PANI-PMMA blend, temperature

Kaiser et al [93] enforced 1995 investigations on the conductivity of PAni/PMMA blends, which contained 60% and 67% by weight of PMMA, measured down to the temperatures of helium (experimental details are similar to those for earlier studies of PAni/PVC blends [80]). Figure 11.52 shows these data compared to earlier measured data [80] for a pellet of compressed PAni powder, similar to that used to make the blends. 

The most remarkable result in this curve is therefore that the conductivity of the PAni/PMMA blend with 60% PMMA exceeds, at all temperatures, that of the pure PAni even though the major component of the blend is a non-conducting component. It seems that the barriers to conductivity in the PAni sample are reduced during the blending process. Kaiser et al found earlier [80] that in PAni/PVC blend with 60% PVC, the conductivity of the blend below temperatures around 100 K was greater, though the room temperature conductivity was depressed relative to that of pure... 

The overall pattern is illustrated in Figure 11.54, which shows, using a logarithmic scale, the enhanced conductivity in the PAni/PMMA blends and the depressed conductivity in the PAni/PETG blends. The approximate linearity of the plots in the figure also shows that the conductivity over a wide temperature... 

The earlier theimopower data for PAni/PVC blends [80] are similar to that for the blends in Figure 11.56, except for negative values at very low temperatures. In the only other data on PAni blends, the theimopower of PAni-CSA/PMMA blends measured by Yoon el al [102] is very close to that of the PAni/PMMA blends investigated here, in magnitude as well as temperature dependence (except for small negative values seen at very low temperatures). These data further emphasise the similarity of PAni blend thermopowers. The theimopower of the pure PAni does appear to be significantly smaller than that of the blends small thermopowers (of either sign) have been seen for some PAni samples by other authors [93,94,95]. 

The different pattern of conductivities, that are shown in Figure 11.54, indicates that the conductivity in the investigated PAni/PETG blends has a relative decrease to that in unblended PAni as the temperature decreases. But with a reduced metallic term and a smaller fraction of material involved in the conduction paths in the blends (consistent with the conclusions of Pelster el al [68]). On the other hand, in the PAni/PMMA blends the conduction barriers that surround the PAni particles appear, especially at lower temperatures, to be lessened. Possible reasons for this effect include better dispersion and optimal network structure for PAni in PMMA, and that hydrolytic effects in PETG-PAni will catalyse the hydrolytic cleavage of ester bonds, which might lead to increased shell dirt . 

FIGURE 1.15 Log-Log plot of W(T) versus temperature for PAni-PMMA blends in the metallic regime. (Reprinted from Wessling, B., Handbook of Nanostructured Materials and Nanotechnology, vol. 5, ed. H.S. Nalwa, Academic Press, New York, 1999, 525. With permission. Copyright 2000 Elsevier Science)... 

Our commercially available polyaniline powder, ORMECON powder, was used for preparing the PAni-PMMA blends. The magnetic susceptibility of unblended PAni and PAni-PMMA blends was measured using a commercial SQUID magnetometer in the temperature range of 2.0-300 K in an applied magnetic field of 100 mT. 

The dopant is p-toluene sulfonic acid (p-TsA), and the core value of PAni-p-TsA (y = 0.5) is calculated to be —206 X 10 emu/mol-2ring. The core susceptibilities of PAni and PAni-PMMA blends are calculated by using the core value of PMMA, which is 62.82 x 10 emu/mol. After subtracting the core value from the experimental values, the total paramagnetic susceptibility of PAni and its blends is plotted as a function of temperature. The data are shown in Figure 1.15 and Figure 1.16. 

All samples show a nearly temperature-independent magnetic susceptibility down to 50 K. Below 50 K, a temperature-dependent Curie-like susceptibility is observed. Figure 1.16 shows the y vs. 1/T plots for unblended PAni and PAni-PMMA blends. The temperature-independent Pauli susceptibility is calculated from the above plot, and the density of the states at the Fermi energy is calculated. 

Figure 23.24 Temperature dependence of peak-to-peak EPR linewidth for unblended PANI (open circles) and PANI-PMMA blend (solid circles), Upper panel, unannealed...

Figure 11.52 also shows that even in the 67% PMMA blend, the conductivity of the blend is greater than that of unblended PAni below temperatures about 150 K. However, as might be expected the reduced quantity of conducting PAni leads to an overall reduction in conductivity compared to the 60% PMMA blend. 

The conductivity of the unblended polyaniline (PAni), PAni-poly(methyimethacrylate) (PMMA) blends and the PAni extracted from blend was measured from room temperature down to millikelvin temperatures at various fields. For PAni (33%)-PMMA (67%), PAni (40%)-PMMA (60%) blends, the reduced activation energy, W = dln(cr)/dln( T), decreases on decreasing the temperature below 1 K and the systems are found to be on the metallic side of the MI transition [25a]. In the case of PAni extracted from the blend, the slope change of Woccurs at 70 K. For unblended PAni, W increases as temperature decreases (Figure 1.43). The temperature dependence of the conductivity is given by... 

Table 2.9 The Room Temperature Conductivity and Resistivity Ratio of PANI-CSA/PMMA Blends at Various Volume Fractions (/) of PANI-CSA...

The temperature dependence of the resistivity for PAN I-CSA/PMMA blends is shown in Fig. 2.56 for 0.(K)2 < f 1 [175]. As a metallic system near the boundary of the metal-insulator transition, p(T) in PANI-CSA is characterized by a positive temperature coefficient [34,35]. Although the positive temperature coefficient is restricted to higher temperatures upon dilution of PAN 1-CSA in PMMA, it is remarkable that this distinctly metallic feature is observed even in samples containing volume fractions of PANI-CSA as low as 0.3%, indicating that even at such dilution the PANI-CSA within the phase-separated network is comparable in quality to that of pure PANI-CSA. 

Again, the subtle variations in the temperature dependence can be most clearly observed from W vs. T plots, as shown in Fig. 2.57. The temperature dependence of the resistivity of PANI-CSA/PMMA blends can be classified into three categories ... 

It is also interesting to note the situation shown in Fig. 19.61b. The pure PAni displays a marked conductivity maximum, whereas the 47% sample shows only a weak maximum and the 20% sample shows none at all. A later study [811 showed that with even better dispersion in a PMMA blend, a higher and broader conductivity maximum is to be observed, and hence a more metallic behavior for the blend than for the pure PAni, Above room temperature the first two samples—both the pure PAni and the 47% PAni blend—behave like metals in a fashion also observed in numerous highly conductive polyacetylenes. Thus in this respect there is no difference between these materials (PAni, PAni blend, and PAc). 

At room temperature and at 10 K, the values of the electrical conductivity of PANI-CSA/PMMA samples with f—f, are of the order of 10 S/cm and 10 S/cm, respectively values which are remarkably high [285] For comparison, polyaniline blends made by dispersing intractable polyaniline in host polymers [286] show percolation only at much higher levels, 4 8.4%. In such samples, the room temperature conductivity at f. is five orders of magnitude lower, of the order of 10 S/cm [286]. 

It was only thanks to many years of cooperation with A. Kaiser that we succeeded in making temperature measurements over the entire temperature range down to 2.7 K and in a broad range of PAni concentrations in a variety of blends [80,101]. The matrix polymer used in the first study was PVC, in the second polyester, PMMA,... 

FIGURE 1.43 Log-LogplotofW(T) versus temperature for unblended PAni, PAni (33%)-PMMA (67%),andPAni (40%)-PMMA (60%) blends and extracted PAni. (Reprinted from Rangarajan, G., Srinivasan, D., Angappane, S., and Wessling, B., Synth. Met, 119, 487, 2001. With permission. Copyright 2001 Elsevier Science)... 

Conclusive additional evidence for the metallic nature of PAni and its blends with PMMA is provided by electron spin resonance (ESR) studies with the observation of a Dysonian line shape [104]. In both cases, the asymmetry ratio (A/B) decreases with decreasing temperature. The observed changes in the line shape from Dysonian to Lorentzian are thus seen to be a manifestation of the variation with temperature of the electrical conductivity (Figure 1.46). The g value is calculated as 2.00191 + 0.00005. The g value, which is close to the fi-ee spin value, confirms that the spins are indeed polarons. 

---