Electrical properties poling

One example has been described of CD (PbS) on a poled ferroelectric substrate. The PbS crystal size was larger (ca. 1 jim) on the poled substrate than on the unpoled (or a glass) substrate (ca. 0.3 jim) [34], Other changes in the electrical properties of the films were noted. The differences were ascribed to the electric field and charge accumulation at the ferroelectric surface (more details can be found in Sec. 5.2.4.3). 

In view of the potential technological importance of noncentrosymmetric organic crystals, several approaches have been evolved to artificially achieve noncentrosym-metry, which include electric field poling of polymers, self-assembly of molecular layers, Langmuir-Blodgett assembly of films and host-guest interaction in noncentrosymmetric hosts (Marder et al, 1994). Prediction and/or control of the three-dimensional structure of crystals, given only the information of molecular properties, however, remains difficult at present. 

Typical values of electrical properties of the homopolymer without additives and treatments are in the Table 3.8. The values can be substantially changed by the type of cooling and post-treatments, which determine the morphological state of the polymer. Dielectric constants as high as 17 have been measured on oriented samples that have been subjected to high electrical fields (poled) under various conditions to orient polar crystalline form.74... 

Second, the development of methods and concrete numerical calculations of the constants (reduced matrix elements of the dipole and quadru-pole moments, polarizability, and hyperpolarizabilities, vibronic constant, etc.) determining the effects of electronic degeneracy on electric properties of molecules predicted in this paper seems to be one of the most up-to-date problem in the topics under consideration. Such calculations are quite possible, in principle, provided that the wave functions of the degenerate electronic term (for the calculation of the dipole moment), as well as the excited ones (for the calculation of the polarizabilities), are known. Considering the advances in quantum chemistry, the solution of the problem is quite possible from the practical point of view, especially if one takes into account that in the cases under consideration one can determine numerically the wave function of the system in the presence of an electric field instead of a calculation of excited states. 

Molecular dynamics employs Newtonian mechanics to model the time evolution of the system. The positions, velocities, and accelerations of each atom in the system are calculated from the force-field potential. Newtonian mechanics describes the relationship between the potential felt by each atom, the forces on each atom, and, therefore, the accelerations, velocities, and positions of each atom at each time step of the simulation. From the time evolution of the system, we can calculate many properties of the system. In this chapter, we describe the history, methods, and results of the work on the electric field poling of nonlinear optical polymeric guest-host systems. 

Attempts have been made to correlate the outstanding electric properties (piezoelectricity, pyroelectricity) of PVDF with parameters obtained from wideline NMR experiments. Samples processed by different electric methods have been included in NMR experiments. Wideline NMR studies of irreversible effects induced by relatively high static electric fields have been reported by several authors. The results of wideline NMR ( H) have been used [53] in a combined investigation with a piezoelectric resonance method to find any effects of electric poling (0.78 MV/cm at 120°C) on structure and/or orientation. However, no indications of structural or orientational changes have been found. 

Fig. 5.15 Light-induced generation of second-order NLO properties in an electric field-poled PMMA film doped with 25 wt% of a spiropyran (see Chart 5.14). Alternating irradiation at 7 = 355 nm and 7 = 514 nm.

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