Nematic phase mesogenic groups

In addition to their unusual rheological properties, the nematic phases of polymers, like those of simple compounds, can be oriented by the application of magnetic or electrical fields. These properties have been more fully examined for comb-type polymers with mesogenic side-chains than for polymers with the mesogenic groups in the main chain, since in the comb polymers it is possible to influence the side-chain orientation independently of the main-chain orientation. 

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... 

Table 3. Phase behavior of polymers having the same mesogenic group linked to the different main chains (AT = extent of the nematic phase)...

The only general statement that might be made is that the specific features of the structure of polymeric nematics are provided for by the ordering effect of the main chain on the packing of mesogenic groups in LC phase. 

The fusion of LC phases above Tcl causes a sharp change in the character of flow and the values of Ef for nematic and smectic polymers become closer. In an isotropic phase Ef for a polymeric smectic ( 140 kJ/mole) is only twice as large as Ef for a polymeric nematic (70—80 kJ/mole). In other words, the transition from LC phase to isotropic melt, accompanied by the liberation of mesogenic groups from the mesophase levels the differences in the character of flow of smectic and nematic polymers. The differences in Ef for isotropic phase are determined only by the differences in chemical nature of the main chain of smectic and nematic polymers. The values of Ef, in this case, are close to the Ef values for poly(butylmethacrylate) and poly(butylacrylate), respectively, which are structurally similar to polymers XI and XII except that they do not contain mesogenic groups. 

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