Group Meetings

Individual presentations on our group meetings in 2011 spring season is listed below. 

Sol-Gel Synthesis and Surface Modification of TiO2 nanoparticles  by Ayca Abakay (Date: August 11, 2011)

Titanium dioxide as a photocatalyst has attracted a great deal of attention due to its unique properties such as low cost, UV stability and high efficiency besides its wide range of application areas such as self cleaning surfaces, water purification and solar energy cells. Among different techniques, sol–gel method is the most common technique for TiO2 nanoparticle synthesis. In this study, nano-sized titanium dioxide particles were obtained by a simple sol-gel procedure. Effects of synthesis parameters on properties of nanoparticles were investigated and nanoparticles were characterized with dynamic light scattering (DLS), X-ray diffraction spectroscopy (XRD) and thermal gravimetric analysis (TGA). Furthermore, surface of TiO2 nanoparticles were modified with aminosilanes and effect of parameters such as aminosilane concentration, reaction time and reaction temperature on modified particles were analyzed.  

Strength of Laminates with Surface Modified Polymer/MWCNT Nano-Composite Interlayers by Kaan Bilge (Date: August 04, 2011)

This study presents a demonstration of the toughening of carbon fiber reinforced structural composites by nano-fibrous interlayers. Specifically, the epoxy cross-linking P(St-co-GMA) and P(St-co-GMA)/MWCNT electrospun nano-fibers were applied on conventional carbon/epoxy prepreg plies prior to curing under vacuum. It was shown that Tg of the polymeric fibers limits the curing temperature to gain from the fibrous interlayer.Strain energy release rate under shear (ENF tests) loading was enhanced by more than %50 without any strength loss in the in plane shear direction.(DNS tests). Transversal impact tests also provided evidence for toughness increase (about %20) by the nano-fibrous interlayers. Additionally, the significant increases in the longitudinal and transversial tensile tests suggested that nano-fibrous interlayers not only act as interlayers but also strengthens the matrix thanks to their compatibility achieved by chemical tailoring with almost no weight penalty. 

Temperature Compensation of FBG sensors embedded inside GFRC and Detection of Damage in GFRC by Talha Boz (Date: July 28, 2011)

The structural health of glass fiber reinforced composites (GFRC) has been monitored by Fiber Bragg Grating (FBG) sensors. FBG sensors are capable of sensing pico-scale strains and heat changes in the region which they're embedded. Therefore in the process of structural health monitoring, the temperature sensitivity of FBG sensors should be compensated. In order to compensate temperature sensitivity of FBG sensors, bending experiments with and without heat change have been conducted. With these experiments, the strain and temperature factors of FBG sensors were found and the bending experiments with temperature compensation was correlated with bending experiments without any heat change. Afterwards GFRC sample used in the experiments have been damaged deliberately to show that FBG sensors are capable of detecting damages occurred in composite structures. The results of the experiments will be presented. 

Can Carbon Nanotubes be Dispersed Using Aromatic Structures? by Elif Ozden Yenigun (Date: July 21, 2011)

In the electrospinning process, various requirements of the initial solution will determine the final composite fiber structure, particularly the diameter of nanofibers. The challenging part in composite nanofiber production is to suspend nanotubes and form homogenous stable suspensions. To achieve well dispersed carbon nanotubes in a polymer matrix is quite difficult, due to CNTs' high affinity for one another (i.e. their tendency to aggregate in bundles and agglomerates) and their rather weak interaction with common polymers. Aromatic compounds are known to interact strongly with graphitic sidewalls of carbon nanotubes through effective π-π stacking. Our experimental observations demonstrated that thermodynamic stabilization of electrospinning solution could be achieved using polymers consisting aromatic structures as in the case of P(St-co-GMA). In order to investigate the effect of the aromatic groups on the dispersion state of CNTs in polymer matrix, the interactions between CNTs and two different polymers ( i.e. one is aromatic and the other is linear) driven by their chemistry is revealed by molecular dynamics simulations. Specifically, radial distribution functions of aromatic groups in the system are analyzed to discuss the effect of various molecular interactions on a molecular basis. 

Monitoring sol-gel synthesis of colloidal silica at different solvent media by Ozge Malay (Date: July 14, 2011)

The Stöber method is a well-known procedure for the sol-gel synthesis of monodisperse colloidal silica. Although this method has been widely investigated, there are very few studies on comprehensive understanding of the time-dependent growth mechanism of silica particles, and a generally accepted scheme has not been presented yet. The experimental procedure is very simple, but the underlying mechanism for the formation and growth of the charge-stabilized silica spheres displays a challenging picture. The standard procedure involves the base-catalyzed hydrolysis and condensation of silicon alkoxides in an alcohol-ammonia-water system. In this study, initially monodisperse approximately 50 nm silica nanoparticles were synthesized in a lower alcohol, ethanol, and the growth of the nanoparticles was monitored by dynamic light scattering (DLS), atomic force microscopy (AFM) and nuclear magnetic resonance techniques (NMR). Ongoing investigation aims to characterize size, distribution and colloidal stability of silica suspensions during growth by using different alcohols as the reaction medium. The effect of the solvent on the reaction rate is a complex phenomenon; however changing the solvent of the system constitutes vast information on particle formation mechanism since it affects the solubility of the polymerized species. Growth of colloidal silica was similarly studied with these solvents and preliminary results showed that an optimum size and distribution can be obtained for different alcohol environments. 

Surface Energy Calculation of Dip Coated Polymer Films by Contact Angle Measurements by Oguzhan Oguz (Date: July 07, 2011)

The interaction between polymer molecules and nanoparticles is essential for composite materials. In this respect, surface energy/tension of materials are used to have initial idea about the compatibility issues. The purpose of this study is to obtain a substantial relation between the interfacial tensions and physical properties for composite materials. Poly(ethylene oxide) and poly(trimethylene oxide) based polyurethane films were prepared by dip coating at various concentrations and surface energy calculations were performed via sessile drop methods. Surface tension of the modifier groups were measured using pendant drop analysis. Further studies involving powder wettability analysis are discussed. 

Using Alternative Reinforcing Fillers in PDMS and Examining their Compatibility via Rheological Measurements by Erim Ulkumen (Date: June 30, 2011)

Polydimethyl siloxane (PDMS) is a thermally, physically and chemically stable polymer due to the silicone-oxygen bonds in the backbone. PDMS has an extraordinary low glass transition temperature of around -100 oC mainly because of the strong ionic nature of the silicon-oxygen bond. Silicone elastomers can be regularly produced when PDMS is crosslinked. However, mechanical properties of silicone elastomers are very poor, resulting from the lack of intermolecular interactions due to low surface energy. Therefore, they must be reinforced to be used in the applications such as medical implants, sealants, automotive, cable industry and etc. Fumed silica is a widely used reinforcing agent for silicone elastomers due to their compatibility with the polymer. Although there are a number of publications that report the use of fumed silica as reinforcement, articles that report the compatibility and comparison of platelet silica is rare. We have found that when PDMS is reinforced with platelet fillers, the modulus of PDMS increase remarkably due to the high aspect ratio of the filler. Tensile and tear strength, filler-matrix interaction and processability of the composite are also enhanced. In this project, the use of platelet silica in silicone elastomer compounds as filler material has been targeted and comparative results are projected to be acquired in terms of physical and economical benefits.

Synthesis of Novel Hydrogels and Investigation of the Swelling Behavior by Kinyas Aydin (Date: June 22, 2011)

Synthesis of hydrogels has been one of the most popular research topics in recent years. A hydrogel is a network of hydrophilic polymer chains, sometimes found as a collodial gel in which water is the dispersion medium, which has a highly absorbent character. In our study, we have focused on the synthesis of a novel hydrogel from acrylamide and 2-acrylamido-2-propane methylsulfonic acid (AMPS). Poly (acrylamide-co-2-acrylamido-2-propanemethyl sulfonic acid) was synthesized in various solvents and cast films were prepared. Water swelling ratio and capacity of the finely prepared films were investigated by using Voigt-Based Viscoelastic method.

Effect of Solvent Acidity on Size and Stability of Anatase Titania Nanoparticles by Deniz Alpay and Ece Alpaslan (Date: May 26, 2011)

Titanium dioxide nanoparticles (TNP) have been widely studied for the past 2 decades due to their unique properties. High photocatalytic and photovoltaic activity, excellent dielectric and gas sensitive properties are some of the particular properties that attract great attention on nano-sized anatase phase titanium dioxide. Aim of this study was to analyze the effect of synthesis conditions on properties of TNPs. In order to do so, nanoparticles were synthesized using a sol-gel method, with titanium isopropoxide (TIPP) as a precursor in 1-propanol solution. A common challenge with sol-gel route is to eliminate the progress of agglomeration of the particles caused by the high coordination number of TIPP precursor. The initial syntheses were repeated from previously reported studies, and after optimizing the reaction time and concentration, effect of pH on nanoparticle size, morphology, and crystalline structure was further analysed. The optimum pH range for synthesis of stable titanium dioxide nanoparticles with <5nm particle size was obtained through investigation of characterization data of TNPs synthesized in different pH levels. Particle size and stability of sub 10nm TNPs were confirmed with kinetic size distribution analyses via dynamic light scattering. Our findings suggest that equilibrium conditions for sub 10nm sized TNP occur only at a very narrow pH range, at which formation of network structures is inhibited. Obtained data indicates that the pH – particle size correlation is not linear, and that gelation occurs at solutions with acidity ratio [H+]/[Ti] lower than 0.7 for the system under investigation. In normal conditions, TNP form amorphous structures, and the transformation from amorphous to anatase phase can be induced in particles synthesized in low pH conditions via calcination. The optimum (i.e. minimum energy consuming) calcination conditions were determined and phase transformation was confirmed via X-Ray Diffractometry. It was observed that the post-calcination crystals of TNP that have been synthesized outside the narrow pH range were not in completely anatase phase. The calcination conditions to synthesize particles comprising >90% anatase and <10% rutile, a composition that is mostly preferred for photovoltaic and photocatalytic applications, have also been determined. Images of particles taken via scanning electromicroscopy and atomic force microscopy were also congruent with our findings. Having focused on the effect of pH, concentration, and equilibrium time, we believe that this ongoing study provides a wide range of information on sol-gel synthesis of TNPs.

Fatigue Monitoring of Glass Fiber Reinforced Composite Using Fiber Bragg Grating by Fazlı Fatih Melemez  (Date: May 12, 2011)

Glass fiber reinforced composites (GFRC) are widely used in all load bearing industrial applications. It is therefore critical to understand the fatigue characteristic of GFRC to improve the material characteristics of these composites and to prevent any untoward accidents. In this presentation we describe the preliminary results of fatigue test on GFRC fabricated using an in-house laboratory scale resin transfer molding system. The fatigue tests are done for different load conditions – tension compression and reverse bending. Fiber Bragg grating (FBG) is a sensor which is embedded into the composite at the manufacturing step for monitoring the strain. Fiber bragg grating sensors are able to sense pico-scale strains in the region which they're embedded. They give more accurate results in comparison to strain gauges. The preliminary results of these experiments will be presented.
See the animation of FBG working principle