Nematic phase birefringence

Nematic phase birefringence, which was observed in the cellulose solutions made from the 24.5/75.5 solvent, appeared immediately following dissolution and persisted throughout a two week period. The uniformly dispersed birefringent patterns were the most prevalent ones in the nematic solutions at all DPs studied, but Schlieren and thread-like patterns indicating nematic phases were readily observed in solutions of DP 450 cellulose. 

N - Yellow, birefringent and transparent disclinati ons present (the original nematic phase)... 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

The birefringence An of a nematic phase depends on the anisotropic polarizabilities a, a L and the degree of order S (Eq. (3), p. 114). The polarizabilities ae and of a nematic phase parallel and perpendicular to the director respectively obey the following relations 53)... 

From the measurements of the birefringence in Fig. 12a further important aspect has to be mentioned. With increasing temperature An does not continuously tend to zero at the phase transformation temperature Tc but vanishes discontinuously. At the phase transformation the nematic phase, having a finite birefringence, coexists... 

A number of characteristic temperatures are important in LC polymer work. The glass transition temperature, Tg, is that temperature below which segmental motion of the main chain of the polymer does not occur, although motions (e.g., rotation) of side-groups may occur. The isotropization or clearing temperature, 7j, is the temperature at which the polymer enters the isotropic melt from one of its mesophases and the birefringence of the mesophase disappears. Temperatures are often quoted more specifically defining where phase transitions occur. For example, 7, would be the temperature where the nematic phase enters the isotropic melt. In this case, of course, TNI is the same as Tr... 

A binary mixture of 4-pentylphenyl 4-pentylbicyclo[2.2.2]octane and 4-heptyl-phenyl 4-pentylbicyclo[2.2.2]octane, see Table 3.8, exhibits a wide nematic phase at room temperature with a high clearing point, a low birefringence (0.08), a moderate viscosity (46 cP) and a weakly negative dielectric anisotropy (Ae —1.1) measured at 20°C. It is a surprisingly good solvent for... 

The use of birefringence to determine the behavior of 5( 7) is a natural choice since the principal characteristic of the nematic phase is optical birefringence i.e., the refractive index differs for light polarized parallel (/ n) or perpendicular (%) to the axis of molecular alignment. Eor a nematic liquid crystal, the director n specifies this optical z axis and / n = and = Dg are called the extraordinary and ordinary refractive indices, respectively. In general, rig > rig and the difference is the refractive index anisotropy (birefringence)... 

The fibers spun from nematic phase systems and solvent composition 25.0/75.0 exhibited twice the tenacity and modulus of those extruded from the system of the solvent composition 29.3/70.7. The two fiber types showed significant differences in birefringence, and in the appearance of their external surfaces and fracture surfaces. The comparable moduli of the fiber spun from a nematic solution with those of Fortisan may result from high uniaxial orientation of cellulose molecules in the former fiber. 

Using n = 1.4, An is estimated as about 0.3 which is of the different order from the birefringence (An = 0.026) measured for the nematic phase in the solution which... 

Due to the clear correlation between molecular properties which can be calculated by computational methods and the physical properties of the nematic phase, the dielectric anisotropy (Ae) and the birefringence (An) can be predicted with reasonable accuracy by molecular modeling [28]. On the other hand, the viscoelastic terms and Kj, K2, are currently not really predictable, even if some recent results based on neural networks [3b], Monte Carlo simulations [29] and molecular mechanics approaches [30] give rise to some careful optimism (Figures 4.8 and 4.9). 

---