Temperature nematic-isotropic

The liquid-crystal transition between smectic-A and nematic for some systems is an AT transition. Depending on the value of the MacMillan ratio, the ratio of the temperature of the smectic-A-nematic transition to that of the nematic-isotropic transition (which is Ising), the behaviour of such systems varies continuously from a k-type transition to a tricritical one (see section A2.5.91. Garland and Nounesis [34] reviewed these systems in 1994. 

Characterization439 Inherent viscosity before and after solid-sate polymerization is 0.46 and 3.20 dL/g, respectively (0.5 g/dL in pentafluorophenol at 25°C). DSC Tg = 135°C, Tm = 317°C. A copolyester of similar composition440 exhibited a liquid crystalline behavior with crystal-nematic and nematic-isotropic transition temperatures at 307 and 410°C, respectively (measured by DSC and hot-stage polarizing microscopy). The high-resolution solid-state 13C NMR study of a copolyester with a composition corresponding to z2/zi = 1-35 has been reported.441... 

Unlike low molar mass liquid crystals, these materials do not undergo a nematic-isotropic transition. Instead, they adopt liquid crystal behaviour throughout the region of the phase diagram for which they are in the melt. Above a particular temperature, rather than adopting an isotropic liquid structure, they decompose. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

Currently, theories are not yet able to predict the transition temperatures based on molecular structure of the constituent molecules. However, for several compounds there is considerable empirical data relating the transition temperature between isotropic and nematic phases (Tni) to molecular structure. Higher implies greater nematic stability. For example, it is... 

One of the primary features of the Gay-Berne potential is the presence of anisotropic attractive forces which should allow the observation of thermally driven phase transitions and this has proved to be the case. Thus using the parametrisation proposed by Gay and Berne, Adams et al. [9] showed that GB(3.0, 5.0, 2, 1) exhibits both nematic and isotropic phases on varying the temperature at constant density. This was chosen to be close to the transitional density for hard ellipsoids with the same ellipticity indeed it is generally the case that to observe a nematic-isotropic transition for Gay-Berne mesogens the density should be set in this way. The long range orientational order of the phase was established from the non-zero values of the orientational correlation coefficient, G2(r), at large separations and the translational disorder was apparent from the radial distribution function. 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

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). 

Rather more surprisingly, there is evidence that even with a fixed rigid-rod length the nematic-isotropic transition temperature can depend upon molecular weight, although to a much lesser extent than in the cases already considered. Thus in the polymer XI the transition temperature rose from 135.5 °C to 165 °C as M was increased from 4,000 to 20,000 62). 

So far we have not considered the influence of the constitution of the polymer main chain on the formation of the nematic phase. If the same mesogenic group is linked to different backbones, the nematic phase can be preserved, as shown for one example in Table 3. Owing to the different flexibilities of the backbones, the nematic state is shifted with respect to the temperature. With falling flexibility of the main chain, as indicated by the increasing glass transition temperature, the phase transformation temperatures nematic to isotropic are shifted towards higher temperatures. This clearly indicates that the restriction of motions, due to the polymer-fixation, directly reflects on the phase transformation temperature. If this restriction... 

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

Figure 2.32 Order parameters calculated for (a) anthracene and (b) anthraquinone dissolved in nematics with different dielectric anisotropy [9]. For the case Ae > 0, the results obtained in the absence of induction effects are also shown (dotted line). The temperature dependence of the dielectric anisotropy is taken into account, with the values eNI = 10.2, AeN/ = 8 and eNI = 5.2, AeN/ = —0.5 for the two cases at the nematic-isotropic transition. Atomic charges in absolute value greater than 0.1 are shown (in e units). The y axis is perpendicular to the molecular plane.

Typical data obtained on 5CB in the nematic-isotropic transition region. The sample is first heated from the nematic into the isotropic phase and then cooled back into the nematic. Note that the detector voltage is independent of temperature and is near zero when Tis above the N-I transition temperature. The sUghtly larger values observed at the minima for the nematic phase result from some residual ellipticity of the beam if the first polarizer and the n director are not oriented at exactly 45 degrees. 

Fig. 9. Reciprocal of the reduced temperature for nematic-isotropic equihbrium plotted against the axial ratio x (From Flory and Ronca >)...

It was shown by Helfrich that at temperatures shghtly above the nematic-isotropic transition temperature, a field induced phase transition from isotropic to nematic can be observed by applying an electric field. According to a thermodynamic consideration, the shift in the transition temperature caused by the electric field E is given by... 

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