MWCNT dispersions

Figure 7.2. SEM of 10 vol. % MWCNT-PMMA composite film showing MWCNT dispersion in PMMA. Reprinted with permission from Springer Science and Business Media (44).

In addition, Song et al. (76) used a one-step condensation synthesis method to prepare the PLA-MWCNTnanocomposites. For detailed experimental conditions, refer to Song et al. (76). The nanocomposites were prepared using pure and carboxylic-acid-functionalized MWCNTs. As with the other studies, the carboxylic-acid-functionalized MWCNTs dispersed better in PLA compared with non-functionalized MWCNTs. Overall, it can be inferred from the studies described in this section that the functionalization of MWCNTs is a very important step in the process of making a nanocomposite, as it will help to disperse the nanotubes uniformly, which will eventually create the superior properties of the resulting nanocomposite. In addition, among the currently available methods for functionalization, carboxylic-functionalization is the best for uniform dispersion. 

Hu et al. [58] reported a well defined peak at 0.68 V for 5.0 x 10 M indole-3-acetic acid (an important hormone present in plants) in pH 2.0 phosphate buffer at GCE modified with MWCNT dispersed in DHP [58]. The oxidation peak current of the indole-3-acetic acid increased gradually with the amount of MWCNT-DHP dispersed at the GCE up to a volume of 15 pL. For higher volumes the thickness of the layer blocked the electron transfer. The oxidation peak current presented a linear relationship with the concentration of the hormone from 1 X 10 M to 5 X 10 M, with a detection limit of 2 x 10 M and a reproducibility of 4.3 % for 36 measurements of 5 x 10 M. The authors extended the use of this sensor to the determination of the hormone in gladiola, apple and phoenix leaves showing a very good agreement with HPLC determinations. 

Other nano-fillers have also investigated. Cao et al. [253] reported the utilization of multiwalled carbon nanotubes (MWCNTs) as filler-reinforcement to improve the performance of plasticized starch (PS). The PS/MWCNTs nanocomposites were prepared by a simple method of solution casting and evaporation. The results indicated that the MWCNTs dispersed homogeneously in the PS matrix and formed strong hydrogen bonding with PS molecules. Besides the improvement of mechanical properties, the incorporation of MWCNTs into the PS matrix also led to a decrease in the water sensitivity of the PS-based materials. 

Another amperometric DA sensor based on PPyox-MWCNT composite was developed by T i et al. [73]. In this work, the deposition of PPyox-MWCNT film on gold substrate was carried out potentiostatically in a solution containing p3rrrole and functionalized MWCNTs dispersed in water. The overoxidation of the PPy in the composite was carried out in a NaOH solution using CV. The PPyox-MWCNT modified electrode shows good sensitivity, selectivity and stability with a LOD of 1.7 nM. 

Sanchez studied the functionalization of oxidized SWCNTs and MWCNTs dispersed in thermoplastic elastomers based on poly(butylene terephthalate) (PBT)/ poly(tetramethylene oxide) (PTMO). These nanocomposites showed good dispersion and enhancement in thermo-oxidative stability [27]. 1 % of pristine multi-walled carbon nanotube (MWCNTs) were dispersed in silicon rubber. The SR nanocomposites showed 28 % better thermal stability and 100 % improvement in the ultimate tensile strength is achieved as compared with the pristine polymer matrix counterpart [28]. Also ionic liquids have been tested to improve the dispersion and thermal stability of MWCNTs in polychloroprene rubber (CR) showing improvement in these properties [29]. On the other hand the effect of carbon nanofiber on nitrile rubber was studied. It has been found that the nanofiber increase the thermal stability and decrease the flammability [4]. 

BoUo et al [53] reported an interesting comparison of the electrochemical behavior of GCE modified with l.Omgml of chemically oxidized MWCNT dispersed in water, dimethylformamide (DMF), chitosan (CHI) (in 1.0%v/v acetic acid solution) and Nafion (Naf) in ethanol (Figure 3.7). 

Fig. IS TEM images of (a) untreated MWCNTs and (b) PS-grafled MWCNTs dispersed in toluene. Reprinted from Luo et al. [166], Copyright 2012, with permission from Wiley Periodicals, Inc...

Multi-wall carbon nanotubes (MWNTs) were also used as the reinforcing phase in LCP nanocomposites prepared by means of melt blending technique in a twin-screw extruder equipped with an ultrasonic unit to facilitate MWCNT dispersion (Kumar and Isayev, 2010). The role of ultrasonication was positive and resulted in increased structural as well as rheological properties because of the better-dispersed nanofiller. MWCNT were also used in LCP blends with polycarbonate (PC) (Mukherjee et al., 2009) and PEI (Nayak, Rajasekar, and Das, 2010). In the first case, PC/LCP/MWNTs nanocomposites containing as-received or modified (COOH-MWNT) carbon nanotubes were prepared through the melt process in an extruder and then compression molded. The incorporation of functionalized MWCNTs improved thermal, structural, dynamic-mechanical, and electrical properties of the composites, in particular in blends with treated MWCNTs. MWCNTs were also used, both unmodified and surface treated with SiC particles, to improve dispersion in PEI/LCP blends prepared by melt blending. In the ternary systan, viscosity in the blend with modified MWCNTs was found to be lower than the ternary blend with pure MWCNTs, probably because modified MWCNTs improved the fibrillation of LCP compared to pure MWCNTs. Nanocomposite matrices have not been used to prepare foams yet. 

Optical microscopy is an easy and useful technique to use to check the quality of the dispersion of both SWCNT and MWCNT dispersions, as it can evidence the presence of aggregated CNTs and clusters that cannot be detected with the unaided eye. Due to its relatively low resolution, however, it cannot provide as detailed information at the "CNT level as electronic microscopy technologies. 

Note that UV-Vis spectroscopy can be used in exactly the same way to monitor the debundling of MWCNTs in an aqueous surfactant solution of SDS-MWCNT dispersions. The procedure described can be directly adapted to the study of systems containing MWCNTs [see Figure 3.11]. ... 

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, which has been already discussed, presents the UV-Vis absorption of SDS-MWCNT dispersions, all initially possessing different concentrations during ultrasonication (0.01-1.5 wt%], but diluted to the same MWCNTs prior to UV-Vis measurement. 

Figure 3.12 shows the plateau value of the absorbance at the end of the debundling of two different SDS-MWCNT dispersions, for which the CNT concentrations range from 0.05 wt% to 1.5 wt%, plotted as a function of the CNT concentration. For all the SDS-CNT dispersions, the UV-Vis absorbance was constant as long as the CNT concentration was kept below a critical concentration, being equal to 1.5 wt% for the batch MWA P041206 (open circles, batch 2) and... 

Even if conductivity results of CNT/polymer nanocomposites are commonly reported in terms of wt%, mainly because of lack of precise CNT density values, percolation is considered to be a volumetric phenomenon. In an attempt to fairly compare conductivity results obtained for systems containing either SWCNTs or MWCNTs dispersed in a given polymer matrix, the percolation threshold values of the various nanocomposites series were calculated in terms of vol% CNT. See ref. [28] for the details of the calculation. 

SEM images of MWCNTs dispersed into epoxy primer used for aircraft coating. 

SEM images of type 1 MWCNTs suspended or dispersed in THE and then deposited on graphite substrates, (a) MWCNTs suspended in pure THE. (b) The as-obtained MWCNTs dispersed in 10vol% TEA in THE. 

In another report presented by Su and Zhitomirsky [54], PPy-MWCNT composites were synthesized using sulfanilic acid azochromotrop (SPADNS) and sul-fonazo ni sodium salt (CHR-BS) as anionic dopants for the chemical polymerization of PPy. The advantage of using CHR-BS was that it got adsorbed on the MWCNT s surface, allowed MWCNT dispersion and promoted the PPy polymerization on the MWCNT surface. The formation of PPy coated MWCNT... 

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