MWCNT concentrations

Jin et al. (26) used melt blending to fabricate MWCNT-PMMA composites with different CNT loadings varying from 0 to 26 wt%. They used a laboratory mixing molder to disperse MWCNT in PMMA at 200°C followed by compression molding at 210°C. Their TEM studies revealed good dispersion even at high MWCNT concentration. The composites showed enhanced mechanical and thermal properties. 

The product of the innovated polymerization procedure described above shows a uniform size distribution of spherical particles, with spheres size of 8 pm, contrary to polydispersed MWCNT/PMMA particles 1—12 pm in diameter. The polydispersity may originate from the presence of MWCNT particles (48), and the size of final MWCNT/PMMA spheres depends on MWCNT concentration and size and also on the level of MWCNT aggregation and the number of individual MWCNTs involved in the formation of composite particles. The presence of nanotubes in PMMA/MWCNT composites was confirmed by SEM analysis, which identified a large amount of MWCNTs at the surface of the composite spheres. Some of them are just adhered on PMMA spheres surface but others come into bulk of PMMA matrix. It was also confirmed by TEM analysis that nanotubes are well embedded in the surface of PMMA particles and even more, they are present inside individual PMMA/MWCNT particles. 

Figure 4.10 SEM Images and fiber diameter distribution histogram of electrospun MWCNT/PS fibers with various MWCNT concentrations (a) pure PS and (b) 1% MWCNT/PS. Reprinted from Ref. 14S with permission from SPIE.

Kashiwagi et al. published the results of study of thermal and combustion properties of PP/MWCNT nanocomposites [11,12]. A significant decrease of maximum rate of heat release (RHR) was detected during combustion research with use of cone calorimeter. A formation of char network stractirre during the combustion process was considered to be the main reason of combustibility decrease. The carbonization influence upon combustibility of polymeric nanocomposites was widely presented in literature [10-13]. Notably, Kashiwagi et al. [11,12] were the first to hypothesize that abnormal dependence of maximum RHR upon MWCNT concentration is closely re-... 

Detailed analysis of TGA graphs (Figure 3) allows claiming that thermal stability increase is achieved even by addition of 1 wt% of MWCNT to PP, while further addition does not lead to such fundamental growth. In addition, Figure 4 shows the comparative results for onset degradation temperatures (T ) and the maximum mass loss temperatures (T ) of PP/MWCNT nanocomposites with the different types and concentrations of MWCNT. One can see nonlinear relation of (T ) and versus MWCNT concentration in the PP compositions (Figure 4). 

Fig. 27 Influence of the MWCNTs concentration on the fiber surface quality of SMPU/MWCNTs composites. Modified from ref. [142] by permission. Copyright 2007, Wiley-Blackwell...

Crystallinity and glass transition temperature of epoxy were increased with increase in multiwall carbon nanotnbe, MWCNT, concentration. MWCNT acted as nucleating agent, initiating formation of new crystallites. ... 

Figure 3.11 displays the dispersion dynamics of different concentrations of MWCNTs in aqueous SDS solutions. For each UV-Vis spectrum, the absolute concentration of the MWCNT dispersion effectively measured was made identical by using the appropriate dilution factor. Therefore, the absorbance measured represents the state of dispersion of the CNTs at the end of the debundling, which proves to be the same for MWCNT concentrations up to 1.4 wt%. Please note that it was verified that, in all the cases, enough SDS molecules were present in order to enable the debundling of all the CNTs that can potentially be exfoliated. 

Figure 3.11 Evolution of the value of the absorbance at 260 nm for different concentrations of thin MWCNTs in aqueous SDS solutions as a function of the energy delivered to the solution. The MWCNT concentrations investigated were 0.01 wt%, 0.5 wt%, (A] 1 wt%, (T) 1.4 wt%, and 1.5 wt%. The weight ratio of SDS to MWCNTs in solution was kept at 1.5. The blank used was the original SDS solution diluted by the same factor. (Reproduced with permission from ref. 92. Copyright 2007 Elsevier Ltd.)...

Figure 5.3 Conductivity (four-point measurements) as a function of MWCNT concentration for MWCNT/iPP-g-MA and MWCNT/PS nanocomposites.

Figure 5.9 shows the electrical conductivity of the MWCNT/ PPO/PS nanocomposites as a function of the weight fraction of the MWCNTs. The percolation threshold is already reached at about 0.3 wt% MWCNT. As for the other series of nanocomposites studied, the conductivity is in the range of several tens of S/m at higher MWCNT concentrations [i.e., higher than 1 wt%]. 

Figure 5.9 Results of four-point conductivity measurements as a function of the MWCNT concentration for MWCNT/PPO/PS nanocomposites.

Figure 5.10 Two-point measurements of the conductivity of MWCNT/PPO/ PS nanocomposites as a function of the MWCNT concentration for 2 and 5 min of mixing in the extruder. The symbol ( ), at 6.7 wt% MWCNTs, corresponds to the conductivity of the pure masterbatch (MWCNT /PS before extrusion).

In Figure 2 the case of a multiwalled carbon nanotubes (MWCNT)/ Polyurethane (PUR) nanocomposite is contemplated. Details on the characteristics of this nanocomposite are given in reference 11. The influence of MWCNT concentration and temperature on tan 5 spectra is shown. The frequency at which the relaxation takes place, (co)Max, shifts to higher frequencies with MWCNT content, reflecting that shorter X= (o)Max times are necessary to block PUR chains motions as the nanoparticles are more abundant. However, at concentrations below 3% MWCNT no relaxation is observed, which indicates that such a reduced number of MWCNTs is incapable of hindering the motion of PUR chains. 

Figure 3.- Variation of (ro)Max with MWCNT concentration at 70°C and 120°C. The data are fitted to the percolation equation (see text). The values of the fitting parameters are presented in the...

Noteworhtly, it is observed that at T= 120°C the relaxation associated with (tan 5)Max occurs for lower MWCNT concentrations 1.5% at 120°C, but 3% at 70°C. This result is explained within the context of the so-called dynamic percolation [14] which accounts for the variation of the percolation threshold depending on the favourable or unfavourable conditions for nanoparticle/polymer interactions. The results of the application of the percolation equation X=Xo, taking X=(o)Max and ( )Max values obtained from Figure 2, are presented in Figure 3. Although the number of data is relatively scarce, it can be deduced that the percolation threshold is reduced as temperature increases, passing from approximately ( )c =2 at T=70°C to < )c =0.96 at T=120°C. This result indicates that MWCNT/PUR interactions are easier formed as temperature is increased. 

Recently, Bhunia et al. prepared a transdermal device from 2-hydroxyethyl methacrylate grafted carnboxymethyl guar gum-functionalized multi-walled carbon nanotube (MWCNT) in situ composite membranes for sustained delivery of diclofenac sodium [65]. Pol5mier matrix-MWCNT interaction at 0.5 and 1 wt% MWCNT concentrations induces excellent copolymer-adsorbed fibrillar orientation of MWCNT compared to that at 2 and 3 wt%. 

Wide angle X-ray diffraction (WAXD) of multi walled carbon nanotube (MWCNTs). Reinforced polyethylene composites showed that the MWCNTs are very well distributed and dispersed in the PE matrix [61], There is a broadening and reduction in intensity of the 110 and 200 PE rejections with increasing MWCNT concentration, indicative of altered amorphous and crystalline phases. XRD results of calcium carbonate and PE composites were studied and the results showed that the adoption of calcium carbonate in polyethylene has two primary effects the reinforcement and the nucleating effect. The reinforcement effect increases the bulk crystallinity and modulus, while the nucleating effect decreases the spher-ulite size. 

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