Nematic characteristic data

Characteristic data of dynamic scattering and twisted nematic displays are listed in Table 1. 

Q = 1.3gcm , lu = 5.2 A and M = 265 (for CA), one obtains = 26 according to Eq. (13), and, from Eq. (11), Vp = 0.284 at incipience of a nematic (or cholesteric) phase. The experimental values in Table 3 are generally somewhat greater than this result, but the differences are not beyond the uncertainty in the characteristic chain dimension, here represented by o/n. Inasmuch as this ratio depends on the solvent, as is well known, dependence of the threshold Vp on the solvent is to be expected. Available data do not permit a definitive correlation, however, between Vp at incipience of the mesomorphic phase and the influence of the solvent on chain flexibility. 

Ratios such as AvilAvg and AvtlAvj i j) are free from the orientational order of the molecular axis Szz and solely depend on the spacer conformation [Eqs. (7.3), (7.6) and (7.7)]. From an analysis of the spectra, these ratios were found to remain nearly invariant with temperature. These observations support the idea that the nematic state is populated with an ensemble of nematic conformers. Since the flexible spacer maintains liquidlike characteristics in this state, the equilibration among the conformers should be affected by the temperature, leading to a small variation in these ratios when the data are collected over a wider temperature range. 

The NMR technique has provided useful information regarding the orientational characteristics of nematic liquid crystals [96]. In many examples, the order parameters of the mesogenic core comprising a linear array of aromatic nuclei have been accurately determined from the observed dipolar (Dhd) and quadrupolar (Av) splittings by using deuterium-substituted samples. An attempt was made to elucidate the nematic conformation of the polymethylene-type spacer involved in dimer compounds, o, ct)-bis(4-cyanobiphenyl-4 -yloxy)alkanes (CBA- , with = 9,10), by the combined use of NMR and RIS analysis. The chemical structures and the phase transition data obtained by differential scanning calorimetry (DSC) are shown in... 

Extensive structure analyses were first carried out for the dimers depicted in Scheme 1 (CBA- , n = 9,10). The same techniques were applied to the main-chain polymer liquid crystals having analogous chain sequences (Scheme 2), leading to the conclusion that conformational characteristics of the spacer involved in the nematic state are essentially the same in both dimer and polymer liquid crystals [99]. The molecular scheme described previously has also been successf ully adopted in the analysis of the observed chemical shift and the dipolar coupling data of the dimer and trimer com-... 

Most of the data published on the viewability of twisted nematic liquid crystal displays (TNLCDs) has been measured in transmission.This approach has been favored because transmissive data is easier to interpret from a fundamental point of view. On the other hand, characterizing displays in reflection is more realistic from the practical standpoint, since TNLCDs are primarily viewed in reflection. It was the purpose of this work to explore the relationship between reflective and transmissive viewability. To this end, data was collected on the angular variation of the contrast as a function of applied voltage. These measurements indicate that the viewability in transmission is much more limited than in reflection. The cause of this difference in viewability is shadows, which occur when the TNLCD is viewed in reflection. The object casting the shadow occurs in the plane of the liquid crystal. We call this object the real image. Ambient lighting projects an apparent shadow of the real image into the plane of the reflector. It should be noted that these results are characteristic of TNLCDs and, therefore, independent of the manner in which the TNLCD is manufactured, or the liquid crystal material used in the display. 

The response time of the nematic to an absorbed energy pulse is determined by the order parameter response. Here a connection can be made to experiments on ultrasonic propagation. The order parameter cannot change faster than a molecular vibration period. Since S is determined by the interactions of many molecules, the characteristic time is much longer than the molecular time scale. Moreover, as the phase transition point is approached, there is a slowing of order parameter response typical of critical phenomena. This has been verified by ultrasonic experiments. Therefore, the thermal grating response time should be comparable with the ultrasonic data. ... 

CD samples must be isotropic, at least in the direction perpendicular to the light path, since linear dichroism (LD, birefringence) causes large distortion of CD spectra. Therefore, samples of solid dispersions, membranes, films, gels, and liquid crystals should be prepared with great care to minimize anisotropy. Furthermore, the measurement of nonsolution samples should be repeated several times after successive rotation of the cell around the light beam axis to check the independency from linear dichroic interference. Some reliable studies show LD data with CD curve in order to examine artifacts. The linear dichroic interference may be too large for solid-state samples, and hence their measurement may require specialized instruments. Chiral liquid crystal (nematic or smectic C phase) exhibits a characteristic circular dichroic phenomenon irrelevant to... 

Spatial variations of the nematic director through the sample. At rest, the textures translate into a polydomain structure where the distribution of nematic directors remains constant within each domain. Few optical data have been gathered on surfactant nematics however. For the calamitic phase at rest, Schlieren textures with characteristic line and point defects were observed in various systems [12,15,16,172,284,288,301]. As samples were allowed to relax for some hours, the Schlieren textures coarsened and an alignment resulted due to the interactions with the glass surfaces of the cell. 

Mossbauer studies of glassy nematic [4-6], cholesteric [7, 8] and smectic [9-12] liquid crystals allow the determination of characteristic Debye temperatures, order parameters, intramolecular and lattice contributions to the nuclear vibrational anisotropy, and molecular orientation [13] in the liquid crystal states. For example, Bekeshev et al. have shown from In/ versus temperature data at very low temperatures that MBBA [A -(/7-methoxybenzylidene)-/7-bu-tylaniline] and some liquid-crystalline di-substituted derivatives of ferrocene have weak intermolecular bonds that result in the appearance of additional degrees of free-... 

The conformational analyses of mainchain LCs have been reported from various laboratories. Although the results seem to vary somewhat depending on the models adopted in the treatment of experimental data, all suggest that flexible spacers prefer to take extended conformations in the nematic state. Efforts to formulate molecular theories to describe the N1 transition characteristics of the mainchain LCs in terms of the molecular parameters have also been reported [7,43,44]. In an ideal crystalline state, molecules are aligned in a perfect order, often only the most preferred conformation being permitted for the spacer [45]. The structural characteristics of chain molecules in the crystalline, liquid crystalline, and isotropic fluid states must manifest themselves in the conformational entropy of the system upon phase transitions. As the DP of the mainchain LC sample increases, however, the degree of crystallinity tends to be lower, and accordingly the CN transition becomes less sharp [11]. 

Figures 11.11 and 11.12 describe the evolution of normalized shear stress o (y,t)/a and first normal stress difference Nj (y, t)/Ni with strain yt for PSHQ9 in start-up shear flow. The flow temperature was 130 °C and shear rate was y = 1 s . The experimental data are shown by dots and simulated curves by dashed line. The simulated overshoots for shear and normal stresses are the same as those for experimental data, but the overshoots in calculated curves occur at a relatively low yt and are very narrow. One can attribute such large values of o (Y,t)/a ratios, characteristic for liquid crystalline polymers, to the existence of a lot of polydomains in the nematic state when the start-up flow initiated. Recall that the theory used for simulation utilizes monodomain approach, whereas PSHQ9 exhibits polydomains in nematic state in start-up flow. Thus, deviation of simulated results from experimental data seems reasonable.

Identification of nematic polymeric mesophases is a more complex problem than identification of polymer smectics. The structural data are usually limited to finding the absence of small-angle reflections in the x-rays of unoriented samples. The low enthalpy of the transition from the anisotropic to the isotropic phase (Table 6.9), close to the corresponding values characteristic of low-molecular nematic liquid crystals, and the absence of layered reflections indicate the one-dimensional type of ordering, although these data are insufficient for a complete description of the structure of nematic polymers, which can be both similar to and (Afferent from low-molecular-weight nematics. 

In FIGURES 4 and 5 the available experimental data for the dielectric relaxation times are collected for 7CB and 8CB, respectively. The general behaviour is very similar in the two cases. In the isotropic phase a single relaxation time is measured, corresponding to the rotational (fynamics of the molecules about their short axes. However, the relaxation becomes bimodal for tempo-atures lower than Tni. In the nematic phase, dielectric data sets have been collected with the electric field parallel (8 ) and perpendicular (ij ) to the nematic director. In the parallel geometry a single relaxation process is observed, characterised by a relaxation time which increases rapidly at low temperatures. A second relaxation process appears in the perpendicular geometry its characteristic time is only... 

---