Iodine critical constants

The critical datum is not a buret reading, as it was in the case of the volumetric method. Rather, the amount of iodine used is determined coulometrically by computing the coulombs (total current over time) needed to reach the end point. The coulombs are calculated by multiplying the current applied to the anode-cathode assembly (a constant value) by the total time (seconds) required to reach the end point. The modern coulometric titrator automatically computes the amount of moisture from these data and displays it. 

The classical work of Eley and collaborators described the vinyl ethers-iodine systems as being characterised by the parallel complexation of the monomer by the catalyst and initiation due to the attack of I onto the double bond. It was on this mechanistic basis that Okamura et applied their scheme to calculate propagation rate constants in 1,2-dichloroethane and -hexane. A criticism of that method has already been given in the previous section and the low values of kp obtained in this study prove that the gross approximations implicit in the method, mainly the neglect of the iodine bound to the polymer, introduce large errors in the estimate of the rate constants. 

For PAN-CSA [116,120], PPyCPFe) [74,118], and iodine doped (CH) [150], the proximity of the material to the insulator-metal transition can be gauged by the resistivity ratio p(l. 4 K)/p(300 K) and a plot of the reduced activation energy W = —7 dln(p(0)/d7 [151]. For a conductor close to the insulator-metal transition, the resistivity follows a power law behavior with T [152] for a critical regime sample, the plot of log W versus log T approaches T =0K at a constant value. The plot of log W versus log T for a critical sample provides a dividing line between the plot of log W versus log T for insulator hopping behavior which increases with decreasing T (i.e., the slope of log W versus log T is equal to y if cr cx exp (To/Ty) and the plot of log W versus log T for metallic samples which decrease with decreasing T. The W plots for selected PAN-CSA materials are shown in Fig. 46.13. 

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