Research

Electrospun Polymer /MWCNTs Nanofibers Reinforced and Toughened Composites

  • Elif Ozden

The focus of this study is to investigate mechanical properties of multi-walled carbon nanotube (MWCNT) reinforced electrospun nanofibers in composites. Neat and MWCNT reinforced composite nanofibers of poly(styrene-co-glycidyl methacrylate) are produced by electrospinning. The process optimization for composite electrospun fibers is also investigated. An emprical relationship between polymer and MWCNT concentration and average fiber diameter is sought by response surface methodology (RSM). The nanofibers, are then embedded into epoxy matrix to form polymer composites. The experimental procedure was designed so as to observe the effects of GMA composition in structure and the effect of additional crosslinker agent by spraying method. The effect of neat and MWCNT reinforced nanofibers in the composites is reported in comparison to neat epoxy.

Development of Shear Thickening Fluid for Body Armors

  • Burcu Ozel

In many industrial applications, colloidal suspensions of dispersed particles in liquid are widely used. Industrial applications of colloidal suspensions range from cement mixing to cosmetics and filled polymers. Most of the chemical and allied processing industries encounter non-newtonian flow behavior; namely shear thinning and shear thickening. Shear thickening is the increase of viscosity with shear rate, which is more common than the opposite effect, shear thinning that occur with various kinds of fluids. The dramatic viscosity increase in shear thickening fluids (STF) can damage processing equipments, such as filters and mixer blades, and cause blocking of transport pipes.1,2 When the colloidal suspensions are subjected to high shear force during processing, or pumping such slurries through pipes, process performance would be affected drastically. On the other hand, applications based on shear thickening response attract a great deal of attention in many areas. Examples are design of biomedical and sports wear, and military applications, particularly the design of body armors due to their ability to absorb high amounts of energy when subjected to high velocity projectiles.3 Lee at al.4 demonstrated that impregnation of shear thickening fluids improve the ballistic performance of Kevlar® fabric. Conventional body armors are composed of around 40 layers of wowen fabric which cause bulkiness and stiffness, thus mobility, agility and comfort are reduced. Hence this kind of protective vest provide torso protection only. Consequently, there has been a requirement of flexible and light-weight protective body armor design because of the particular risk of extremities. Employment of STFs in armor fabrics provides comparable ballistic properties with higher flexibility, but STF impregnated Kevlar® has no weight advantage over neat Kevlar®. Therefore, this project focuses on the investigation of the control parameters of STFs in order to understand the origin of the shear thickening behavior for the design of the optimized liquid body armor systems using STF/woven fabrics. In addition, a two-dimensional simulation model based on an Eulerian multiphase flow approach is investigated to predict the flow characteristics of STFs through filter during polymer processing.

Application of Epoxy Based Carbon Nanotube Nano-Composites in Health Structural Monitoring Systems

  • Shalima Shawuti

Carbon nanotubes (CNT) have been intensively investigated due to their mechanical, chemical, and electrical properties.1 CNTs are known as effective fillers in polymeric matrices which introduce strong materials at low loadings2,3 because of their excellent mechanical properties in combination with very high aspect ratio (as high as 1000 for multi walled carbon nanotubes (MWCNT)), which enables percolation of the filler at low MWCNT loading. It is also promising to use CNTs as additives in polymer blends for electrically conductive applications.3 The potential of CNTs for application as reinforcements, however, highly depends on the ability to disperse the nanotubes homogenously throughout the matrix and on the transfer of the mechanical load from the matrix to nanotube. Our current research includes improving dispersion of MWCNTs in epoxy-polymer matrix system by various methods. Synthesis of MWCNT/Epoxy composite films are synthesized via various processing methods, such as resin transfer mold and conventional open mold. Characterization of mechanical, electrical and thermal properties as well as solvent resistance are performed. REFERENCES 1. Xiong et al. Carbon 2006, 44, 2701. 2. Lu et al. Carbon 2007, 45, 2621 3. Cooper et al. Composites Sci. and Tech. 2002, 62, 1105

Glycidyl-Methacrylate-Based electrospun Mats and Catalytic Silver Nanoparticles

  • M. M. Demir, G. Ugur, M. A. Gulgun, Y. Z. Menceloglu. Macromol. Chem. Phy. 209, 508 (2008)

P(AN-GMA) and PGMA fibers coated with monodisperse silver nanoparticles have been prepared by a combination of electrospinning and electroless plating. The morphology of the electrospun fibers remains unchanged after surface hydrazination. Oxidation of hydrazine in an ammoniacal solution of AgNO3 reduces and deposits silver atoms along the fiber surface, which then coalesce to Ag particles. The size of the silver nanoparticles is varied between 20-60 nm. Since the density of the active sites for silver reduction is lower in P(AN-GMA), a smaller particle size could be obtained. The catalytic activity of the silver nanoparticles has been confirmed. Catalytic silver nanoparticles were selectively deposited onto the surface of glycidyl methacrylate (GMA) based electrospun fibers by a redox reaction. The process takes place in an aqueous solution of AgNO3 at ambient conditions using electroless deposition. GMA as comonomer is rather useful, since the pendant oxirane ring can be opened and a range of functionalities can be introduced to the polymer backbone. Modification of the fiber surface with the reducing agent hydrazine provided binding sites for nucleation of metal atoms. Silver cations were reduced into metallic silver that diffused and coalesced into particles along the fiber's surface. Control of particle sizes in the range of 20-60 nm was achieved by varying the GMA fraction of the copolymer and/or the deposition time. Silver particles can be deposited evenly along edges, inside cavities, and cover irregularly shaped objects that are otherwise difficult to coat evenly. Control of particle sizes in the range of 20-60 nm was achieved by varying the GMA fraction of the copolymer and/or the deposition time.

Development of Novel Materials for Proton Exchange Membrane Fuel Cells

  • Burak Birkan, Cenk Gumeci

Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. Among them, proton exchange membrane fuel cells (PEMFC) deliver the highest power density, while offering low weight, cost, and volume. For these reasons, PEMFC’s are broadly applicable as an environmentally friendly energy source, but their larger-scale commercial use is limited due to poor ionic conductivities at low humidity and/or elevated temperatures, a susceptibility to chemical degradation at elevated temperatures, and finally, membrane cost. In response to all these factors, low cost, high proton-conducting, as well as mechanically and thermally stable PEM’s were produced by using the ATRP (Atom Transfer Radical Polymerization) technique in supercritical carbon dioxide (Sc-CO2) as the polymerization method for the synthesis of polymers with fluorinated backbones and ionic pendant groups on the side chain. Secondly, one of the most important components of PEMFC’s is the electrode, which typically consists of a Pt catalyst, either Pt black or Pt on a carbon support. The catalytic behavior of a precious metal is determined mainly by its distribution, the metal oxidation state, and sensitivity to the fuel quality. A unique process was developed, which involves producing nano-sized metal-supporting carbonfibers that could be used as the catalyst on electrodes at PEMFCs, thus increasing the catalytic material concentration by up to 0.2 mg catalytic material/cm2 area of the carbon fiber structure.

Nonisocyanate Based Polyurethane/Silica Nanocomposites

  • Ozge Malay

There is a growing intense effort to develop new chemical processes using natural resources due to the prediction that the petroleum resources will be exhausted within the next century.1 One such resource of particular interest is CO2, a nontoxic, nonflammable, naturally abundant carbon feedstock. Chemical addition of CO2 into valuable organic compounds is useful in the terms of both resource utilization and pollution prevention.2 One of the most promising methodologies in this area is the direct synthesis of carbonates via coupling of CO2 with epoxides under supercritical conditions. Cyclic carbonates have been widely used as polar aprotic solvents, intermediates for organic and polymeric synthesis, and ingredients for pharmaceutical/fine chemicals in biomedical applications.3 Many organic and inorganic compounds, such as metal oxides, quaternary ammonium salts, transition metal complexes, and ionic liquid, have been developed to be used as catalysts for the cycloaddition of carbon dioxide with epoxides for the synthesis of cyclic carbonates.4 From the standpoint of environmental protection and resource utilization, the development of an environmentally benign process using carbon dioxide, which is the largest single source of greenhouse gas, could be utilized in the nanocomposite synthesis that uses cyclic carbonates as intermediates for the production of polyurethane polymers. Polyurethane polymers are used world-wide on a large scale for foam, fiber, adhesive and coating applications so forth. The involvement of toxic components, such as isocyanates, in their fabrication process makes the production extremely toxic and dangerous. More recently, non-isocyanate polyurethanes were formed by the reaction of cyclocarbonate oligomers and primary aliphatic amine oligomers.5 Among many studies, the reaction of oxiranes and CO2 to provide the five-membered cyclic carbonate has received much attention because of its simple reaction, high yield and harmless nature of the reagents. Our current research includes optimization studies on cyclic carbonate synthesis from epoxides by the use of supercritical carbon dioxide both as a medium and as a raw material, and synthesis and modification of silica nanoparticles, as different parts of a comprehensive work on development of environmental-friendly polyurethane/silica nanocomposite coatings with superior thermal and mechanical properties and also excellent fire retardance ability. REFERENCES 1. Gross, R. A.; Kalra, B. Science 2002, 297, 803 2. Anastas, P. T.; Lankey, R. L. Green Chem. 2000, 2, 289 3. Shaikh, A.-A. G.; Sivaram, S. Chem. Rev. 1996, 96, 951 4. Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. Rev. 1996, 153, 155 5. Kihara, N.; Takeshi, E. J. Polym. Sci. 1993, 31, 2765

Production and Processing of Transparent Biodegradable Nanocomposites

  • Funda Inceoglu

The environmental pollution from consumed plastics becomes very serious due to the increasing usage of the synthetic plastics, especially ones used in the packaging materials. With tighter environmental regulations and increasing waste disposal costs, plastic manufacturers are forced to seek new alternatives. One of the challenging ideas in this aspect is to increase the production and usage of biodegradable polymers (like starch, cellulose, lignin, poly(lactic acid), polyhydroxyalkanotes , etc.) with the goal of replacing non-biodegradable plastics, at least partially. However, biodegradable plastics cannot compete with conventional petroleum-based plastics because they alone are usually brittle and sensitive to water. Therefore, it must be combined with other materials, like synthetic polymers, to produce satisfactory plastic. The important question here is that "what is happening to the degradation rate of synthetic polymers in the environment after addition of natural biopolymer?" Recent studies have shown that when natural biopolymer is included in the synthetic one, the degradation rate of the synthetic polymer is also increasing. It was suggested that microbes first create pores on consumption of the biopolymer and increase the surface area of the composite. Increased surface area enhances oxygen-based reactions, which could increase synthetic polymer chain oxidation and susceptibility to biotic reactions . The main problem, however, in using natural polymer with synthetic polymer is the reduction of the mechanical and optical properties of the latter one with the addition of natural polymer. In our project, we are not only trying to prevent this drawback but also further enhance the mechanical strength of the composite material by the help of nano-science.

We use reactive twin screw extruder in order to modify polyolefins to provide compatibility between synthetic and natural polymers and to prepare biodegradable nanocomposite formulations.

Tunable, Superhydrophobically Stable Polymeric Surfaces by Electrospinning

  • Kazim Acatay, Eren Simsek

The Lotus Effect: Thanks to Nature... High water repellence of superhydrophobic surfaces is an outstanding result of the limited contact area between the solid surface and water. Such surfaces are characterized with high advancing water contact angle (AWCA) and low tilt (sliding) angle values. The solid-liquid interfacial energy can be minimized by engineering both the chemistry and the topography of the solid surface. For instance, when nano-sized crystals of epicuticular wax, an intirinsically hydrophobic material, cover micron-level roughness on the lotus leaf, AWCA is increased to 160o and water droplets roll of the surface easily. To mimic the topography of the lotus leaf and achieve a water repellent, solvent resistant, superhydrophobically stable surface, we fabricated a polymeric film surface with a high degree of roughness through a reactive electrospinning process.

One parameter that allowed the tunability of the electrospun film topography was the viscosity of the solution, which was altered by mixing the polymer and the crosslinker in various solvent quantities. As the viscosity increased, the morphology of the microtextured films changed from predominantly beads to fiber-only structure. Use of viscosities lower than 31 mPas for electrospinning suprisingly resulted surfaces with low roughness because of the large percent of solvent reaching the grounded plate. However, by synthesizing low molecular weight (LMW) copolymer, we could achieve surfaces having bead-only morphology after electrospinning. We measured ~167o AWCA and ~4.3o sliding angle values on this surface.

Solvent resistivity of the surfaces was investigated by exposing them to various solvents (hexane, toluene, dichloromethane, chloroform, acetone, DMF, DMSO, water) for 10 days. Mass loses % were observed to be between 1.8 and 11.7 which indicated that they are quite resistant to corrosion.

Side Chain Liquid Crystalline Polycarbonate with a Chiral Backbone

  • Yesim Muge Calik Sahin

The experimental demonstration of a novel and environmentally benign supercritical carbon dioxide(ScCO2) technique that sheds light on the design of an optically active side chain liquid crystalline poly(ether-carbonate) in a single-step reaction. Opticallyactive side chain liquid crystal polymers have attracted much attention due to the wide range of their potential applications, in particular in the fields of optics and electro optics. By a purposeful design of a chiral monomer and an understanding of the copolymerization mechanism in ScCO2 media simultaneously, we easily obtain highly stereoregular (isotactic), optically active polymers. The obtained copolymers exhibit another practical benefit by widening the mesomorphic temperature range compared to its acrylic analogues. This can be a good basis in the material design for preparing the practical materials suitable for device applications. Consequently, this work is important to the materials chemistry and to the synthesis community; it is the insightful combination of material processing and chemical design that not only enables the production of a valuable material, but also elucidates the advantages of ScCO2 application in terms of liquid crystallinity and the tacticity of the obtained polymer.

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Ultraviolet Curable Nanocomposite Coatings

  • Cahit Dalgicdir, Funda Inceoglu

The UV-curing process is increasingly used for applications in the coating industry as paints and inks because the process offers ultra-fast, solvent-free, ambient-temperature curing, advantages, which lead to an energy-saving of up to 70%. Recent advances in improved mechanical properties and cost-effective preparation have increased the suitability of UV-curable nanocomposite films for a widening variety of applications. In order to achieve the homogeneous dispersion necessary for the enhanced properties, we demonstrated that the molecular structure of the silicate filler should be adjusted for the polymer type used, by attenuating the adhesive forces between silicate layers. Otherwise, phase-separation would prevent nano-sized dispersion and give rise to micron-sized agglomerates. The presence of clay up to 10 wt% was found to significantly increased the tensile strength, thermal stability and barrier properties of the neat resin. The clay delayed the initial decomposition by increasing the pathway of the volatile products, while it acted as a heat barrier and lad to the heat build-up at high temperature, thereby increasing the char residue. Most interestingly, the optical appearance, i.e. the transparency, of the base resin did not change noticeably with the clay addition. All of these improvements are the result of a very homogeneous, nano-scale distribution of clay layers in the polymer matrix.

Intumescent Flame Retardant Synthesis for Coating Applications

  • B. Kerem Goren, Funda Inceoglu

Apart from a few exceptions, most polymer materials are combustible. It is therefore necessary to modify these polymer materials to make them fire-retardant for many applications such as the construction, furniture, vehicle, or electronics industries. To satisfy regulations and standardized tests, organohalogen or organophosphorus compounds are added in large quantities as flameproofing agents to plastics. However, while organohalogen compounds themselves are non-toxic, they can release corrosive pyrolytic components that are a health hazard in the event of fire. Consequently, we are seeking safer alternative flame retardants by using materials such as metallic hydroxides. Unfortunately, their low activity requires usage in high concentrations, which negatively impacts the mechanical properties of the matrix into which they are incorporated. Flame retardants based on intumescency are preferred because of their moderate minimum effective concentration, which does not have a negative impact on the mechanical properties of the polymer matrix, in contrast to metal hydroxides. An intumescent system is active in the condensed phase and does not release corrosive and toxic gases while still maintaining flame retardancy. Moreover, intumescent systems evolve less corrosive gases, since nitrogen’s higher decomposition temperature is the main reason favoring their use in the cable jacketing industry. The main ingredients for an intumescent additive include an acid source, a carbon source, and a blowing agent. The solution employed here was the incorporation of a phosphorus- and nitrogen-containing intumescent additive into the polyethylene resins homogeneously, and the flame resistance of the resulting resin was demonstrated to meet the stand testing methods, the Limiting Oxygen Index (LOI) and the UL-94.

Smart Materials and Structures

  • Collaborative research with Dr. Papila group of Mat. Sci. and Eng., Sabanci University

The inspiration for materials for new technologies often comes from natural structures and systems, which have evolved to optimize a specific physical property. Two such examples include the wing motions of birds for improved flight and leg movements of insects for all-scale stable robotics systems. Successfully engineered systems depend heavily on the performance of engineered materials such as smart materials, active materials that have undergone a change in one or more properties in response to an external stimulus of chemical, electrical, thermal or optical nature. Electroactive polymers (EAP) are smart materials that appeal to numerous industries because of their inexpensive, lightweight, biologically compatible and mechanically stable structures. They can tolerate considerable deformation, while maintaining ample power. They have a fast response time, very low density, high strain output, and outstanding pliability when compared to ceramics and shape memory alloys. The performance of piezoelectric nano/micro-fiber reinforced thin films was successfully demonstrated in a micro air vehicle wing structure. Other research projects currently emphasize the optimization of EAP processing, electromechanical characterization and sensor-actuator applications.

In-vitro and in-vivo Degradation of Non-woven Materials Prepared by Electrospinning

  • Collaborative research with Dr. Piskin group of Hacettepe University, Ankara, Turkey

The aim of this study is to prepare non-woven materials from a biodegradable polymer, poly(ε-caprolactone) (PCL) by electrospinning. PCL was synthesized by ring-openning polymerization of ε-caprolactone in bulk using stannous octoate as the catalyst under nitrogen atmosphere. PCL was then processed into non-woven matrices composed of nanofibers by electrospinning of the polymer from its solution using a high voltage power supply. The effects of PCL concentration, composition of the solvent (a mixture of chloroform and DMF with different DMF content), applied voltage, and tipcollector distance on fiber diameter and morphology were investigated. The diameter of fibers increased with the increase in the polymer concentration and decrease in the DMF content significantly. Applied volatage and tip-collector distance were found critical to control “bead” formation. Elongation at break, ultimate strength and Young modulus were obtained from the mechanical tests, which were all increased by increasing fiber diameter. The fiber diameter influenced both in vitro degradation (performed in Ringer solutions) and in vivo biodegradation (conducted in rats) rates significantly. In vivo degradation was found to be faster than in vitro. Electrospun membranes were more hydrophobic than PCL solvent-casted ones, therefore their degradations were much slower.