Mircea Chipara

Nanodiamond-Based Thermal Fluids

Dispersions of nanodiamond (average size ∼6 nm) within dielectric insulator mineral oil are reported for their enhanced thermal conductivity properties and potential applications in thermal management. Dynamic and kinematic viscosities-very important parameters in thermal management by nanofluids-are investigated. The dependence of the dynamic viscosity is well-described by the theoretical predictions of Einsteins model. The temperature dependence of the dynamic viscosity obeys an Arrhenius-like behavior, where the activation energy and the pre-exponential factor have an exponential dependence on the filler fraction of nanodiamonds. An enhancement in thermal conductivity up to 70% is reported for nanodiamond based thermal fluids. Additional electron microscopy, Raman spectroscopy and X-ray diffraction analysis support the experimental data and their interpretation.

Thermal property of regioregular poly(3-hexylthiophene)/nanotube composites using modified single-walled carbon nanotubes via ion irradiation

The effects of radiation-induced modifications on the thermal stability and phase transition behaviour of composites made of 1% pristine or ion irradiated single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) are reported. Thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), Raman spectroscopy and electron spin resonance (ESR) were used to investigate the radiation-induced functionalization of carbon nanotubes and to assess the effect of ionizing radiation on the adhesion between macromolecular polymer and carbon nanotubes. Irradiation was used to introduce defects in a controlled way solely within pristine nanotubes before composite synthesis. The addition of irradiated SWNTs to a polymer matrix was found to enhance thermo-oxidative stability and phase transition behaviour. Further, ESR studies demonstrate the electronic interaction through charge transfer between filler and matrix. These results could have immense applications in nanotube composite processing. Based on the experimental data, a model for the interaction between polymeric chains and carbon nanotubes is proposed.

Comparison of the π-conjugated ring orientations in polyaniline and polypyrrole

Abstract While both polyaniline and polypyrrole are similar conducting polymers, from angle-resolved photoemission, they exhibit key structural differences. A temperature dependent change in the orientation of the aromatic ring orientation is observed in polypyrrole doped with the perchlorate (ClO 4 − ) counter ion. This structural change occurs at about 220–240 K and is associated with a change in carrier mobility and ring orientation, but is not associated with a significant change in metallicity. Polyaniline exhibits a change in aromatic ring orientation with films thickness, but not with temperature.

Polyvinylchloride-single-walled carbon nanotube composites: thermal and spectroscopic properties

Nanocomposites of single-walled carbon nanotubes dispersed within polyvinylchloride have been obtained by using the solution path. High-power sonication was utilized to achieve a good dispersion of carbon nanotubes. Thermogravimetric analysis revealed that during the synthesis, processing, or thermal analysis of these nanocomposites the released chlorine is functionalizing the single-walled carbon nanotubes. The loading of polyvinylchloride by single-walled carbon nanotubes increases the glass transition temperature of the polymeric matrix, demonstrating the interactions between macromolecular chains and filler. Wide Angle XRay Scattering data suggested a drop of the crystallite size and of the degree of crystallinity as the concentration of single-walled carbon nanotubes is increased. The in situ chlorination and amorphization of nanotube during the synthesis (sonication step) is confirmed by Raman spectroscopy.

Spin-wave modes in magnetic nanowires

Spin-wave modes in magnetic transition-metal nanowires having diameters of about 10 nm and lengths on the order of 1 μm are investigated by model calculations. There are quasicontinuous modes with k vectors parallel to the wires axis and discrete modes with k vectors perpendicular to the wire axis. Due to the small cross section of the wires, the perpendicular modes can be ignored in many cases and the low-temperature behavior of the wires is quasi one-dimensional. Using an analytic approach and exploiting the analogy between micromagnetism and quantum mechanics it is shown that all spin-wave modes with k vectors parallel to the wire axis are localized. The spin-wave localization is a micromagnetic analog to the Anderson localization of conduction electrons due to randomness in less than two dimensions, and, as in the electron analogy, arbitrarily small disorder is sufficient to cause the localization of all modes.

ON THE RADIATION INDUCED DEGRADATION OF NBR-EPDM RUBBERS

Abstract The modification of the elongation at break for NBR–EPDM rubbers subjected to gamma irradiation at various integral doses ranging from 10 to 200 kGy, at different dose rates (0.14, 0.47, 1, 3, 4.7 kGy/h) are reported. The presence of the dose rate effect is noticed.

On styrene-butadiene-styrene-barium ferrite nanocomposites

Abstract Magnetic investigations on a nanocomposite material obtained by spinning solutions of styrene–butadiene–styrene block copolymer containing barium ferrite nanoparticles onto Si wafers are reported. The effect of the spinning frequency on the magnetic features is discussed. It is observed that the magnetization at saturation is decreased as the spinning frequency is increased as the centrifuge force removes the magnetic nanoparticles from the solution. This is supported by the derivative of the hysteresis loops, which show two components, one with a high coercive field and another with a small coercive field. Increasing the spinning frequency increases the weight of the low coercive field component. The anisotropy in the distribution of magnetic nanoparticles, triggered eventually by the self-assembly capabilities of the matrix, is revealed by the difference between the coercive field in parallel and perpendicular configuration. It is noticed that increasing the spinning frequency enhances this difference. The effect of annealing the nanocomposite films is discussed.

ESR investigations on irradiated polystyrene

Abstract Electron spin resonance investigations on the free radicals generated in polystyrene by gamma irradiation, in the temperature range 300–460 K, are reported. Assuming the competition between first and second order recombination processes, the isothermal decay of free radicals has been accurately fitted. It is proved that both the first and the second order isothermal reaction rates are well described by a Vogel–Fulcher–Tamman–Arrhenius equation.

Stereoselectivity of cycloolefin polymerization with WCl6-based catalytic systems

Abstract The stereochemistry of ring-opening metathesis polymerization of several cycloolefins (cyclopentene, cyclooctene, 1,5-cyclooctadiene, cyclododecene) in the presence of WCl6-based catalysts (WCl6/iBu3Al/epichlorohydrin, WCl6/iBu3Al/chloranil, WCl6/phenylacetylene, WCl6/Ph4Sn, WCl6/allyl4Si, WCl6/iBu2AlOAliBu2) has been carefully examined. The product stereoselectivity and polymer microstructure were greatly dependent on the nature of the catalytic system and reaction conditions; the nature of the cycloolefin also influenced the steric course of the reaction. An important stereochemical consequence was that the cis and trans content of the polyalkenamer greatly depended on the oxidation state of the W-atom, catalyst activity and reaction temperature. The relationship between the microstructure of several polyalkenamers (polypentenamer, polyoctenamer, polydodecenamer) obtained with the above catalytic systems and cycloolefin reactivity or catalyst nature is fully illustrated. The role of the ligand as well as of the cocatalyst and activator is also shown. The particular stereochemical behaviour of cycloolefins in the presence of the studied WCl6-based catalytic systems is discussed in terms of the metallacarbene—metallacyclobutane mechanism. The steric effects induced by the coordinating cycloolefin and the existing ligands at the transition metal are also considered.

Non-isothermal crystallization kinetics of polyethylene/carbon nanofiber composites

The effect of carbon nanofibers on the crystallization of polyethylene using differential scanning calorimetry operating in dynamical mode at various cooling rates (2, 4, 8, 15, and 30°C/min) is reported. Experimental data were analyzed using the Avrami, Ozawa, and the combined Avrami–Ozawa methods. It is concluded that the addition of carbon nanofibers modifies the crystallization process of polyethylene (changing the value of the Avrami exponent), and that the combined Avrami–Ozawa approach describes more accurately the crystallization of carbon nanofiber reinforced polyethylene composites. The analysis of the experimental data indicates that the dispersion of carbon nanofibers within polyethylene leads to complex changes in the crystallization process such as increased crystallization temperature, longer times to reach 50% crystallization, higher crystallization rates, and lower degrees of crystallinity of the polymeric component.