Filler networks conductive

The reduced value of the scaling exponent, observed in Fig. 29 and Fig. 30a for filler concentrations above the percolation threshold, can be related to anomalous diffusion of charge carriers on fractal carbon black clusters. It appears above a characteristic frequency (O when the charge carriers move on parts of the fractal clusters during one period of time. Accordingly, the characteristic frequency for the cross-over of the conductivity from the plateau to the power law regime scales with the correlation length E, of the filler network. 

This paper represents an overview of investigations carried out in carbon nanotube / elastomeric composites with an emphasis on the factors that control their properties. Carbon nanotubes have clearly demonstrated their capability as electrical conductive fillers in nanocomposites and this property has already been commercially exploited in the fabrication of electronic devices. The filler network provides electrical conduction pathways above the percolation threshold. The percolation threshold is reduced when a good dispersion is achieved. Significant increases in stiffness are observed. The enhancement of mechanical properties is much more significant than that imparted by spherical carbon black or silica particles present in the same matrix at a same filler loading, thus highlighting the effect of the high aspect ratio of the nanotubes. 

Figure 9.11 Schematic representation of inter-particle contact and formation of conducting filler networks as a function of filler loading (v), before percolation (vv. ...

In the case of HAF N330 filler in its virgin first cycle that the resistivity increased up to 20% strain. They have attributed this initial increase in resistivity to the breakdown of the filler network structure in the rubber. They observed that when the applied tensile strain increases above this 20% strain, the resistivity with strain graph reaches a plateau. They suggested the occurrence of this phenomenon is a consequence of the orientation effects of filler imder strain and also the effect of the reformation of some of the conduction paths. When the load is removed, the resistivity does not return to its original value but increases further. This indicates that some of the breakdown of the filler agglomerate structure is permanent. 

Mohamed et al. [149] evaluated the use of several types of sulfosuccinate anionic surfactants in the dispersion of MWCNTs in NR latex matrices. Sodium l,5-dioxo-l,5-bis(3-phenylpropoxy)-3-((3-phenylpropoxy)carbonyl) pentane-2-sul-fonate showed the best dispersion capabihty and improved the electrical conductivity of the resulted composites. These results have significant implications in the development of new materials for aerospace applications because the filler s dispersiou directly influences the properties of the final material. Jo et al. [150] obtained pristine MWCNt-Ti02 nanoparticles filled with NR-CllR and epoxidized NR-CUR, concluding that the second blend proved higher thermal conductivity because the epoxy branches in ENR and the functionalized MWCNT form a stronger network. Conductivity in CNTs reinforced with rubber-based blends can be improved when reaching a critical concentration of the filler known as the percolation threshold, when a continuous network structure is formed. Thankappan Nair et al. [151] discussed the percolation mechanism in MWCNT-polypropylene-NR blends. 

In the case of PHA/cellulose whisker materials, studies were conducted using a latex of poly(3-hydroxyoctanoate) (PHO) as a matrix and a colloidal suspension of hydrolyzed cellulose whiskers as natural and biodegradable fillers. Due to the geometiy and aspect ratio of the cellulose whiskers, the formation of a rigid filler network, called the percolation phenomenon, was observed, leading to higher mechanical PHO properties. ... 

Thermal transport in such multiphase materials can be made more efhdent by aligning the nanofiUer in a certain direction. Alignment is understood as a preferred orientation of a fiber or tube (in its longitudinal axes) within a 3D sample. This may provide a great advantage when thermal conductivity enhancement is desired in a preferential direction, which is similar to the case of a pure filler network (Barzic and Barzic 2015). Fillers with anisotropic shape, such as CNTs or CNFs, can be oriented in polymer composites by processing, CNT arrays, or by external force. 

Fig. 8.8 (Left) Electrical conductivity as a function of filler content for EBA with carbon black, graphene platelets, and hybrid systems of mixtures between carbon black and graphene platelets. Adopted from Oxfall et al. (2015). (Right) schematic representation of a CB/CNT hybrid indicating the preferential localisation of the filler particles, the existence of active (conductive) and dead (non-conductive) network branches and the conductive bridges between the two filler networks existing in the matrix...

Besides the interaction between the polymer and the filler, an interaction between filler particles occurs, predominantly above a critical concentration threshold, the percolation threshold. The properties of the material change drastically, because a filler-filler network is established. Eor example an over proportional increase of electrical conductivity of a carbon black filled compound. But even at... 

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