Rahmi Ozisik

Role of thermal boundary resistance on the heat flow in carbon-nanotube composites

We use classical molecular dynamics simulations to study the interfacial resistance for heat flow between a carbon nanotube and octane liquid. We find a large value of the interfacial resistance associated with weak coupling between the rigid tube structure and the soft organic liquid. Our simulation demonstrates the key role played by the soft vibration modes in the mechanism of the heat flow. These results imply that the thermal conductivity of carbon-nanotube polymer composites and organic suspensions will be limited by the interface thermal resistance and are consistent with recent experiments.

Effect of chemical functionalization on thermal transport of carbon nanotube composites

We use molecular dynamics simulations to analyze the role of chemical bonding between the matrix and the fiber on thermal transport in carbon nanotube organic matrix composites. We find that chemical bonding significantly reduces tube-matrix thermal boundary resistance, but at the same time decreases intrinsic tube conductivity. Estimates based on the effective medium theory predict increase, by about a factor of two, of the composite conductivity due to functionalization of single-walled nanotubes with aspect ratios within 100–1000 range. Interestingly, at high degree of chemical functionalization, intrinsic tube conductivity becomes independent of the bond density.

Polymer nanocomposite foams

Polymer nanocomposite foams, polymer foams with nanoparticles, are an intriguing class of materials with unique structure and properties. The shape, size and surface chemistry of nanoparticles can be tailored to control the foam structure, and therefore, foam properties. Nanoparticles also add functionality to polymer foams. In this paper, we briefly review the recent developments in polymer nanocomposites and nanocomposite foams. This is followed by an extensive discussion regarding the role of nanoparticles in foam morphology and properties. Finally, the current and future trends of polymer nanocomposite foams are summarized. Both challenges and opportunities in this field are discussed.

Effect of Surface Modification on Magnetization of Iron Oxide Nanoparticle Colloids

Magnetic iron oxide nanoparticles have numerous applications in the biomedical field, some more mature, such as contrast agents in magnetic resonance imaging (MRI), and some emerging, such as heating agents in hyperthermia for cancer therapy. In all of these applications, the magnetic particles are coated with surfactants and polymers to enhance biocompatibility, prevent agglomeration, and add functionality. However, the coatings may interact with the surface atoms of the magnetic core and form a magnetically disordered layer, reducing the total amount of the magnetic phase, which is the key parameter in many applications. In the current study, amine and carboxyl functionalized and bare iron oxide nanoparticles, all suspended in water, were purchased and characterized. The presence of the coatings in commercial samples was verified with X-ray photoelectron spectroscopy (XPS). The class of iron oxide (magnetite) was verified via Raman spectroscopy and X-ray diffraction. In addition to these, in-house prepared iron oxide nanoparticles coated with oleic acid and suspended in heptane and hexane were also investigated. The saturation magnetization obtained from vibrating sample magnetometry (VSM) measurements was used to determine the effective concentration of magnetic phase in all samples. The Tiron chelation test was then utilized to check the real concentration of the iron oxide in the suspension. The difference between the concentration results from VSM and the Tiron test confirmed the reduction of magnetic phase of magnetic core in the presence of coatings and different suspension media. For the biocompatible coatings, the largest reduction was experienced by amine particles, where the ratio of the effective weight of magnetic phase reported to the real weight was 0.5. Carboxyl-coated samples experienced smaller reduction with a ratio of 0.64. Uncoated sample also exhibits a reduction with a ratio of 0.6. Oleic acid covered samples show a solvent-depended reduction with a ratio of 0.5 in heptane and 0.4 in hexane. The corresponding effective thickness of the nonmagnetic layer between magnetic core and surface coating was calculated by fitting experimentally measured magnetization to the modified Langevin equation.

Sulfated levan from Halomonas smyrnensis as a bioactive, heparin-mimetic glycan for cardiac tissue engineering applications.

Chemical derivatives of levan from Halomonas smyrnensis AAD6(T) with low, medium and high levels of sulfation were synthesized and characterized by FTIR and 2D-NMR. Sulfated levan samples were found to exhibit anticoagulation activity via the intrinsic pathway like heparin in a dose-dependent manner. Exceptionally high heparin equivalent activity of levan sulfate was shown to proceed via thrombin inhibition where decreased Factor Xa activity with increasing concentration was observed in antithrombin tests and above a certain concentration, levan sulfate showed a better inhibitor activity than heparin. In vitro experimental results were then verified in silico by docking studies using equilibrium structures obtained by molecular dynamic simulations and results suggested a sulfation dependent binding mechanism. With its high biocompatibility and heparin mimetic activity, levan sulfate can be considered as a suitable functional biomaterial to design biologically active, functionalized, thin films and engineered smart scaffolds for cardiac tissue engineering applications.

Rapid and efficient fabrication of multilevel structured silica micro-/nanofibers by centrifugal jet spinning

A rapid and efficient method consisting of two simple steps, centrifugal jet spinning (CJS) and annealing, is introduced to fabricate multilevel structured silica micro-/nanofibers. Using this technique, which is 500 times faster than electrospinning, silica micro-/nanofibers with a hollow or porous internal structure are formed as a result of non-solvent evaporation induced phase separation in the spinning solution. Silica nanofibers with solid cross sections (364 nm and 781 nm), hollow cross sections (outer and internal diameters of 458 nm and 216 nm respectively), and encapsulated voids (outside diameter of 1.4 μm where bi-continuous nano-pores 118 nm are observed) are fabricated by tuning the amount of non-solvent in the spinning solutions. This technique can be readily extended to large-scale and efficient fabrication of various ceramic materials with multileveled fibrous structures.

Controlling Foam Morphology of Poly(methyl methacrylate) via Surface Chemistry and Concentration of Silica Nanoparticles and Supercritical Carbon Dioxide Process Parameters

Polymer nanocomposite foams have received considerable attention because of their potential use in advanced applications such as bone scaffolds, food packaging, and transportation materials due to their low density and enhanced mechanical, thermal, and electrical properties compared to traditional polymer foams. In this study, silica nanofillers were used as nucleating agents and supercritical carbon dioxide as the foaming agent. The use of nanofillers provides an interface upon which CO2 nucleates and leads to remarkably low average cell sizes while improving cell density (number of cells per unit volume). In this study, the effect of concentration, the extent of surface modification of silica nanofillers with CO2-philic chemical groups, and supercritical carbon dioxide process conditions on the foam morphology of poly(methyl methacrylate), PMMA, were systematically investigated to shed light on the relative importance of material and process parameters. The silica nanoparticles were chemically modified with tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane leading to three different surface chemistries. The silica concentration was varied from 0.85 to 3.2% (by weight). The supercritical CO2 foaming was performed at four different temperatures (40, 65, 75, and 85°C) and between 8.97 and 17.93 MPa. By altering the surface chemistry of the silica nanofiller and manipulating the process conditions, the average cell diameter was decreased from to  μm, whereas, the cell density was increased from to cells/cm3. Our findings indicate that surface modification of silica nanoparticles with CO2-philic surfactants has the strongest effect on foam morphology.

Nanoporous Polymer Films from Immiscible Polymer Blends: Pore Size and Composition Dependence

Nanoporous polymer films have been prepared using immiscible blends of polyetherimide (PEI) and poly(caprolactone diol) (PCLD). The films were prepared by spin-coating from a common solvent – dichloromethane (DCM). To create the nanoporous films, PCLD was removed by immersing the films in acetone, which dissolves PCLD only. Field emission scanning electron microscopy was used to study the porous structure. The pore structure of the films was affected by many factors such as composition of the blend, molecular weight, and various processing parameters. The formation of nanometer size pores was mainly due to the fast phase separation process during spin-coating. The pore size and pore size distribution was found to depend on the blend composition.

Monte Carlo simulation of the structure of mono- and bidisperse polyethylene nanocomposites

The structure of bidisperse polyethylene (PE) nanocomposite mixtures of 50:50 (by mole) of long and short chains of C160H322/C80H162 and C160H322/C40H82 filled with spherical nanoparticles were investigated by a coarse-grained, on lattice Monte Carlo method using rotational isomeric state theory for short-range and Lennard-Jones for long-range energetic interactions. Simulations were performed to evaluate the effect of wall-to-wall distance between fillers (D), polymer-filler interaction (w) and polydispersity (number of short chains in the mixture) on the behavior of the long PE chains. The results indicate that long chain conformation statistics remain Gaussian regardless of the effects of confinement, interaction strength and polydispersity. The various long PE subchain structures (bridges, dangling ends, trains, and loops) are influenced strongly by confinement whereas monomer-filler interaction and polydispersity did not have any impact. In addition, the average number of subchain segments per filler in bidisperse PE nanocomposites decreased by about 50% compared to the nanocomposite system with monodisperse PE chains. The presence of short PE chains in the polymer matrix leads to a reduction of the repeat unit density of long PE chains at the interface suggesting that the interface is preferentially populated by short chains.

Interfacial surfactant competition and its impact on poly(ethylene oxide)/Au and poly(ethylene oxide)/Ag nanocomposite properties

The structure and properties of nanocomposites of poly(ethylene oxide), with Ag and Au nanoparticles, surface modified with a 1:1 (by volume) oleylamine/oleic acid mixture, were investigated via transmission electron microscopy, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry (DSC), infrared spectroscopy, dynamic mechanical analysis, and static mechanical testing. Results indicated that there was more oleylamine on Ag nanoparticles but more oleic acid on Au nanoparticles. This difference in surfactant populations on each nanoparticle led to different interfacial interactions with poly(ethylene oxide) and drastically influenced the glass transition temperature of these two nanocomposite systems. Almost all other properties were found to correlate strongly with dispersion and distribution state of Au and Ag nanoparticles, such that the property in question changed direction at the onset of agglomeration.