Key Engineering Materials Vols. 334-335

Abstract: Tensile strength distributions of multi-walled carbon nanotubes (MWNTs), single-walled carbon nanotube (SWNT) ropes and sub-bundles are studied by statistical approach based on the experimental data sets. It shows that the distributions can be adequately described by a two-parameter Weibull model. Considering further their nonlinear stress-strain behavior, the force-strain relation and tensile strength of SWNT bundles are studied. The Weibull modulus of the sub-bundles in a SWNT bundle can be estimated in terms of the maximum sustained force and the failure strain of the bundle. A SWNT bundle was subjected to tensile testing using a nano-mechanical testing device. The result suggests that the nonlinear behavior of SWNTs does affect the force-strain relation of SWNT bundles, mainly at large strain.

Abstract: This paper presents a method for chemical functionalization of CNTs through the combined process of UV/O3 treatment and silanization process. FT-IR and TEM were employed to characterize the changes in surface functionalities and morphology. The results indicate improved dispersion and attachment of silane molecules on the surface of CNTs. Epoxy matrix nanocomposites containing functionalized CNTs showed much better dispersion with associated higher mechanical properties than those without functionalization. These findings confirmed the improved interfacial interactions due to covalent bonding between the functionalized CNTs and epoxy resin.

Abstract: This paper reports a study based on a novel concept of ‘self-healing’ coatings applied onto the brittle fibre surface to reduce the stress concentrations and thus to improve the reinforcing efficiency in a composite. The individual E-glass fibres as well as rovings were coated with a carbon nanotube (CNT) reinforced epoxy composite. The tensile strengths were measured for the individual and bundle fibres, which were treated statistically to determine the Weibull parameters and thus to evaluate the notch sensitivity of the fibres with and without coating. The results indicate that the tensile strength of the individual fibre increased by 10% after coating with neat epoxy. Coating with epoxy nanocomposite containing 0.3wt% MWNT further improved the tensile strength. However, increasing the nanotube content was not necessarily beneficial due to the formation of nanotube agglomerates within the matrix. The tensile tests on fibre roving also showed a clear trend of beneficial effect of nanocomposite impregnation on tensile strength. The rovings impregnated with nanocomposite exhibited a more uniform strength distribution and higher strengths than those impregnated with neat epoxy. Changes in prevailing failure mechanisms influenced by the epoxy and nanocomposite coatings are identified.

Abstract: Thin films of poly(methyl methacrylate) (PMMA)，poly(vinyl acetate) (PVAc) and carbon nanotube composites were produced by different coating methods. The best way to produce the carbon nanotube / PMMA / PVAc composite film with conductive network is dispersing carbon nanotubes in PMMA and PVAc by ultrasonic and by solution casting. Electrical resistance responses of carbon nanotube / PMMA / PVAc composite sensors against various organic vapors at low concentrations are investigated. The experimental results indicate that the composites have high selectivity to various organic vapors at the same concentration. In addition, the electric resistance response of the composites against organic vapors takes place in step with their vapor adsorption procedure. Compatible blends of poly(methyl methacrylate) and poly(vinyl acetate) would be a good candidate to produce a series of electrically conducting carbon nanotubes composite film whose resistance is sensitive to the nature and concentration of an analyte in the vapor phase. The results indicate that the carbon nanotube / PMMA / PAVc composite film can be used as a novel organic vapor sensor to detect, quantify and discriminate various organic vapors.

Abstract: A simple method is described for preparing silver nanoparticles via photoreduction in layered laponite suspensions without any additional reduction agent or heat treatment. The properties of silver nanoparticles were studied as a function of the UV irradiation time. Transmission electron microscopy (TEM) was employed to study the variation of particle shape, size and size distribution. The crystal structure of silver nanoparticles was characterized powder Xray diffractometry (XRD). The optical absorption spectra of silver nanoparticles were obtained by ultraviolet-visible (UV-Vis) spectroscopy.

Abstract: Mono-dispersed silver nanoparticles with various morphologies were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinylpyrrolidone) (PVP). The effects of the concentration of AgNO3, temperature, adding rates of PVP and AgNO3 injected into ethylene glycol as well as molar ratio between the repeating unit of PVP and AgNO3 were discussed. By varying the temperature and reaction time, silver nanocubes, n
noprisms and nanospheres were obtained. It is indicated that at elevated temperatures, the ethylene glycol can reduce Ag+ ion into Ag atoms, and thereby inducing the nucleation and growth of silver nanostructures in the solution phase. In the polymer-mediated polyol process, PVP plays a critical role in producing silver nanostructures with good stability and size/shape uniformity. As a result, both the crystallinity of a seed and extent of PVP coverage on the seed were instrumental in controlling the morphology of Ag nanoparticles.

Abstract: In order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices, polyimide nanofoamed films has been prepared from the polyimide precursors (PMDA-ODA) and poly(ethylene oxide) (PEO) in N,N-dimethylacetamide. The synthesization process included blending polyimide as the major phase with a minor phase of the thermally labile PEO blocks. The foamed films were characterized by a variety of experiments including TG and SEM, and the experimental results indicated that the labile PEO would undergo oxidative thermolysis to release small moleculars from the matrix so as to leave voids into the polyimide matrix. The dielectric properties of the films were studied over broad frequency ranges. The dependences of dielectric constant on the PEO content in the films and frequency were discussed. The films with a proper amount of PEO displayed relatively low dielectric constant compared to the pure polyimide film. Thin film foams with high thermal stability and low dielectric constants can be prepared using the approach.

Abstract: It was shown that chitosan and hydroxy-aluminum pillared montmorillonites were excellent materials for the removal of dyes and metal ions from effluent of dying and finishing. Chitosan/ hydroxy-aluminum pillared montmorillonite nanocomposites are expected to play a multiplex role in the treating process. In this study, the nanocomposite was prepared by incorporating hydroxy-aluminum pillared montmorillonite into chitosan solution that diluted acetic acid was used as solvent for dissolving the chitosan. The ratio of chitosan to the cationic exchange capacity of the montmorillonite was about 1:1, 2:1, 3:1, 4:1 and 6:1, respectively. The nanocomposites were characterized by XRD (X-ray diffraction), SEM (Scanning Electron Microscopy). The experimental results indicated that the presence of hydroxy-aluminum cation was in favor of the chitosan intercalation and the interlayers of MMT was intercalated with the bilayers of chitosan sheets. and they can be used in absorption of organic and metal ions for dying and finishing effluent.

Abstract: This study describes a method of good dispersion and alignment of VGCFs, and examines the effect of nanofiber content on the mechanical properties of nanocomposites. The dispersion of nanofibers was carried out by solution blending, mechanical mixing, and sonication. Levels of 4% – 31% volume content of VGCFs were mixed with polypropylene (PP) powder, and then were melt-mixed using a twin-screw extruder. For the further alignment of fibers, extruded rods were stacked in the mold cavity for the compression molding. In the case of 31% volume content, tensile modulus and strength improved by 100% and 40%, and the flexural modulus and strength increased by 120% and 25%, respectively. The shear modulus showed 65% increase, but the strength dropped sharply by 40%. In the transverse direction, the tensile, flexural, and shear strength decreased as more fibers were added. The matrix modification by maleic anhydride (MAPP) increased the tensile and flexural properties of VGCF/PP by 20% - 30% in the longitudinal direction, and 40% - 250% increase in the transverse direction. The fiber surface treatment by plasma improved tensile and flexural properties of untreated VGCF/PP (18 % vol) composites by 10% - 30% in the longitudinal direction, but strength in the transverse direction decreased by 30% - 40%.