Performing Dielectric Experiments

Fig. 20 Charge carrier mobility in P3HT as a function of the charge carrier concentration. Squares refer to an experiment performed on a field effect transistor while circles refer to experiments done on an electrochemically doped sample. In the latter case the mobility is inferred from the steady state current at a given doping level. Solid and dashed lines have been fitted using the theory of [101]. The fit parameters are the site separation a, the prefactor Vq in the Miller-Abrahams-type hopping rate, the inverse wavefunction decay parameter y and the dielectric constant e. From [101] with permission. Copyright (2005) by the American Institute of Physics...

Values of the dipole moment ratio of PNS are obtained from dielectric measurements. From thermoelastic experiments, performed on polymer networks, the temperature coefficient of the unperturbed dimensions is determined. Analysis of these results using the RIS model is performed leading to the parameters given above. 

The broad-band dielectric study of highly filled PDMS is complementary to the NMR study of molecular motions in filled PDMS. The dielectric experiments were performed in the frequency range of 10" -10 Hz [27], A combined analysis of the dielectric spectra both for filled PDMS and the pure components of the mixtures was used to assign the dielectric losses to motions of adsorbed and non-adsorbed PDMS chain units. As discussed above, the interpretation of the results is based on a two-phase model assiuning the exchange of chain units at the surface of Aerosil between adsorbed and non-adsorbed states. 

In the extended CM, the JG relaxation is just part of the continuous evolution of the dynamics. The JG relaxation should not be represented by a Cole-Cole or Havriliak-Negami distribution, as customarily assumed in the literature, and considered as an additive contribution to the distribution obtained from the Kohlrausch a-relaxation. Nevertheless, the JG relaxation may be broadly defined to include all the relaxation processes that have transpired with time up until the onset of the Kohlrausch a-relaxation. Within this definition of the JG relaxation, experiments performed to probe it will find that essentially all molecules contribute to the JG relaxation and the motions are dynamically and spatially heterogeneous as found by dielectric hole burning [180,283] and deuteron NMR [284] experiments. This coupling model description of the JG relaxation may help to resolve the different points of view of its nature between Johari [285] and others [180,226,227,280,281,283,284],... 

Fig. 7a shows a schematic of the arrangement for the spectro-electrochemical experiment performed by Su et al. The lowest layer is the bulk Pt substrate on which a thin layer of Pt nanoparticles is deposited. In this case, CO is adsorbed on the Pt nanoparticles and the system is immersed in water. The adsorbed CO molecules and water are treated as the mixed phase. In the absence of CO, the mixed phase is simply water, and CO adsorption only adds a component to Bmag- In the theoretical study of Su el al, a three-layer model was used to simulate the experimental system in which the first layer is water, the third layer is the substrate, and the layer between them is an effective layer composed of Pt nanoparticles, adsorbed CO and water, as shown in Fig. 7b. For each layer, an optical constant obtained from the literature was given to describe its optical property and the dielectric constant was calculated as the square of refractive index. Since the size of the Pt nanoparticles is much smaller than the wavelength of the incident IR radiation, EMT could be used to calculate the effective dielectric constant of the second layer. Although this layer consists of three phases, namely Pt nanoparticles, adsorbed CO molecules and water, inclusion of the three phases separately in these calculations led to an excessively complicated computation, so CO molecules and water were treated as a mixed phase and Pt nanoparticles were immersed in this mixed phase.

We define the electret state of a dielectric material, in agreement with the definition of Refs. 9 and 10, as a metastable state characterized by a stored polarization. Its decay time must be assumed long with respect to the characteristic time of experiments performed on the material. 

The a transition, which involves motion in long segments of the main polymer chain, is related to the Tg. The P transition involves rotation of short-chain ester side groups in PMMA and therefore occurs below the Tg. The frequency dependency of the p-Tg can be used to calculate the activation energy for the molecular motion, which provides important information for characterising the structure and predicting the performance of polymeric materials. In a dielectric experiment, the calculated activation energy for the P transitions in PMMA was 17.7 kcal/mol. This correlates well with the values calculated from DMA and creep experiments. 

Results of experiments performed by Durand et al. [5] are presented in Figure 2.15. They show excellent agreement with the theory. These authors have also verified that the critical field varies as the inverse of the pitch p. The helical unwinding can also be observed when an electric field is applied to a cholesteric liquid crystal with a positive dielectric anisotropy [6],... 

The nematic-isotropic transition temperature [13] for 7CB is 315.7 K, and for 8CB Tm is 314.0 K. 8CB also has a smectic A phase, with a nematic-smectic transition of 294.3 K crystallisation takes place at 302.7 K for 7CB and at 302.5 K for 8CB. Dielectric experiments have been performed by different authors for both compounds using frequency domain [14-16] and time domain [17,18] techniques. 

Ion pairing should be taken into account when adduct formation involves charged species and occurs in low dielectric media. In particular, if the formation of inclusion complexes involves previous ion-pair dissociation, the apparent stability constants are known to be concentration dependent [33]. In the present case, however, similar binding constant values have been obtained by independent titration experiments performed in air-equilibrated CH2CI2 or CD2CI2 solutions at 298 K and with concentration values that range between 10 to 10 " M and 5 X 10 " to 5 X 10 M by using the spectrofluorimetric technique and H NMR measurements, respectively [13, 14]. It follows that complex formation does not require ion-pair dissociation and ion-paired adducts are formed ... 

The presence of surface conductance behind the slip plane alters the relationships between the various electrokinetic phenomena [83, 84] further complications arise in solvent mixtures [85]. Surface conductance can have a profound effect on the streaming current and electrophoretic mobility of polymer latices [86, 87]. In order to obtain an accurate interpretation of the electrostatic properties of a suspension, one must perform more than one type of electrokinetic experiment. One novel approach is to measure electrophoretic mobility and dielectric spectroscopy in a single instrument [88]. 

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