Time-dependent electric fields reorientation, dielectric

Previous investigations of helix-coil transition kinetics, which used a variety of fast relaxation methods (electric field jump, ultrasonic absorption, dielectric relaxation and temperature jump), encountered many difficulties (12). The systems studied were long homopolymers (>200 residues) that often had hydrolyzable side chains. Controversial results have been reported, depending on the experimental technique employed, because unwanted side chain reactions or molecular reorientation were often difficult to distinguish from the helix-coil conformational change. However, as observed here, a maximum in the relaxation times was detected for these experiments ranging from 15 ps to 20 ns and was attributed to the helix-coil transition. 

The first process prevails at relatively low frequencies. The electric component E of radiation orients dipole moments p along the field direction, while chaotic molecular motions hinder this orientation p and E are the vectors, and the field E is assumed to vary harmonically with time t. Due to inertia of reorienting molecules the time dependence of the polarization lags behind the time dependence E(f), so that heating of the medium occurs (the heating effect is not considered in this work). The dielectric spectrum obeys the Debye relaxation, for which the absorption monotonically increases with frequency. 

Upon application of a dc field across a dielectric current appears and immediately starts to decrease exponentially with time until it attains a stationary value, the time inversely depends on tanperature. The decrease in conductivity depends on sample thickness and the electrical field strength but is usually marginal compared to decrease due to the reaction. Electrode polarization, accumulation of opposite charges at the electrodes and dipole reorientation, contributes to the gradual decrease of current with time. Polarization can be reduced by choice of an appropriate electrode material for example, aluminum electrodes were shown not to be completely blocking for epoxy resins (Ulanski 1997). 

Since most ER suspensions don t contain the charging agent, the electrode polarization can be ruled out as contributing to the ER effect. The next question that arises is whether the Debye or the interfacial polarization would control the ER effect or if they would jointly control the ER effect. As is known, the Debye polarization is generated due to the dipole orientation in an electric field. For most solid materials, the dipole is almost unable to reorient because the solidification usually fixes the molecule with such rigidity in the solid lattice that there is little or no orientation of the dipoles even in an extremely strong electric field [34]. If the ER effect stems from the polarization of the solid particulate material, it seems unlikely that the Debye polarization would make any contribution, and the interfacial polarization will probably be responsible for the effect. Hao experimentally clarified this issue by measuring the ER response time [26] and temperature dependence of the dielectric property 35, as the ER response time and the... 

Molecules consisting of atoms with different values of the electronegativity are polar. The dipole moment of a chemical bonding is a vector and therefore, a compensation or increase of the bond moments in a molecule can be observed [1, 2], Furthermore, the overall dipole moment of a molecule depends on the life time of different conformations. By the dielectric method, only a very small orientation of the molecular dipoles and the time for its reorientation can be measured as static dielectric constant and relaxation time T, respectively. Thereby the static dielectric constant can be produced by switching off an external electrical field in different steps as demonstrated in Fig. 1. 

In many practical situations, the current is alternating (ac)—that is, an applied voltage or electric field changes direction with time, as indicated in Figure 18.23a. Consider a dielectric material that is subject to polarization by an ac electric field. With each direction reversal, the dipoles attempt to reorient with the field, as illustrated in Figure 18.33, in a process requiring some finite time. For each polarization type, some minimum reorientation time exists that depends on the ease with which the particular dipoles are capable of reahgmnent. relaxation frequency The relaxation frequency is taken as the reciprocal of this minimum reorientation time. 

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