Molecular strain, electric polarization

The new polar symmetry allows for the existence of macroscopic polarization, large or small, depending on the magnitude of the strain and molecular dipole moments shown by small arrows. Due to the distortions, the densest packing of our pears and bananas results in some preferable ahgimient of molecular skeletons in such a way that molecular dipoles look more up than down. By definition, the dipole moment of the unit volume is electric polarization. These simple arguments brought R. Meyer to the brilliant idea of piezoelectric polarization [25] ... 

The influences of nonunifoim strains on die electric polarization in materials with molecular dipoles and die possibly resulting piezoelectric effects were first considered for liquid crystalline (LC) materials (Meyer 1969 HeUrich 1971). The arrangements of the individual LC molecules are classified according to the orientation distribution of the so-called molecule director, a vector diat represents the main axis of the molecule in question, but not necessarily its dipole moment Basically, three nonunifoim deformation modes can be identified (1) Splay is a deformation, in which the directors of die individual LC molecules assume a radial fanUke pattern with their tips further away from each odier dian dieir ends. (2) Bend is a deformation, in which the LC directors are arranged like die rail cars of the trains on parallel tracks in a curve. (3) Twisf is an LC director arrangement, in which the directors of subsequent LC-molecule layers are rotated eidier all clockwise or all counterclockwise relative to each odier. Combinations of die diree fundamental deformation modes are also known. Many additional details and die state of the art may be found in the pertinent literature and on die Internet, e.g., in Andrienko (2006). 

To understand the mechanisms of the electrostriction in the G-elastomers, a model has been established (Wang et al. 2003). The model simulation indicated that the mechanisms of the electric field-induced strain in the electrostrictive G-elastomers are attributed to the rotation of the polar crystal domains that formed by grafted molecular chains and the reorientation of the flexible backbone chains when an electric field is applied. 

By symmetry, not only solids can represent linear coupling between electric (polarization, electric field) and mechanical (stress, strain) quantities, but liquid crystals and other organized fluids can do so, too. Structured fluids have low symmetry, and a rich variety of piezoelectric type coupling constants may exist. Lack of inversion symmetry can be due to chirality or special molecular shapes and asymmetric packing. Chiral liquid crystal phases are the cholesterics, chiral smectics and chiral colmnnar phases. 

Electroporation. When bacteria are exposed to an electric field a number of physical and biochemical changes occur. The bacterial membrane becomes polarized at low electric field. When the membrane potential reaches a critical value of 200—300 mV, areas of reversible local disorganization and transient breakdown occur resulting in a permeable membrane. This results in both molecular influx and efflux. The nature of the membrane disturbance is not clearly understood but bacteria, yeast, and fungi are capable of DNA uptake (see Yeasts). This method, called electroporation, has been used to transform a variety of bacterial and yeast strains that are recalcitrant to other methods (2). Apparatus for electroporation is commercially available, and constant improvements in the design are being made. 

The optical anisotropy, as characterized by the difference between the absorption of IR light polarized in the directions parallel and perpendicular to the reference axis (i.e., the direction of applied strain), is known as the IR linear dichroism of the system. For a uniaxially oriented polymer system [10, 28-30], the dichroic difference, A/4(v) = y4 (v) - Ax v), is proportional to the average orientation, i.e., the second moment of the orientation distribution function, of transition dipoles (or electric-dipole transition moments) associated with the molecular vibration occurring at frequency v. If the average orientation of the transition dipoles absorbing light at frequency is in the direction parallel to the applied strain, the dichroic difference AA takes a positive value on the other hand, the IR dichroism becomes negative if the transition dipoles are perpendicularly oriented. 

Figure 33 illustrates the schematic set up for a DIRLD spectroscopy experiment. A small-amplitude oscillatory tensile strain is applied to a sample, and the time-dependent fluctuations of IR dichroism signals corresponding to the dynamic reorientations of electric dipole transition moments associated with the molecular vibrations of various constituent chemical groups of the system induced by the applied strain are monitored with a pair of polarized IR beams oriented in directions parallel and perpendicular to the strain direction. Under a small-amplitude dynamic strain, the time-dependent dichroic difference can also be treated as the sum of a quasistatic component AA(v) and a dynamic component AA(v, t) induced by the strain s(t), similar to the stress response described in eqn [25]. 

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