Fevzi Çakmak Cebeci

Layer-by-layer assembled polyaniline nanofiber/multiwall carbon nanotube thin film electrodes for high-power and high-energy storage applications

Thin film electrodes of polyaniline (PANi) nanofibers and functionalized multiwall carbon nanotubes (MWNTs) are created by layer-by-layer (LbL) assembly for microbatteries or -electrochemical capacitors. Highly stable cationic PANi nanofibers, synthesized from the rapid aqueous phase polymerization of aniline, are assembled with carboxylic acid functionalized MWNT into LbL films. The pH-dependent surface charge of PANi nanofibers and MWNTs allows the system to behave like weak polyelectrolytes with controllable LbL film thickness and morphology by varying the number of bilayers. The LbL-PANi/MWNT films consist of a nanoscale interpenetrating network structure with well developed nanopores that yield excellent electrochemical performance for energy storage applications. These LbL-PANi/MWNT films in lithium cell can store high volumetric capacitance (~238 ± 32 F/cm(3)) and high volumetric capacity (~210 mAh/cm(3)). In addition, rate-dependent galvanostatic tests show LbL-PANi/MWNT films can deliver both high power and high energy density (~220 Wh/L(electrode) at ~100 kW/L(electrode)) and could be promising positive electrode materials for thin film microbatteries or electrochemical capacitors.

Electrochemical synthesis and structural studies of polypyrroles, poly(3,4-ethylene- dioxythiophene)s and copolymers of pyrrole and 3,4-ethylenedioxythiophene on carbon fibre microelectrodes

Polypyrrole, poly(3,4-ethylenedioxythiophene) and the copolymer of pyrrole and 3,4-ethylenedioxythiophene films were synthesized electrochemically on carbon fibre microelectrodes (CFME). Deposition conditions on the carbon fibre and the influence of monomer concentrations on the copolymerization, as well as the electrochemistry of the resulting polymers and copolymers, were studied. Structural studies of the polymers were conducted using different techniques such as cyclic voltammetry, in situ spectroelectrochemistry, FTIR and scanning electron microscopy. The effect of the monomer ratio on the formation of copolymer is reported. A high level of stability to overoxidation was also observed for poly(3,4-ethylenedioxythiophene) as the polymer on this CFME substrate shows limited degradation of its electroactivity at potentials 1.2 V above its half-wave potential.

Graphene-reinforced poly(vinyl alcohol) electrospun fibers as building blocks for high performance nanocomposites

Graphene-containing fibrous structures with a high level of affinity towards a polymer matrix solution have been proved to be promising for high performance macroscopic nanocomposite reinforcement purposes. In this study, for the first time, co-solvent assisted well-dispersed thermally reduced graphene oxide dispersions were successfully prepared. Graphene solutions were then further dissolved in poly(vinyl alcohol) solutions and electrospun, respectively, producing hollow nanofibers, for which key properties, such as solution extensional viscosity and fiber mechanical properties, were studied. The hollowness of the as-spun fibers were comprehensively investigated through a focused ion beam-based advanced cutting and cross-section technique in a dual beam instrument. The effect of reduced graphene oxide content on individual fiber alignment of mats subjected to tension forces was studied via scanning electron microscopy. The analysis revealed that the optimum alignment was achieved at 0.6 wt% graphene-content as-spun mats, in which the Youngs modulus was improved by over 60% compared to the neat PVA as-spun mats. The mentioned phenomenon was found as responsible for superior mechanical properties.

Halloysite Nanotubes/Polyethylene Nanocomposites for Active Food Packaging Materials with Ethylene Scavenging and Gas Barrier Properties

Novel polymeric active food packaging films comprising halloysite nanotubes (HNTs) as active agents were developed. HNTs which are hollow tubular clay nanoparticles were utilized as nanofillers absorbing the naturally produced ethylene gas that causes softening and aging of fruits and vegetables; at the same time, limiting the migration of spoilage-inducing gas molecules within the polymer matrix. HNT/polyethylene (HNT/PE) nanocomposite films demonstrated larger ethylene scavenging capacity and lower oxygen and water vapor transmission rates than neat PE films. Nanocomposite films were shown to slow down the ripening process of bananas and retain the firmness of tomatoes due to their ethylene scavenging properties. Furthermore, nanocomposite films also slowed down the weight loss of strawberries and aerobic bacterial growth on chicken surfaces due to their water vapor and oxygen barrier properties. HNT/PE nanocomposite films demonstrated here can greatly contribute to food safety as active food packaging materials that can improve the quality and shelf life of fresh food products.

Synthesis, Characterization and Application of Cellulose Based Nano-Biocomposite Hydrogels

Nano-hydroxyapatite/cellulose-graft-polyacrylamide biocomposite hydrogels of different molar ratios were prepared to examine their potential application as a carrier for colon targeted drug delivery in vitro. The particle size of the synthesized nano-hydroxyapatite was found to be 122 nm. The swelling behavior of the composite hydrogels was observed in acidic and basic aqueous solution that simulated lower small intestine, colon and stomach fluids. The hydrogel could be applied in drug-delivery systems and acetylsalicylic acid was used as a model compound to test such a possibility. Finally, the synthesized biocomposite hydrogels with the 96.97% maximum encapsulation and 85.67% release efficiency in the basic medium were found to be a suitable candidate to carry and release of colon-targeted drugs.

Multilayer transfer printing of electroactive thin film composites

We demonstrate the high fidelity transfer printing of an electroactive polymer nanocomposite thin film onto a conductive electrode. Polyelectrolyte multilayer thin films of thickness ∼200 nm containing 68 vol % Prussian Blue nanoparticles are assembled on a UV-curable photopolymer stamp and transferred in their entirety onto ITO-coated glass creating ∼2.5 μm-wide line patterns with ∼1.25 μm spacing. AFM and SEM are used to investigate pattern fidelity and morphology, while cyclic voltammetry confirms the electroactive nature of the film and electrical connectivity with the electrode. The patterning strategy presented here could be used to pattern electroactive thin films containing a high density of nanoparticles onto individually addressable microelectrodes for a variety of applications ranging from biosensor arrays to flexible electronics.

ELECTROINDUCED DISPERSIVE POLYMERIZATION OF METHYL METHACRYLATE IN AQUEOUS MEDIA

The electroinduced and chemical dispersion polymerization of methyl methacrylate was performed in aqueous methanol solution (water/methyl alcohol ratio 50/50 by wt%) in the presence of poly(vinyl pyrrolidone) as a stabilizer with ceric sulfate as an initiator. The effects of various polymerization parameters, for example concentrations of recipe ingredients, temperature, potential, and duration, on polymerization yield were examined. In all cases higher yield was obtained with the electrochemical polymerization procedure. Initiation rates of polymerization processes, thermal properties of polymers, interaction of initiator with monomer and stabilizer were investigated in detail. Possible structure of polymer is proposed, and a tentative polymerization mechanism is suggested.