Orientation of molecular dipoles

Application of an electric field normal to the plates (typically the plates are coated with thin films of conducting glass such as indium-tin oxide) unwinds the helix if there is one, and also may cause the polar axis to orient normal to the plates (along the field), or even flatten the chevrons. It should be stressed that any added orientation of molecular dipoles along the field direction should be a weak secondary effect — the polar order occurring in the FLC phase is a thermodynamic property of the phase and not dependent upon applied fields. 

As first realized by Meyer in 1974, when the molecules making up the C phase are non-racemic, the resulting chiral C phase can possess no reflection symmetry. Thus, the maximum possible symmetry of a C phase is C2, and the phase must possess polar order (21). One of the macroscopic manifestations of polar order can be a macroscopic electric dipole moment (the polarization P) associated with orientation of molecular dipoles along the polar axis. While the existence of polar order is not sufficient to assure an observable polarization (just as chirality does not assure optical activity), in fact many FLC materials do possess an observable P. 

Fig. 17. (a) Orientation of molecular dipole moments in 3-methyl-4-nitropyridine. Y-oxide fih molecular dipole moment from direct integration methods fi2, from multipolar model and Hj from semi-empirical calculation, (b) Electrostatic potential around the molecule in the plane of ring atoms. Contours at 0.2kcal/mol (reproduced with permission from Hamazaoui et al. [79]). 

Several distinct relaxation processes are usually present in a solid polymeric material, and these are dielectrically active if they incur significant orientation of molecular dipoles. The multiplicity of relaxation processes is seen most easily in a scan of dielectric loss at constant frequency as a function of temperature (Fig. 3.6). As the temperature is raised, molecular mobilities of... 

End-over-end rotation of whole polymer molecules becomes a much more accessible mechanism for orientation of molecular dipoles when the material is in the liquid state, especially when it is dissolved in a solvent of low molar mass. 

Fig. 9. Schematic diagrams of the three types of polarisable potentials. The left-hand diagram shows a point polarisability model (e.g. SK [35] and DC potentials [36]). The centre diagram shows die polarisation on the two 0-H bonds (e.g. NCC potential [37]). The right-hand diagram shows the all-atomic (or three-) polarisation models (e.g. Bernardo et al [44] and Burnham [26]). The lower diagram schematically illustrates the relative orientations of molecular dipole moments of the four nearest neighbour molecules would in possible to cancel out due to the ice rule and give rise a strong local field.

The outer potential is due to the free or excess charge on the surface of phase a and can be measured experimentally. The surface potential is due to the dipolar distribution of charge at the interface due to the unequal adsorption of ions and orientation of molecular dipoles. It cannot be measured experimentally. Since these quantities are defined with respect to the process of bringing a charged species from infinity into the phase, the surface potential is positive when the positive end of the dipolar charge points toward the center of the solution and the negative end toward charge-free infinity. 

The dielectric permittivity of a medium (relative to the permittivity of free space, 8q = 8.85 X 10 F/m) is given by e and measures the polarization of the medium per unit applied electric field. The dielectric loss factor arises from energy loss during time-dependent polarization and bulk conduction. The loss factor is written as a". The loss tangent or dissipation of the medium, tan<5 is defined by e"/e. The orientation of molecular dipoles has a characteristic time r. Typically is short early in the cure but grows large at the end of the cure. 

The orientation of molecular dipoles cannot take place instantaneously when an electric field is applied. This is the exact analogy of a fact discussed in chapter 7, namely that the strain in a polymer takes time to develop after the application of a stress. In fact the two phenomena are not simply analogous the relaxation of strain and the rotation of dipoles are due to the same types of molecular rearrangement. Both viscoelastic... 

For long-chain molecules there are different geometric possibilities for the orientation of molecular dipole vectors with respect to the backbone. Following the notation of Stockmayer (1967), polymers are classified as type A (with dipoles fixed parallel to the mainchain, e.g., ds-l,4-polyisoprene and polyethers), type B [with dipole moments rigidly attached perpendicular to the mainchain e.g., poly (vinyl acetate) and most synthetic polymers], or type C [with a more-or-less flexible polar sidechain e.g.,poly(n-alkyl methacrylate)s]. However, a polymer possessing only one type of dipole moment is an exceptional case. The timescale (speed) of each polarization (and subsequent relaxation) process will determine whether this process will be monitored by a particular dielectric technique. Characteristics and fundamental peculiarities of relaxations generally found in polymers are discussed hereafter. Note that cases where finite polarization is present even in the absence of an external field (e.g., the permanent polarization in ferroelectrics) are not considered. 

The electrospun PVDF and P(VDF-TrFE) fibrous webs have good piezoelectric properties, showing that they have preferred orientation of molecular dipoles. In order to prove this, P(VDF-TrFE) webs were characterized by infrared (IR) spectroscopy and the piezoelectric signal from the electrospun nanofiber web. The result provides the evidence of the preferential orientation of CF2 dipoles in the P(VDF-TrFE) nanofiber web during electrospinning. 

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