Key Engineering Materials Vols. 334-335

Abstract: The multi-walled carbon nanotubes (MWNTs) and carbon fibers (CFs) were added to the phenolic resin to fabricate MWNTs/phenolic, MWNTs/CFs/phenolic nanocomposites and CFs/phenolic composites by hot press method. The differential scanning calorimetry (DSC) test was performed for the above-mentioned three kinds of composites. The valley points on the slope of endothermic responses correspond to the glass transition temperatures of the composites. The MWNTs/phenolic nanocomposites had the lowest glass transition temperature among the three kinds of composites discussed, which indicated a better thermal conductivity property of MWNTs. Phenolic-based composites reinforced by different weight percentages of MWNTs and CFs were also investigated. The tensile failure morphologies of nanocomposite specimens were examined using a scanning electron microscope to evaluate the possible effects on the glass transition temperature of nanocomposites..

Abstract: To prepare polymer nanocomposites with enhanced performance, well dispersion of nanoparticles in matrices is necessary. In this work a new route that combines graft pre-treatment and drawing technique with melt mixing was applied. That is, nano-SiO2 particles were firstly modified by graft polymerization and then the grafted nanoparticles were melt-compounded with polypropylene (PP) producing composites filaments via drawing. Finally, the filaments were injection molded into bulk materials. The resultant PP based nanocomposites are much tougher than the unfilled polymer as characterized by either static or dynamic test, besides showing a simultaneous increase in strength and stiffness. Morphology studies indicated that drawing induced extension and separation of the grafted nano-silica agglomerates in PP matrix during making the filaments are frozen to a certain extent after nanocomposites manufacturing. In this way, the nanoparticles are well distributed in the matrix and correlated with each other throughout the entire composites, which might ensure an overall improvement of mechanical properties. Besides, β-crystal of PP developed in the drawing process can be retained in the nanocomposites, which also contributes to the toughening of PP. In view of these, the proposed drawing aided dispersion of nanoparticles might also be applicable to the preparation of other nanoparticles/polymer composites.

Abstract: In this study, poly (ether ether ketone) /carbon nanotubes (CNTs) nano composites were fabricated by using multi-walled carbon nanotubes (MWNTs) as the filler. Tensile properties of nano composites at before and after glass transmission temperature were studied. At room temperature, the addition of 15 wt% MWNTs increased the tensile modulus (0.1
0.3% strain) by 89% and the yield stress by 19%. At after glass transmission temperature (200) , they increased by 163% and 42%, respectively. The experimental results showed that the reinforcement of CNTs are more effective for high temperature than room temperature. DMA results showed an increase in storage modulus with increasing MWNTs loading fraction at both below and above the glass transition temperature Tg (around 145oC). Scanning electron microscope and transmission electron microscope images showed that the MWNTs were well dispersed in the matrix and orientated one direction.

Abstract: To promote dispersion of nano-silica in polypropylene (PP), a polymerizable foaming agent p-vinylphenylsulfonylhydrazide was synthesized and grafted onto the nanoparticles via free-radical polymerization. The results of thermogravimetric analysis (TGA) showed that the sulfonyl hydrazide groups of poly(p-vinylphenylsulfonylhydrazide) acquired the desired thermal decomposition ability, which might be related to their internal oxidation-reduction. Electron microscopy observation indicated that the grafted nanoparticles exhibit greatly improved dispersion in PP owing to the fact that the sulfonyl hydrazine groups on the grafted polymer inside the agglomerates decomposed like blowing agent to form polymer bubbles, leading to rapid inflation of the surrounding matrix that pulled apart the agglomerated nanoparticles during melting mixing.

Abstract: In our previous works, a double percolation mechanism of stress volumes was proposed to explain the special effects generated by small amounts of grafted nanoparticles. Accordingly, it is inferred that strengthening nanoparticle agglomerates and enhancing nanoparticles/polymer matrix interfacial interaction are the key issues to improve mechanical performance of the matrix polymer. To confirm this idea, in-situ crosslinking was adopted to prepare nanocomposites by adding reactive monomers and crosslinking agents during melt compounding of nano-silica with polypropylene (PP). It was found that the grafted polymer chains were successfully crosslinked and chemically bonded to the nano-silica forming crosslinked networks. Meanwhile, matrix molecular chains penetrated through the networks to establish the so-called semi-IPN structure that interconnected nanoparticles by the networks and improved filler/matrix interfacial interaction. As a result, the tiny nanoparticles were well distributed in the matrix and the toughening and reinforcing effects of the nanoparticles on the matrix were brought into play at rather low filler loading, as evidenced by mechanical performance tests. Besides, β-crystal was detected in the nanocomposites experienced in-situ crosslinking reaction.

Abstract: This paper presents results of a study carried out to evaluate the effects of an organomodified nanoclay, either on its own or in combination with a polyimide, upon the fire performance of a commercially available nylon. The fire performance, as determined using cone calorimetry showed that up to 40% improvement in the peak heat release rate could be achieved at addition levels of only around 5wt% of nanoclay. The level of improvement was shown to be strongly dependent upon nanoscale dispersion with a more highly exfoliated morphology, as determined using transmission electron microscopy, which showed a greater reduction in the peak heat release rate compared to a more ordered intercalated structure. Investigation of the mechanism of fire retardancy showed that the reduction in the heat release rate is due to the nanoclay reinforcing the char layer which prevented combustible products from entering in to the gaseous phase. Generally, though, the time to ignition is unaffected by nanoclay additions. The addition of the polyimide to the nanoclay reinforced nylon was inconclusive showing little evidence of further improvements in fire performance.

Abstract: Despite of the excellent properties of carbon nanofiber, the properties of carbon nanofiber filled polymer composite were not improved as much as expected. The usual reason may be not sufficient dispersion of the nanofibers within the composites. For the improvement in the mechanical properties of composites, the carbon nanofiber reinforced hybrid composites was investigated. For the dispersion of the carbon nanofiber, the solution blending method using ultrasonic was used. The hybrid composite was manufactured by the Solution-Dip-Type prepreg manufacturing machine. This machine is consisted of resin bath, curing tower that evaporates solvent and process controller for manufacturing speed. The prepregs were cured in an Autoclave. 3 wt% of carbon nanofiber containing hybrid composite, Carbon Fabric/Ep, was tested by Universal Testing Machine. The tensile strength and modulus were improved by 25% and 35%, respectively. In-plane shear strength and modulus were improved by 45% and 78%, respectively.

Abstract: An elastic double-shell model based on continuum mechanics is presented to study the dynamic torsional buckling of an embedded double-walled carbon nanotube (EDWCNT). Based on the model, a condition is derived to predict the buckling load of the EDWCNT. It is shown that the buckling load of the EDWCNT for dynamic torsional buckling is no less than that for the static torsional buckling. Further, the effect of the van der Waals forces is discussed when an inner nanotube is inserted into an embedded outer one. In particular, the paper shows that, in the absence of the initial van der Waals forces, the buckling load of an EDWCNT is always in-between that of an isolated inner tube and of an embedded outer nanotube, which is different from the result obtained by neglecting the difference of radii. This indicates that disregarding the difference of the radii of the double-walled nanotubes can not properly describe the effect of the van der Waals forces between interlayer spacing.

Abstract: In efforts to improve the thermal conductivity (TC) of Ultem™ 1000, it was compounded with three carbon based nano-fillers. Multiwalled carbon nanotubes (MWCNT), vapor grown carbon nanofibers (CNF) and expanded graphite (EG) were investigated. Ribbons were extruded to form samples in which the nano-fillers were aligned. Samples were fabricated by compression molding where the nano-fillers were randomly oriented. The thermal properties were evaluated by DSC and TGA, and the mechanical properties of the aligned samples were determined by tensile testing. The degree of dispersion and alignment of the nanoparticles were investigated with high-resolution scanning electron microscopy. The thermal conductivity was measured in two directions using the Nanoflash technique.