Isotropic-nematic transition temperatur

This PLC has an isotropic-nematic transition temperature (T ) of 374 K, a nematic—smectic-A transition temperature of 345 K and a... 

As discussed in Section 1.5.2, e// + 2cx = constant, namely, 3e// — 2As = constant and therefore // = (2/3)(A + constant). Equation (1.114) shows that the dielectric anisotropy Ae is a linear function of the order parameter S. At a temperature below the isotropic-nematic transition temperature and under zero applied field, when the order parameter is S , the dielectric anisotropy is (Ac)o. Approximately we have... 

The effect of the partial orientation is demonstrated in Fig. 9 for the perinaphthenyl radical in azoxyanisol. This spectrum has been taken at the isotropic-nematic transition temperature [72] of the solution. Due to a small temperature gradient in the resonator the sample contained both isotropic and nematic regions on recording the spectrum. Fig. 9b gives therefore a superposition of the spectra of both the randomly and the partially oriented radicals. 

The necessary protocol to fabricate a good POLICRYPS template is the following a mixture of NLC, photoinitiator and monomer is heated above the nematic-isotropic transition temperature of the NLC component (65 °C) in order to exploit the high diffusivity of the NLC molecules in the isotropic state. In this condition, the mixture is cured with a UV interference pattern radiation. When the curing process is over, the sample is slowly cooled down to room temperature, below the isotropic-nematic transition temperature. The optical holographic setup for UV curing is reported in Fig. 1.6. 

Depression of the nematic-isotropic phase transition temperature(Tjjj) is caused by the addition of cis-BMAB. Sudden phase transition occurs when the content of cis isomer reaches the critical... 

Yoshida et al. recently disclosed an alternative method that allowed them to produce stable suspensions of gold nanoparticles (1-2 nm in diameter) in nematic liquid crystals [315]. They used a simple sputter deposition process, which allowed them to prepare thin liquid crystal films of well-dispersed gold nanoparticles in both 5CB and E47 (available from Merck) with a nanoparticle size depending on the used nematic liquid crystal. Unfortunately, the authors did not provide any details on whether the nanoparticles were capped with a ligand or bare, non-coated particles, which makes it difficult to assess and compare the reported thermal as well as electro-optic data. However, very similar effects were found as a result of nanoparticle doping, including lower nematic-to-isotropic phase transition temperatures compared to the used pure nematics as well as 10% lower threshold voltages at nanoparticle concentrations below 1 wt% [315]. 

In addition to lowering V th, ferroelectric nanoparticles such as BaTi03 or Sn2P2S6 [144, 156, 318-323] have also been shown to increase the nematic-to-isotropic phase transition temperature (TN/Iso) and the order parameter of the nematic host [142, 320, 324-326], which are thought to have their origin in a coupling of the electric dipole moment of the particles with the orientational order of the surrounding nematic molecules (Fig. 6). 

Figure 11.5 Complex dynamic viscosity as a function of temperature for a main-chain polyether consisting of a methyl stilbene mesogen and a mixture of seven-and nine-carbon aliphatic spacers. The polymer has a molecular weight of 36,000. The diamonds and squares are for temperature ramp rates of 0.1 °C and 2.0°C/min, respectively the open and closed symbols are for heating and cooling, respectively. The dashed line marks the isotropic-nematic transition. (From Gillmor et al. 1994, with permission from the Journal of Rheology.)...

Figure 12. Orientational dynamics of the discotic system GBDII (N = 500) at several temperatures across the isotropic-nematic transition along the isobar at pressure P — 25. (a) Time evolution of the single-particle second-rank orientational time correlation function in a log-log plot. Temperature decreases from left to right, (b) Time dependence the OKE signal at short-to-intermediate times in a log-log plot. Temperature decreases from top to bottom on the left side of the plot T = 2.991,2.693,2.646, and 2.594. The dashed lines are the simulation data and the continuous lines are the linear fits to the data, showing the power law decay regimes at temperatures. (Reproduced from Ref. 115.)...

Figure 20. (a) Orientational correlation time t in the logarithmic scale as function of the inverse of the scaled temperature, with the scaling being done by the isotropic to nematic transition temperature with Ti-N. For the insets, the horizontal and the vertical axis labels read the same as that of the main frame and are thus omitted for clarity. Along each isochor, the solid line is the Arrhenius fit to the subset of the high-temperature data and the dotted line corresponds to the fit to the data near the isotropic-nematic phase boundary with the VFT form, (b) Fragility index m as a function of density for different aspect ratios of model calamitic systems. The systems considered are GB(3, 5, 2, 1), GB(3.4, 5, 2, 1), and GB(3.8, 5, 2, 1). In each case, N = 500. (Reproduced from Ref. 136.)... 

Nematic-isotropic phase transition change in nematic-isotropic phase-transition temperature... 

T, nematic liquid crystal to isotropic melt transition temperature... 

Fig. 2.1.4. (a) Variation of the equilibrium values of q and s with zW/k T for V = 1.3. (6) Theoretical isotherm for v = 1.3 showing solid-nematic and nematic-isotropic transitions. (The isotherm is drawn for the solid-nematic transition temperature T/e = 0.678.) (After reference 5.)... 

Warner et al. [88,89] give a full description of the free energy and recover, by minimization, the spontaneous strain and the mechanical critical point. They also show that, if the network is crosslinked in the nematic phase, a memory of-the nematic state is chemically locked. This causes a rise in the nematic-isotropic phase transition temperature compared with the uncrosslinked equivalent. After crosslinking in the isotropic state, the transition temperature (on the contrary) is lowered. 

Optical techniques to investigate the isotropic-nematic transition in liquid single crystal ela.stomers have been used [10, 11, 18]. The nematic order parameter (the modulus of the order parameter gy) has been determined by measuring the absorbances AII and Aj using the integrated intensities of the CN absorption band with the incident light polarized parallel and perpendicularly to the director of the monodomain [10, 18]. The result is plotted in Fig. 15 as a function of reduced temperature. It is similar to these obtained for polydomains or-... 

In equations (5)-(8), i is the molecule s moment of Inertia, v the flow velocity, K is the appropriate elastic constant, e the dielectric anisotropy, 8 is the angle between the optical field and the nematic liquid crystal director axis y the viscosity coefficient, the tensorial order parameter (for isotropic phase), the optical electric field, T the nematic-isotropic phase transition temperature, S the order parameter (for liquid-crystal phase), the thermal conductivity, a the absorption constant, pj the density, C the specific heat, B the bulk modulus, v, the velocity of sound, y the electrostrictive coefficient. Table 1 summarizes these optical nonlinearities, their magnitudes and typical relaxation time constants. Also included in Table 1 is the extraordinary large optical nonlinearity we recently observed in excited dye-molecules doped liquid... 

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