Isotropic transition temperature

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... 

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. 

A hot-stage-equipped polarizing microscope was used for measurement of these parameters. The anisotropic melting temperature (Tn) was determined as the onset temperature of stir-opalescence observed on the hot-stage. The liquid crystalline-isotropic transition temperature (71) was also determined by the use of the hot-stage-equipped microscope. 

Dynamic polymers having C=N bonds responding to changes in a neat/solution environment were reported by Lehn et al. [22]. Polymers 9-12 were synthesized by the corresponding dialdehydes and diamines in the presence of anhydrous Na SO (Fig. 8.3). The isotropic transition temperatures of polymers 9 and 10 were obtained at 72.5 and 68.4°C, respectively. Polymer 11 was not a film or solid but viscous oil with an isotropic phase at less than 40°C. In contrast, polymer 12 showed the highest isotropic transition temperature at about 160°C due to the existence of... 

Note Mesophase X should be stable at lower temperatures than phase Y. For example, the wewai/c-isotropic transition temperature would be denoted as Tni. 

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). 

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... 

The degree of orientational order in a uniaxial nematic is given by the order parameter S, defined by Eq. (2-3). S is zero in the isotropic state, and it approaches unity for hypothetically perfect molecular alignment (i.e., all molecules pointing in the same direction). In single-component small-molecule nematics, such as MBBA, S varies with temperature from 5 0.3 at Tni, the nematic-isotropic transition temperature, to S 0.7 or so at lower... 

Change in selectivity from H2O selective to ethanol selective at the nematic-isotropic transition temperature... 

Like ebulliometry, the cryoscopic method is also limited to relatively low-molecular-weight polymers with up to 50,000. A newer variation of this method involves the solute-induced depression of the nematic-isotropic transition temperature for a liquid crystal solvent such as p-azoxyanisole [2]. This technique appears to be particularly convenient for high-molecular-weight polymers and may be applicable for M values as high as 10 . 

S is a function of temperature. The higher the LC to isotropic transition temperature, the higher the degree of order at room temperature. 

Fig. 2. The dependence of the melting points (O) and the nematic-isotropic transition temperatures ( ) on the nmnher of methylene units in the flexible spacer, n, for the BCBOn series [18]. Also shown are the nematic-isotropic transition temperatures ( ) and the smectic A-isotropic transition temperatures ( ) for the nOCB series...

Thus on varying the length and parity of the flexible spacer in dimers the nematic-isotropic transition temperature exhibits a dramatic odd-even effect which attenuates as the length of the spacer is increased while the nematic-isotropic entropy also exhibits the same pronounced alternation but which appears not to attenuate as the spacer length is increased. Such behaviour is also observed for semi-flexible main chain liquid crystal polymers for example, Figs. 5 and 6 show the dependence of and ASni/R, respectively, on the length of the flexible spacer for the poly a,co-[4,4 -(2,2 -dimethylazoxy-phenyl)]alkandioates [9],... 

Fig. 5. The dependence of the nematic-isotropic transition temperature, T i, on the length of the flexible spacer, n, for the poly a,(u-[4,4 -(2,2 -dimethylazoxyphenyl)]alkandioates [9]...

Quite different behaviour is observed if we now compare the ether and carbonate linked series. The nematic-isotropic transition temperatures for the carbonates decrease essentially without alternation as the spacer length is increased while the entropies show a much reduced odd-even effect when compared to the ether linked series [16, 53]. It should be noted, however, that the nematic-isotropic entropies for the carbonate series are still several times larger than those typically observed for conventional low molar mass mesogens. Similar weak odd-even effects in the transitional properties on varying the length of the spacer have also been reported for other dimer series... 

Figure 11 shows the dependence of the transition temperatures on the number of methylene units in the spacer, n, for the CBOnO.lO series [69]. Immediately apparent is that the clearing temperatures show a dramatic alternation which attenuates quite rapidly on increasing n. A dramatic alternation is also exhibited by the nematic-isotropic entropies but which is not attenuated on increasing n (see Fig. 12). As we noted earlier this dependence of the nematic-isotropic transition temperatures and associated entropy changes on varying the length and parity of the flexible spacer is...

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