Relaxation dipole reorientation

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

The new interaction is unstable. To reach stability, fluorophore molecules will use some of their energy to reorient the dipole of the microenvironment (solvent and surrounding amino acids). Dipole reorientation is called relaxation. Emission occurs after the relaxation phenomenon. 

When a fluorophore is bound to a protein, its fluorescence will be dependent on the polarity of the surrounding amino acids. Fluorescence spectra are also dependent on the rigidity of the medium. The relaxation phenomenon (reorientation of the dipole environment) occurs much more easily in a fluid medium. In such a case, emission will occur after relaxation. This is the case when relaxation is faster than fluorescence, i.e., the relaxation lifetime rr is shorter than the fluorescence lifetime to- This occurs when the binding site is flexible, and the fluorophore can move easily. Emission from a relaxed state does not change with excitation wavelength. This can be explained by the fact that whatever the value of the excitation wavelength, the emission will always occur at the same energy level. 

We propose that following the onset of the phase transition the small Cu+ ions residing in the large K+ sites are shifted to an off-center position and produce a distortion in the neighboring unit cells. This imparts to each of these unit cells the ability to behave as a relaxing dipole. In a pure (copper-free) crystal these dipoles are closely interlaced with the complementary part of their respective unit cells and, hence, are unable to reorient. Thus the relaxation process that is linked with these dipoles is not observed in the pure KTN crystal. 

With an alternating current (AC) field, the dielectric constant is virtually independent of frequency, so long as one of the multiple polarization mechanisms usually present is active (see Section 8.8.1). When the dominating polarization mechanism ceases as the frequency of the applied field increases, there is an abmpt drop in the dielectric constant of the material before another mechanism begins to dominate. This gives rise to a characteristic stepwise appearance in the dielectric constant versus frequency curve. For each of the different polarization mechanisms, some minimum dipole reorientation time is required for reahgnment as the AC held reverses polarity. The reciprocal of this time is referred to as the relaxation frequency. If this frequency is exceeded, that mechanism wUl not contribute to the dielectric constant. This absorption of electrical energy by materials subjected to an AC electric held is called dielectric loss. 

Coming to the present volume, one aim has been to provide a basis on which the student and researcher in molecular science can build a sound appreciation of the present and future developments. Accordingly, the chapters do not presume too much previous knowledge of their subjects. Professor Scaife is concerned, inter alia, to make clear what is the character of those aspects of the macroscopic dielectric behaviour which can be precisely delineated in the theoretical representations which rest on Maxwell s analysis, and he relates these to some of the general microscopic features. The time-dependent aspects of these features are the particular concern of Chapter 2 in which Dr. Wyllie gives an exposition of the essentials of molecular correlation functions. As dielectric relaxation methods provided one of the clearest models of relaxation studies, there is reason to suggest that dipole reorientation provides one of the clearest examples of the correlational treatment. If only for this reason, Dr. Wyllie s chapter could well provide valuable insights for many whose primary interest is not in dielectrics. 

In a series of publications by Ishii et al. [54-57], effects of the electric field on structural changes in the amorphous regions, accompanied by an additional relaxation process, were discussed. These effects are reflected in the angular dependence of the second moment at different temperatures. The separation of any orientational effects due to poling from stretching effects were made by the preparation of different sample types. The complications for such a separation arose from the facts that (i) mechanically induced effects on chain orientations are much larger than that of the (electric) dipole reorientation and (ii) after poling only a small irreversible electric polarisation remains. 

Relaxations tend to divide into two types those that obey a simple Arrhenius temperature dependence and those that do not. For simple thermally activated processes Arrhenius behaviour is observed. The probability of the dipole reorientating depends directly on the thermal energy distribution. The relaxation time is related to the frequency of maximum dielectric loss ... 

Historically, dipole reorientation was one of the first molecular relaxation processes to be systemmatically investigated. The permittivity of a molecular system can be described in terms of three contributions ... 

A pioneer effort to the account for electrostatic interaction effects in dipole reorientations and correlation functions was made by Brot and Darmon (39) in their Monte Carlo simulations for the partially ordered solid phase of 1 2 3 trichloro 4 5 6 trimethyl benzene (TCTMB) using the point charge model already mentioned in 2.4. Calculations of transition rates between 6 fold rotational wells of fluctuating depth as a result of changing neighbor orientations resulted in essentially Debye relaxation at 300 Kt but a second simulation at 186 K for which considerable rotational ordering is present produced very nearly a circular arc with od = 0.28 as compared to the experimental Ad = 0.39. 

So-called atomic polarization arises from small displacements of atoms under the influence of the electric field at a frequency of approximately 10 Hz (optical infrared range). The atomic polarizability cannot be determined directly but is normally small compared to the electronic polarizability. Electronic and atomic polarization occur in all types of polymer, even polymers with no permanent dipole moments. Polymers with permanent dipoles show no macroscopic polarization in the absence of an external electric field. If an alternative electric field is applied and if the electric field frequency is sufficiently low with reference to the jump frequency of segments of the polymer, the dipoles orient in the field and the sample shows not only electronic and atomic polarization but also a dipolar polarization. DETA, which operates in a frequency range from 10 to 10 Hz, is used to monitor dipole reorientation induced by conformational changes. These are referred to as dielectric relaxation processes. 

The time t is the relaxation time for dipole reorientation in an electric field of frequency (0 (radians s ). For real systems there may be a number of contributions to the electric permittivity, each relaxing at a different frequency, for example due to internal dipole motion in flexible molecules or collective dipole motion. If these contributions to the electric permittivity are at sufficiently different frequencies, they can be separated in the dielectric spectrum, and it is possible to apply Eq. (9) to each relaxation process. At low frequencies (g)->0), the orientation polarization contribution to... 

The location and shape of the current peaks are quite different for the various components (Fig. 45) and their laminated systems (Fig. 46 and Table 4). The results seem to indicate that the relaxation of space chitge polarizatioo is predominant in the polarized a-b-a system, while in the a-c-a laminate, containing PMMA. the main peak appears from dipole reorientation. In the case of a-d-a system containing PMMA/BaTK), composite, both the relaxatioo of dqmlar (or spontaneous) polarization and relaxation of the space charge polarization can have comparable significance. 

At the glass transition (a-relaxation) the centres of gravity of the mesogenic groups and the polymer chain become mobile. Dipole reorientations associated with this motion contribute to the dielectric relaxation. 

The anisotropy of the a. relaxation in form II PVDF was studied by Miyamoto et They conclude that molecular orientation cannot be explained by whole-chain rotation in the crystalline phase, and propose a model in which dipole reorientation in the crystals takes place by conformational changes assisted by defect motions. 

Given the specific, internuclear dipole-dipole contribution terms, p,y, or the cross-relaxation terms, determined by the methods just described, internuclear distances, r , can be calculated according to Eq. 30, assuming isotropic motion in the extreme narrowing region. The values for T<.(y) can be readily estimated from carbon-13 or deuterium spin-lattice relaxation-times. For most organic molecules in solution, carbon-13 / , values conveniently provide the motional information necessary, and, hence, the type of relaxation model to be used, for a pertinent description of molecular reorientations. A prerequisite to this treatment is the assumption that interproton vectors and C- H vectors are characterized by the same rotational correlation-time. For rotational isotropic motion, internuclear distances can be compared according to... 

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