN-co-MA)], BaTiO3–PPy and PTBPy nanocomposites were synthesized by in-situ emulsion polymerization .Then particle size, yield, crystalline structure (XRD), chemical structure (FT-IR ATR) (Raman) (UV-Vis), morphology (SEM-EDX, TEM) and thermal properties (DSC, TGA), of the nanocomposites were fully characterised. Uniform particle size distributions of the nanocomposites were obtained. The size of the nanocomposites decreased with the increase of PPy and PTBPy concentrations. The spectroscopic characterizations during the formation of (BaTiO3-[P(AN-co-MA)]-PPy) and (BaTiO3- [P(AN-co-MA)]-PTBPy) nanocomposites demonstrated that there is a strong interaction between the [P(AN-co-MA)]-PPy and BaTiO3 and also [P(AN-co-MA)]-PTBPy and the BaTiO3. The morphological characterizations of the nanocomposites were indicated that efficient coverage was occurred for BaTiO3 nanoparticles with polymer shell. Electrochemical impedance spectroscopy (EIS) and electrical conductivity study of synthesised nanocomposites were performed in the second section of the thesis for investigation of individual effects of BaTiO3, PPy, PTBPy, and [P(AN-co-MA)] on obtained nanocomposite conductivity and electrochemical behaviour. (EIS) and equivalent electrical circuit modelling (ECM) were used to investigate the changes in electrical properties of nanocomposites in the presence of BaTiO3, [P(AN-co-MA)], PPy and PTBPy. ECM describes the electrochemical properties and simulates the impedance behaviour of the nanocomposites with charge transfer resistance (Rct), double layer and specific capacitance values of nanocomposites by usage of EIS data. The EIS data suggested that PPy nanocomposites act like capacitors and that capacitance values of nanocomposites were increased with BaTiO3, and conductivity increased due to the interaction between P[AN-co-MA] and BaTiO3. On the other hand, capacitance values of PTBPy nanocomposites were increased with P[AN-co-MA] and BaTiO3. The ECM was utilised using experimental EIS spectral data, and an excellent agreement between experimental results and the parameters was observed with Chi-square (χ2) around 10−3. The increase in total Rct value was found to be dependent on the size and surface area of nanoparticles. The conductivities of the nanoparticle containing solutions indicated that conductivity results have been enhanced by the addition of BaTiO3, PPy and PTBPy. Further, atomic force microscope (AFM) measurements illustrated that BaTiO3-P[AN-co-MA]-PPy nanocomposites show a good ordering of the aggregates and nodules with semi-circular shapes. In the last section of thesis, PPy was deposited separately on BaTiO3, BaTiO3-[P(AN-co-MA)] nanocomposite-coated fabrics by an in-situ chemical polymerization process. Electromagnetic Shielding Effectiveness (EMSE), electrical conductivity, chemical structure, colour measurements and morphology of fabrics were fully characterized and systematically studied for investigation of individual effects of BaTiO3, PPy and BaTiO3-[P(AN-co-MA)] on obtained fabrics' conductivity and shielding behaviour. Shielding properties were measured according to the ASTM D4935-10 protocol, in the frequency range of 15–3000 MHz which includes cell phone, wireless and other electronic equipment working frequency. PPy-coated fabrics showed the highest EMSE performance at 2000-2300MHz. The fabric conductivity values indicated that the use of BaTiO3 enhanced conductivity and PPy-BaTiO3 coated fabrics more conductive than PPy-coated fabrics. Conductivity increased due to the relationship between PPy and BaTiO3-P[AN-co-MA]. Because of high electrical, capacitive and shielding properties, these synthesized nanocomposite based materials can be used in technical textiles such as covering materials of baby rooms, sensitive laboratories or other working areas such as carpets, curtains, military cloths, and radar absorbing materials.