Birefringence temperature dependence

The idea that the critical behaviour of LCEs may be influenced by varying the geometry and density of the crosslinkers was introduced in an early theoretical work [23]. Indications that an increased crosslinking density leads to a more gradual (more supercritical-like) thermoelastic response and birefringence temperature dependence may be found in various publications [7, 18]. Recently, the first systematic and dedicated experimental investigation, by means of H-NMR and high-resolution calorimetry [4,5], demonstrated that the increase in the crosslinkers... 

The dependence of birefringence on the temperature seems to be correlated to the temperature dependence of the lattice parameters ratio c/a shown in Fig. 102. 

Except for coi (transition frequencies of the nuclear spin Hamiltonian) all values are temperature-dependent. From the previous subsection the behaviour of a>s is known. From the anomalous contribution to the birefringence which is proportional to (Sp ) we get the information concerning Ai. If we assume that the damping of the soft mode is non-critical (which is generally accepted), Eq. 10 describes a transition from an under-damped mode to an over-damped one as Tc is approached from either side. 

Glazer AM, Zhang N, Bartasyte A, Keeble DS, Huband S, Thomas PA (2010) Observation of unusual temperature-dependent stripes in LiTa03 and LiTa Nbi-xOs crystals with near-zero birefringence. J Appl Crystallogr 43 1305-1313... 

A detailed insight into the freezing-in process is given by optical investigations. As described in 2.3.1.4 for nematics, 2.3.2.3 for cholesterics and 2.3.3.3 for smectics, the optical uniaxial character of the polymers in the liquid crystalline state has been proved by birefringence measurements and the state of order was calculated from these measurements. This method also provides information about the glassy state. For conventional l.c s it has been demonstrated, that the temperature dependence of... 

Birefringences are mostly observed in condensed phases, especially pure liquids or solutions, since the strong enhancement of the effects allows for reduced dimensions (much shorter optical paths) of the experimental apparatus. Nowadays measurements of linear birefringences can be carried out on liquid samples with desktop-size instruments. Such measurements may yield information on the molecular properties, molecular multipoles and their polarizabilities. In some instances, for example KE, CME and BE, measurements (in particular of their temperature dependence) have been carried out simultaneously on some systems. From the combination of data, information on electric dipole polarizabilities, dipole and quadrupole moments, magnetizabilities and higher order properties were then obtained. 

Based on a study of the temperature dependence of strain-induced birefringence in amorphous polymers, Raha and Bowden [12] suggested that the thermal dissociation of entanglements can be described by... 

In order to produce black-and-white as well as full-colour STN-LCDs, the monochrome interference colours must first be eliminated. This was achieved initially by using two STN-LCDs in a combined double-layer (DSTN) LCD configuration. This involves the use of another non-addressed, passive STN cell in addition to the active display STN-LCD. However, the non-addressed cell has an opposite sense of twist of the nematic director in the cell to that of the addressed STN-LCD. The second STN-LCD, which is identical to the first, but not addressed at all, acts as a retardation compensation layer. The use of an identical second STN-LCD in combination with the active STN-LCD has the advantage that both displays exhibit exactly the same temperature dependence of the birefringence with the same dispersion, assuming that both cells are filled with the same liquid crystal mixture. The second STN-LCD is not addressed and, therefore, there is no increase in power consumption. However, the use of two identical STN-LCDs instead of only one clearly increases the cost and weight of the final product significantly. 

Schematic diagrams of die cell and apparatus for measuring die temperature dependence of die optical rotation of an aligned nematic liquid crystal cell. The birefringence cell is formed from microscope slides and die lines in the cell denote the rubbing direction for the glass plates and thus the direction for the nematic director n.

Figure 2. Temperature dependence of birefringence for PDMS network containing 90.8 mol % short chains. Each curve is labeled by the value of the elongation or relative length of the sample.

This suggestion is in agreement with the well-known fact that the equilibrium rigidity of cellulose ether and ester molecules changes greatly with solvent composition and is also confirmed by the stroi n ative temperature dependence of the statistical size of their chains. The latter property is manifested in a decrease of the intrinsic viscosity, translational friction - and the flow birefringence ) with increasing temperature (Fig. 32) ... 

This expression leads to a temperature dependence for Q that is distinctly different from that implicit in Equation (5) above. For small values of h, Q decays steeply near the critical temperature and then levels off, creating a characteristic defect tail (Fig. 8). These defect tails are commonly observed in the temperature evolution of properties in impure solids that are proportional to the square of the order parameter, such as birefringence and spontaneous strain. To the extent that the sudden strengthening in the dependence of Q with temperature near Tc diverges from the dependence at lower temperatures, two transition temperatures must be differentiated the critical temperature extrapolated from lower temperatures (r ), and the observed f as measured experimentally ff"). 

The selection rules allow us to choose the systems in which the orien-. tational mechanism of birefringence is effective. On the other hand, the explicit form of the temperature dependence Anor(/3) is of no less interest. Its general investigation for any external field and for arbitrary molecular... 

Besides being enormously expanded and spheroidal, the mucins also appear very flexible evidence for this derives from the ease with which these molecules are deformed with shear, as shown in streaming-birefringence experiments, from the concentration dependence of viscosity, and the temperature-dependence of sedimentation coeflScients. Mikkelsen and co-workers confirmed these observations by using the dependence on field strength of electric birefringence relaxation-phenomena. Evidence for local flexibility in the carbohydrate side-chains has come from, for example, n.m.r.-spectral studies. ... 

The order parameter S is a very important quantity in a partially ordered system. It is the measure of the extent of the anisotropy of the liquid crystal physical properties, e.g., elastic constants, viscosity coefficients, dielectric anisotropy, birefringence, and so on. S is temperature dependent and decreases as the temperature increases. The typical temperature dependence of S is shown in Figure 1.16. 

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