Field-induced birefringence results

The field-induced birefringence results demonstrate that the transient birefringence is relatively insensitive to aggregation and that the decay time in the dilute limit is independent of the concentration. Since the orientational relaxation time is a... 

The rod-to-coil transition, i.e. a pure single chain process was first considered from spectroscopic results in the visible and infrared which, however, could not prove alone the absence of aggregation. A more convincing experiment, in which experimental results contain some structural information, is the field-induced birefringence. From a field-induced birefringence experiment in yellow and red solution at concentration as low as 5 x 10 to 5 X 10 g cm, it has been argued that a rod-like conformation was observed when the solution turned... 

In electric-field-induced birefringence (or electrical birefringence, quadratic electrooptic effect, Kerr effect), an isotropic transparent substance becomes birefringent when placed in an electric field . The sample assumes the characteristics of a uniaxial crystal, the optical axis of which is parallel to the direction of the applied field. When the sample is illuminated normal to , the resulting two indices, and wj (which can be thought of as He and ), are associated with the parallel and perpendicular orientations, respectively. 

A well-known nonlinear process taking place in the liquid state of anisotropic molecules is the optical-field induced birefringence (optical Kerr effect ). This nonlinearity results from the reorientation of the molecules in the electric field of a light beam. In the isotropic phase the optical field perturbs the orientational distribution of the molecules. In the perturbed state more molecules are aligned parallel to the electric field than perpendicularly to it and as a consequence the medium becomes birefringent. On the other hand in liquid crystals the orientational distribution of the molecules is inherently anisotropic. The optical field, just as a d.c. electric or magnetic field, induces a collective rotation of the molecules. This process can be described as a reorientation of the director. 

Optical-field—induced birefringence in nematic 4-cyano-4 -pentylbiphenyl was measured with cw pump and probe beams, and the optical-field-induced Freedericksz transition was observed for the first time. The results are in quantitative agreement with the theoretical prediction. 

In this review of work at the Institute for Polymers and Organic Solids at UCSB, we summarize the results obtained from spectroscopic studies, quasi-elastic light scattering and field induced birefringence of dilute solutions. We then turn to more recent studies which focus on gels made up of networks of the rodlike molecules. 

Several factors contribute to the field-induced structural anisotropy that leads to optical anisotropy and hence to birefringence. All involve the particles polarization by the field and the partial alignment of their resultant dipole moments parallel to E. The resultant dipole moment / of a particle is the vector sum of its permanent and induced dipole moments. At the molecular level, electronic and atomic polarization occurs, the extent of which depends on the nature and symmetry of the molecule and on its polarizabilities (a and ax) along the parallel and perpendicular directions relative to the electric field or, for cylindrical symmetry, along the molecular axes a and b (a and a ). Naturally, the concept of the polarizability tensor is applicable to an assembly of molecules as a whole, e.g., a colloidal particle, as well. For such systems, and also for macromolecules and polyelectrolytes in an insulating medium, interfacial polarization may also have a major or even dominant contribution to the resultant dipole moment. 

The induced birefringence is a function of the polarizabilities and the extent of alignment of the resultant dipole moments // of the particles in the solution by the external field E. Under the assumption that interparticle interaction is negligible (dilute solutions) and the energy of interaction U between E and fi is less than the thermal energy kT, the following expressions can be derived [3] for the low-field limit ... 

The utility of laser-induced birefringence is based on the high frequency of the perturbation field used, which allows only electronic polarization to occur. If the duration of the laser pulse is sufficiently long, the induced dipoles are partially reoriented parallel to the E of the laser pulse. Comparison of the rate and amplitude of the resulting birefringence with that observed in square-pulse (dc) experiments permits elucidation of the contributions of different polarization mechanisms to the induced dipole moment of the particles [27]. 

Shear flow-induced birefringence measurements of an isotropic solution of PpPTA in concentrated sulfuric acid with a clearing point of 45 °C were also performed by Picken [77]. As shown in Fig. 5 the flow-induced birefringence increases strongly when the isotropic-nematic transition is approached. The results demonstrate that the application of a relatively small shear rate already leads to a degree of orientational order, in the initially isotropic solution, that is comparable with the order in the nematic phase. This points to a strong coupling between the orientation and the external flow field, and to the occurrence of a shear-induced phase transition. 

The field distorts the cubic lattice and results in a change in the angular (or spectral) positions of the Bragg reflections. Moreover, field-induced phase transitions into novel phases have been observed [91, 96]. The field can also induce birefringence parallel to the field direction due to the optical biaxiality of the distorted cubic lattice [97]. 

Field-induced distortions of the blue phases decay rather fast. In the geometry of a Kerr cell (the field is perpendicular to the light wave vector) the field induces the birefringence, 6n, which results in the electrooptical modulation AI of transmitted light. For nearly crossed polarizers... 

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