SWCNT/PS composites

The inclusion of a conductive polymeric component, namely PEDOT PSS, in PS/SWCNT composites to reduce the non-contact resistivity limiting is shown to reduce the percolation threshold and simultaneously increase the ultimate composite conductivity. The ability of PEDOT PSS to stabilize SWCNT dispersions (individualized SWCNTs] was shown. PEDOT PSS/PS/SWCNT composites showed lower percolation thresholds as compared to PS/SWCNT composites. This reduction was modeled assuming a homogeneous deposition of PEDOT PSS over the SWCNT surface. 

A control series of PS/SWCNT composites (no QDs present] and three series of PS/QD/SWCNT composites (given below], were... 

The introduction of AuNPs into PS/SWCNT composites has not shown to significantly alter the ultimate conductivity of these composites. In the case of only one studied hybrid system made from PSS-stabilized SWCNTs decorated with AuNPs, the addition of the gold particles to the PS/SWCNT composite seems to slightly reduce the percolation threshold. But in all other systems, no effect of the addition of AuNPs... 

Based on TGA and elemental analysis, up to one in 20 nanotube carbons (5 %) has been found to possess an aryl addend [174], For the preparation of PS-CNT nanocomposites, the same authors functionalized SWCNTs by in-situ generation of a diazonium compound from 4-(10-hydroxydecyl)aminobenzoate [175], From rheology data it was suggested that the reinforcement and dispersibility of the thus functionalized SWCNT-composites were improved over PS composites with pristine SWCNTs. 

Figure 2.15 Four-point conductivity measurements of SDS-SWCNT composite films in PS (squares) and in PMMA (rhombi). The molar mass distribution of PS is relatively broad and consists of a mixture of high (about 1,000,000 g/mol for the peak molecular weight) and low molecular weight oligomeric fraction. For the PMMA, the molecular weight distribution is relatively narrow with a main peak located at 1,000,000 g/mol. (From Ref. 100. Reprinted with the authorization of the Royal Society of Chemistry)...

The fact that a minimal change in the SWCNT percolation threshold is observed when introducing QDs raises the question if the SWCNT and QDs are at all interacting. To determine the distribution of SWCNTs and QDs within the PS matrix, scanning electron microscopy (SEM] micrographs of the surface of the PS/ QD/SWCNT composite films were taken. One such micrograph is shown in Figure 6.21. 

PS/SWCNT polymer composites can be prepared with QD-containing latexes. Minimal changes in the percolation threshold were demonstrated using the formulation strategy discussed here. This could be attributed to the low concentrations of filler particles and/or aggregated nature of the filler particles. 

The improved properties of modified CNTs achieved by polymerization technique, e.g. better polymer-CNT interaction, can be also detected by other techniques. As an example, we present here behavior of solutions of the prepared composites. Figure 8.8 represents photomicrographs of two different SWCNT/PS samples dissolved in toluene (right parts) and optical transmission microscopy of casted PS thin films from two different dispersions. The first one (a) represents PS solution in toluene where raw SWCNTs were only added and dispersed by ultasonication for two hours. The second (b) is... 

Modification of the electrical properties of PS by the intfoduction of SWCNTs was described by Wang et al. [199]. The SWCNT/PS nanocomposites with 0-1.0 wt% content of SWCNTs were successfully fabricated by an in-situ suspension polymerization method. DC resistivity and AC impedance measurements performed on the nanocomposites showed that the presence of SWCNTs significantly modifies the electrical impedance of the composites. For instance, for a loading of 1.0 wt% CNT, the resistivity value dropped by over ten orders of magnitude [199]. 

Figure 2.10 SEM image of the surface ofa SWCNT/PS composite containing 5 wt% of SWCNTs, coated with poly(phenylene-ethynylene) (PPE). (From Ref. 86. Reprinted with permission of the American Institute of Physics]...

Regarding the second route mentioned, the colloidal CNT-polymer particle system can be obtained by in situ polymerization. Barraza et al. demonstrated the possibility of preparing SWCNT-filled thermoplastic PS and elastomeric styrene-isoprene copolymer by using a modified version of mini-emulsion polymerization with a cationic surfactant, whereas Deng et al. synthesized CNT/ polyaniline [PANI) composites by in situ emulsion polymerization. Alternatively, the colloidal system can be prepared by direct mixing of the CNTs and the polymer host particles, after the polymerization has already been carried out. 

After freeze-dryingandsubsequentmeltprocessing(compression molding], a composite consisting of homogeneously dispersed SWCNTs in a polymer matrix of choice was finally obtained [see Figure 2.14, ° for which a high moiar mass polystyrene latex was used]. Subsequently, the increase of the conductivity was measured as a function of the wt% of SWCNTs, and a maximum value of about 1 S/m was obtained for a SWCNT/PS nanocomposite. 

Figure 4.5 Four-point DC conductivity profile for composites made using latexes 1 to 6 and SDS-stabilized CNTs i) PS matrix with MWCNTs, (ii) PMMA matrix with MWCNTs, itt) PS matrix with SWCNTs, and [iV) PMMA matrix with SWCNTs. [Reprinted from reference 20, with permission of...

Changes in the percolation threshold and critical exponent for composites prepared via a latex-based route using different processing techniques have been observed. Composites prepared with both a latex of high-Tg [PS) and low-Tg [poly[methyl acrylate) [PMA)) polymer, and SWCNTs and MWCNTs were compared, and the results are presented in the following sections. 

SWCNT/PS composites prepared using three processing techniques are addressed. For one series, the latex-based route, described in Chapter 2, was strictly followed. For two other series of composites, alternative processing techniques were used [see Table 4.2), namely, film casting/compression moulding and spin coating. 

Figure 4.10 Results from [i) Four-point conductivity measurements for SWCNT/PS composites prepared by various processing techniques and ii) respective data fits of the equation given in Table 4.1 (R values of 0.99 for all fittings. Only points close to the percolation threshold were taken into account for the fitting). Lines given in (i) are fittings using the value of t and (pp determined in (//).

The percolation thresholds observed for composites prepared with a PS matrix and SDS- and PEDOT PSS-stabilized SWCNTs are shown in Figure 6.4 [Q. The linear fittings oft and cpp (ii) using the statistical percolation law are given in Figure 6.4 (ii). The statistical percolation law is given as ... 

Figure 6.5 SEM micrograph of a SWCNT/PEDOT PSS/PS composites with a loading of 0.7 wt% SWCNTs and 2.4 wt% PED0T PSS. Scale bar represents 2 pm. (Reprinted with permission from RSC Publishing).

Charge-contrast imaging via SEM of uncoated samples allows for visualization of CNT networks within an insulating matrix when the composite is conductive. It is apparent that it will be difficult to elucidate the true composite morphology since the SEM technique inherently raises the apparent SWCNT thickness by chaining part of the matrix material around the CNTs. Micrographs of SWCNT/PS composite films prepared with SDS-stabilized SWCNTs look similar to that shown in Figure 6.5. 

Implicit in our previous discussions is the presumption that the role of the SWCNT is that of a conductive filler, i.e., the conductive nature of the SWCNT is a prerequisite. However, it could be argued that the SWCNTs in the presence of PEDOT PSS might actually be merely fulfilling the role of a template, or scaffold, within the PS matrix on which the PEDOT PSS adsorbs. In other words, the conductive contribution of the SWCNT might just be negligible. The co-operative contribution to the composite s final conductivity of the SWCNTs and the conductive polymer is investigated in the following sections. 

A different batch of SWCNTs, namely Carbolex SWCNTs, were used in the preparation of SWCNT/PS/PEDOT PSS composites.Carbolex SWCNTs are produced by arc-discharge technology. This technique is used to produce SWCNTs on a large scale but with the disadvantage... 

Composites with varying loadings of SDS- and PEDOT PSS-stabilized Carbolex SWCNTs were prepared. Percolation thresholds, along with the control system (PS/PEDOT PSS blend), were constructed and are given in Figure 6.15. 

The ultimate conductivity of the SWCNT/PEDOT PSS/PS composites is similar to that of the control blends (PEDOT PSS/ PS). It is important to note that the ultimate conductivity of HiPCO SWCNT/PS/PEDOT PSS composites is less than one order of magnitude higher than the corresponding conductivity of poor quality Carbolex SWCNT/PS/PEDOT PSS composites. The difference between HiPCO SWCNT/PS and Carbolex SWCNT/PS composites without the presence of PEDOT PSS, however, is approximately two... 

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