Conducting polymers kinetic experiments

FIGURE 170 Stopped flow kinetics experiment conducted at 95 °C using Cr/AlP04 catalyst (P/AI atomic ratio of 0.8, 600 °C). The reactivity decayed normally even in the absence of polymerization, which rules out polymer accumulation as the cause. 

For concentrated systems, in situ time-dependent experiments have been performed. The mechanisms and the kinetics of the electrochemical inclusion of copper particles inside an organic conducting polymer yield a complete illustration of the typical experiments one can consider. The time scale attainable of the order of a tenth of a second restricts this time-resolved spectroscopy to materials science where mass transportation is involved. ESRF can open the millisecond time scale giving access to more dilute samples and perhaps visualisation of changes of conformation of large molecules as found in biophysical related cases. 

In the previous section it was shown that detailed information about electrochemical processes and the kinetics of follow-up chemical reaction steps can be investigated by UV/Vis/NIR spectroelectrochemical experiments in transmission mode in the diffusion layer at optically transparent or microstructured non-transparent electrodes. Many metal electrodes show a high reflectivity and therefore optical spectra may also be recorded under in situ conditions in reflection mode [70, 71]. This approach is essential for the study of adsorbed species, the formation of solid layers at the electrode surface, reactions of solids [72-74], and the redox behaviour of conducting polymer layers [75-77]. Furthermore, in reflection mode, the angle of incidence may be modified and polarised light maybe used in ellipsometry studies [78]. 

In many classical thermoelectrochemical experiments in open cells, which are directed at modem substances or actual processes, transport properties were of interest [89-93]. In a comprehensive impedance study of a conducting polymer, poly(tetracyanoquinodimethane) (polyTCMQ), Inzelt determined fundamental constants like diffusion coefficient, double-layer capacity and other quantities of the semiconductor layer as a function of temperature [89], Reduction kinetics of buckminsterfullerene Ceo has been studied [90] diffusion coefficients of different systems have been determined [91-93], among them/) of ferrocene/ferricinium in acetonitrile [92] and of vanadium ions [93]. In the latter example, activation energy of diffusion was also of interest. 

The various physical methods in use at present involve measurements, respectively, of osmotic pressure, light scattering, sedimentation equilibrium, sedimentation velocity in conjunction with diffusion, or solution viscosity. All except the last mentioned are absolute methods. Each requires extrapolation to infinite dilution for rigorous fulfillment of the requirements of theory. These various physical methods depend basically on evaluation of the thermodynamic properties of the solution (i.e., the change in free energy due to the presence of polymer molecules) or of the kinetic behavior (i.e., frictional coefficient or viscosity increment), or of a combination of the two. Polymer solutions usually exhibit deviations from their limiting infinite dilution behavior at remarkably low concentrations. Hence one is obliged not only to conduct the experiments at low concentrations but also to extrapolate to infinite dilution from measurements made at the lowest experimentally feasible concentrations. 

Researchers have already invested several decades to elucidate the effect of input variables on the polymerization kinetics and the polymer structures. Many research groups have devoted their resources to obtaining reproducible data on polymerization kinetics. One of the methods to achieve that is to conduct several experiments in parallel to keep most reaction inputs constant and to minimize unpredictable environmental effects. In these series of experiments it appeared to be necessary to apply... 

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