Influence of Matrix Polarity on the Properties of Ethylene Vinyl Acetate–Carbon Nanofiller Nanocomposites

A series of ethylene vinyl acetate (EVA) nanocomposites using four kinds of EVA with 40, 50, 60, and 70 wt% vinyl acetate (VA) contents and three different carbon-based nanofillers—expanded graphite (EG), multi-walled carbon nanotube (MWCNT), and carbon nanofiber (CNF) have been prepared via solution blending. The influence of the matrix polarity and the nature of nanofillers on the morphology and properties of EVA nanocomposites have been investigated. It is observed that the sample with lowest vinyl acetate content exhibits highest mechanical properties. However, the enhancement in mechanical properties with the incorporation of various nanofillers is the highest for EVA with high VA content. This trend has been followed in both dynamic mechanical properties and thermal conductivity of the nanocomposites. EVA copolymer undergoes a transition from partial to complete amorphousness between 40 and 50 wt% VA content, and this changes the dispersion of the nanofillers. The high VA-containing polymers show more affinity toward fillers due to the large free volume available and allow easy dispersion of nanofillers in the amorphous rubbery phase, as confirmed from the morphological studies. The thermal stability of the nanocomposites is also influenced by the type of nanofiller

Particulate Fillers in Thermoplastics

The characteristics of particulate filled thermoplastics are determined by four factors: component properties, composition, structure and interfacial interactions. The most important filler characteristics are particle size, size distribution, specific surface area and particle shape, while the main matrix property is stiffness. Segregation, aggregation and the orientation of anisotropic particles determine structure. Interfacial interactions lead to the formation of a stiff interphase considerably influencing properties. Interactions are changed by surface modification, which must be always system specific and selected according to its goal. Under the effect of external load inhomogeneous stress distribution develops around heterogeneities, which initiate local micromechanical deformation processes determining the macroscopic properties of the composites

Isotactic polypropylene/carbon nanotube composites prepared by latex technology. Thermal analysis of carbon nanotube-induced nucleation

During nonisothermal crystallization of highly dispersed polypropylene/carbon nanotube (CNT) composites, considerable heterogeneous nucleation is observed to an extent scaling with the CNT surface area. Saturation occurs at higher loadings, reaching a plateau value for the crystallization onset which is 15 °C higher than in the unfilled matrix. Polymorphic behavior does not occur, as revealed from wide-angle X-ray diffraction. Upon subsequent heating, an increase in the melting temperature is observed due to increased crystalline perfection in the presence of CNTs. The complex multiple melting behavior is interpreted in terms of recrystallization phenomena. A study at varying heating and cooling rates reveals that CNTs affect the chain segment mobility of the matrix and largely inhibit recrystallization upon heating. TEM observation of the nanocomposite morphology evidences the occurrence of a transcrystalline layer around the CNTs. A structure model is presented, in which individually dispersed CNTs are separated from a bulklike polymer phase by a highly ordered crystalline interface with reduced polymer mobility

On the crucial role of wetting in the preparation of conductive polystyrene-carbon nanotube composites

Polystyrene-single-wall carbon nanotube (PS-SWNT) nanocomposites were prepared by directly mixing aqueous suspensions of exfoliated SWNTs and PS latex particles. After freeze-drying and compression molding, homogeneous polymer films were obtained with well-dispersed carbon nanotubes, as evidenced by scanning electron microscopy imaging. The nanocomposite films display a low percolation threshold and high levels of electrical conductivity. Simultaneously, a considerable increase in the glass-transition temperature of PS is achieved, provided that a sufficient amount of low-molar-mass PS is present in the matrix material. It is suggested that a certain extent of molar mass segregation occurs in the samples, with shorter PS chains preferentially adsorbed onto the nanotube surface. The latter wetting mechanism is indispensable for obtaining favorable electrical and thermal properties

Conductive carbon-nanotube/polymer composites: Spectroscopic monitoring of the exfoliation process in water

A novel and extremely simple UV–Vis spectroscopy technique for monitoring the ultrasound-driven exfoliation of single wall carbon nanotubes (SWNTs) in an aqueous solution of a surfactant is described. The principle of the method is based on the fact that only individual SWNTs absorb UV and visible light. Approaching a plateau value in the UV–Vis absorption, recorded as a function of time and/or the total ultrasonic energy supplied to the system, indicates that the maximum degree of exfoliation has been achieved. Accordingly further energy input should be avoided in order to prevent undesired damage of the SWNTs. Scanning electron microscopy pictures confirm that the leveling off of the absorbance corresponds to the maximum exfoliation of SWNTs. The proposed combination of absorption and imaging techniques offers a cheap, reliable and a viable method for monitoring SWNT exfoliation, assessable to almost all research groups

Modeling of the conductivity of a composite nanotube-polystyrene made with a latex-based process.

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Carbon nanotube/isotactic polypropylene composites prepared by latex technology: Morphology analysis of CNT-induced nucleation

The crystallization behavior of isotactic polypropylene (iPP) in the vicinity of single-wall and multiwall carbon nanotubes (SWCNTs and MWCNTs) has been studied. Combined DSC and transmission electron microscopy (TEM) investigations of bulk composite materials reveal that CNTs nucleate iPP when crystallizing from the quiescent melt and that iPP crystals form a transcrystalline layer of aligned iPP lamellar crystals around the nucleating CNT. The pronounced nucleation effect and the formation of a transcrystalline layer is observed also for ultrathin film CNT/iPP samples. Corresponding diffraction studies show that in bulk as well as in the case of the ultrathin film samples only the ?-phase of iPP exists. The transcrystalline layer is highly oriented around the nucleating CNTs, and the crystallographic c-axes of the lamellae are oriented perpendicular to the long axis of the nucleating CNT, which is in contradiction to assumptions done in other studies. This crystallization behavior is discussed and a possible explanation is provided based on iPP macromolecules wrapped around rather than aligned along the CNTs prior to formation of the nucleus. © 2008 American Chemical Societ

Isotactic polypropylene/carbon nanotube composites prepared by latex technology: Electrical conductivity study

Several series of nanocomposites were prepared using a latex-based process, the main step of which consisted of mixing an aqueous suspension of exfoliated carbon nanotubes (CNTs) and a polymer latex. In the present work, a systematic study on the electrical properties of fully amorphous (polystyrene – PS) as well as semi-crystalline (isotactic polypropylene – iPP) nanocomposites containing either single-wall (SWCNTs) or multi-wall carbon nanotubes (MWCNTs) has been conducted. Percolation thresholds as low as 0.05 wt.% or 0.1 wt.% were observed for SWCNT/iPP and MWCNT/iPP nanocomposites, respectively. The formation of a conductive percolating network at such a low CNT concentration is favored by the high intrinsic conductivity and the low viscosity of the polymer matrix. The electrical percolation threshold of the iPP-based system was found to be lower than its rheological percolation threshold. Beyond the percolation threshold, MWCNT-based nanocomposites generally exhibited higher conductivity levels than those based on SWCNTs, most probably due to the higher intrinsic conductivity of the MWCNTs as compared to that of the SWCNTs. These excellent electrical properties, associated with the strong nucleating effect of the CNTs reported earlier [1] and [2], render this type of nanocomposites extremely attractive from a technological point of view