Temperature nematic

Surface Stabilised Ferroelectric Super Twisted Nematic temperature (degrees centigrade)... 

The order parameter in this sample as determined from the NMR spectra varied from 0.88 to 0.72 over the nematic temperature range. This value is higher than the usual values for low molecular weight nematic liquid crystal compounds. 

It has been shown, for a number of fixed values of the compression, that the length of a cylindrial sample under compression shrinks as one cools from the nematic-isotropic transition temperature towards the glass transition [5]. Conversely the diameter of these samples grows throughout the nematic temperature range. 

To obtain a more quantitative picture of the temperature dependence of tr and R IR, specimens of TPBIO were dissolved in a proprietary mixture of low-molar-mass nematics, designated E5, which has a broad nematic temperature range [Chiang et al., 2000]. Experimental results for two TPBIO specimens having DP= 18 and 63 are shown in Eigure 1.10. Eirst, the temperature dependence of R and R may be considered in the context of pertinent statistical theories. An analysis by Halperin and William [1992] of the conformation of main-chain LCPs using an Ising chain model led to the conclusion that the chain dimensions, and R , each exhibit exponential temperature dependence ... 

A great improvement in mesomorphic properties was achieved with the related bis[cj5-2-(4 -alkylphenyl)-3-mercaptopropenato]nickel(II) complexes ((201) R = C H2 +i, >2), and related alkoxy derivatives ((201) R = OC H2 +i, n> 3). Both the alkyl and alkoxy complexes displayed wide nematic temperature ranges typically between about 100 °C and 200 °C the alkyl derivatives possessing lower transition temperatures. 

It is convenient to use the reduced temperature T/T, where is the nematic temperature, instead of temperature T, in the temperature range near Fig. 6 shows the observed values of 1/S versus T/Tj, for PBDG in chloroform, at various concentrations. 

This is also supported by the fact that a X-like endothermic peak was observed near the nematic temperature in the differential scanning calorimetry. 

The dependence of nematic temperature on the size of solvents is illustrated in fig. 15. The largest interatomic distances of the solvent molecules are taken as their sizes. The larger the solvent molecule, the higher is the nematic temperature in general. Further, the solvents which have dipole moment parallel to the long axis depress nematic temperature and others which have dipole moment perpendicular to it, elevate T. ... 

As stated already, the sign of cholesteric twist of the solution of PBLG in CHCl is opposite to that in dichloroethane. The cholesteric twist of PALG also behaves in the same manner, that is, the sign is different to each other in the two solvents. However, the nematic temperature of PALG in EDC increases with the increase of alkyl chain length in the side chain. The constant n, then can be obtained from the plot of log 1/S against log C in EDC. It... 

Some usability problems did, however, occur with the BCLCE too. Excessive heat cycles above the isotropic transitions deteriorated the alignment of the BCLCE this, however, had to be risked since the isotropic phase was one of the control groups of the experiment. Flexing at or near room temperatm-e (in the glassy phase of the elastomer) created tears and cracks in the system as well. In other words, the BCLCE has to operate in the nematic temperature range not only because this is where the gi-... 

Mesophase morphology becomes progressively homogenized upon isothermal aging in the nematic temperature range. The peak maximum moves to higher temperatures and the peak itself becomes... 

Therefore we again obtain the first order transition for jAi — Ci >0 and second order for IB jA2 — Ci < 0 and a tricritical point for IB /Ai — C =0. The tricritical point (TCP) is located in the continuous phase transition line separating the nematic and smectic A phases [12], see a phase diagram schematically shown in Fig. 6.12. Such a point should not be confused with the triple point common for the isotropic, nematic and SmA phases. In Fig. 6.12, for homologues with alkyl chains shorter than l , the N-SmA transition is second order and shown by the dashed curve. With increasing chain length the nematic temperature range becomes narrower (like in Fig. 6.1) and, at TCP, the N-SmA transition becomes first order (solid curve). 

In this paper, we describe the measurement of the temperature dependence of the pretilts for twisted as well as non-twisted nematic samples, using some conventional nematic LCs. In order to have a unified understanding of the temperature dependence of the pretilts of the different samples, an adequate normalization of both the pretilts and the nematic temperature range has been made. Then the differences in the changes of the temperature-dependent pretilt angles between non-twisted and twisted samples were compared. 

In the second, more accurate and detailed investigation a homeotropic OCB sandwich-like cell of 150 /xm thickness was studied. The aim of this work was to investigate the dependence of the threshold laser power first on the spot size of the focused laser beam in the middle of the nematic temperature range (36.5°C) and then its dependence on temperature in the nematic range as well as its dependence on the wavelength in the visible region. 

If LC monomers are used as starting materials, it is very important to crmsider that monomer and polymer networks can differ in their phase behavior as previously mentioned. This is a particular issue for nematic elastomers. Only very few examples are known in which the temperature regime of a nematic phase of a monomer overlaps with the nematic temperature regime of the polymer. The systematic that was hereby obtained for the LC phase behavior of linear polymers also holds for LC polymer networks because for elastomers the cmicentration of the mesogens is much higher than that of the crosslink. The chemistry that can be used for the crosslink is determined by the chemistry of the polymerization technique. 

Figure 2. Concentration dependence of the nematic temperature for mixtures of...

The field-induced cholesteric-nematic transition is most important for the application of compensated mixtures of cholesteryl derivatives as anisotropic solvents. If electric fields of the order of 10,000 V/cm are used for the orientation of the hquid crystal it is not necessary to adjust the temperature, T, of the sample exactly to the characteristic nematic temperature, of the mixture. Good alignment can still be achieved if T - < 5°C. 

The latent heat of this transition is usually small and may even vanish if the width of the nematic temperature range is sufficiently large [23]. Thus, the transition can be either of first or second order. For the second-order transition the discontinuity in the orientational order parameter S and, hence, the dielectric (or diamagnetic) susceptibility, disappears and the field influence on both phases is the same. Thus we do not anticipate any field-induced shift in the N-A transition temperature. For the weak first-order transition there is a small discontinuity in both S and dielectric (and magnetic) susceptibilities, and the shift depends on the competition between two small quantities the difference in susceptibilities for the nematic and smectic A phases on the one hand and transition enthalpy on the other. In particular, the field may induce a change in the phase transition order, from first to the second order, as shown in Fig. 4 [24]. 

Some rather unusual fluoro-substituted terminal units have been used to provide nematic liquid crystals. Compound 48 [42] has a wholly alicyclic structure and hence gives a high smectic B phase stability in addition to a short nematic temperature range. Compound 49 [43] has a low melting point and a room temperature nematic phase however, it is likely to be very unstable. Likewise, acetylenic compounds 50 [44] and 51 [45] are interesting nematogens, but may have stability problems. 

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