Nematic-isotropic phase

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... 

The Nematic - Isotropic Phase Transition. For nematic solutions kept free from moisture, the phase transformations described in the preceding were not observed, but the nematic phase could be reversibly transformed to the isotropic phase over a temperature interval Tj - Tj lOK. For the sample with w = 0.041, this transition occurred over the range T = 92 C to Tj = 101 0. For temperatures between T and Tj, the sample was biphasic, with the isotropic and nematic phases coexisting. This behavior is similar to that observed in previous studies, in which Tj - Tj is observed to be independent of w over a range of w for which Tj increases with increasing w (3,4). 

It is observed that in the nematic phase of a liquid crystal, the solvation dynamics of coumarin 503 are biexponential [184a]. The slowest time constant decreases from 1670 ps at 311.5 K to 230 ps at 373 K. The solvation time is not affected by the nematic-isotropic phase transition. Thus, it appears that the local environment and not the long-range order controls the time-dependent Stokes shift. A theoretical model has been developed to explain the experimental findings. This model takes into account the reorientation of the probe as well as the fiuctuation of the local solvent polarization. Similar results are also obtained for rhodamine 700 in the isotropic phase of octylcyanobiphenyl [184b]. 

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

All physical parameters mentioned above are material specific and temperature dependent (for a detailed discussion of the material properties of nematics, see for instance [4]). Nevertheless, some general trends are characteristic for most nematics. With the increase of temperature the absolute values of the anisotropies usually decrease, until they drop to zero at the nematic-isotropic phase transition. The viscosity coefficients decrease with increasing temperature as well, while the electrical conductivities increase. If the substance has a smectic phase at lower temperatures, some pre-transitional effects may be expected already in the nematic phase. One example has already been mentioned when discussing the sign of Ua- Another example is the divergence of the elastic modulus K2 close to the nematic-smecticA transition since the incipient smectic structure with an orientation of the layers perpendicular to n impedes twist deformations. 

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. 

X=LILq is plotted as a function of the reduced temperature red at constant nominal stress CTn = 2.11xlO N mm . Here Lg is the loaded sample length at Tred l-OS. These results will also be used below to establish a close connection between the strain tensor and the nematic order parameter. It has also been shown that a quadratic stress-strain relation yields in the isotropic phase above the nematic-isotropic phase transition a good description of the data for ele-ongations up to at least 60% [4]. 

Static mechanical properties in the vicinity of the nematic-isotropic transition in liquid single crystal elastomers (LSCEs) have been investigated [10, 11]. In Fig. 5 the deformation L/Lq (mon) is plotted as a function of the reduced temperature red- Here Lo(mon) denotes the length of the LSCE at the phase transition temperature of the nematic-isotropic phase transition and... 

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... 

In the nematic phase, this ratio is larger than unity (R /R 1.3) (Warner and Terentjev, 1996), but after a nematic-isotropic phase transition, this ratio approaches unity as a result of the formation of a random coil of polymer chains, which makes the polymer material contract along the director axis of LCEs. In the smectic A phase, the ratio R /R is in general smaller than unity because the polymer chains are likely to exist between the smectic layers (Cotton and Hardouin, 1997). 

A new system has been developed in which every mesogen in the LC or LC polymer is photosensitive (Ikeda, 2001 Tsutsumi et al., 1998b Tsutsumi et al., 1997 Ikeda and Tsutsumi, 1995). For example, the azobenzene moiety could play roles as both a mesogen and a photosensitive moiety (Fig. 3.20) in azobenzene derivatives that form LC phases. These azobenzene LCs show a stable LC phase only when the azobenzene moiety is in the trans form, but do not show an LC phase when all the azobenzene moieties are in the cis form. Examination of these azobenzene LCs revealed that a nematic-isotropic phase transition is induced in the azobenzene LC polymers within 200 ns over a wide temperature range imder optimized conditions (Ikeda, 2001). 

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