Chemical Structure and Transition Temperatures

The current state of the molecular-statistical theory of liquid crystals does not yet permit the reliable prediction of transition temperatures between 

The influence of terminal substituents on the thermal stability of the nematic phase has been studied, in particular, for compounds of the type 

From Table 1.3 it can be seen that there is a definite correlation between the increase in polarizability of the substituent (Aa) and the increase in Tni- Such a dependence would, generally speaking, be expected from the Maier-Saupe molecular-statistical theory if the role of steric factors were not allowed for. In the case of reasonably bulky substituents these factors prove to be significant, and, in particular, they can explain the fall-out points for CH3O and C2H5O groups. This correlation is also confirmed for compounds of the form (l.xx), where X, Y = —COO—, —OCO, —N=N—, or a simple bond [63]. 

In general, the thermal stability Tni of the nematic phase of (l.xix) decreases in the series of bridge groups X [60]  

As to the lateral L groups in (l.xix), even small ones usually perturb the structure of a mesophase and cause a significant depression in the clearing point. Moreover, they have a different effect on the thermal stability of the nematic and smectic mesophases. Smectic phases are much more infiuenced because lateral substituents strongly prevent side-by-side packing of rigid molecular cores favorable for the smectic ordering. 

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Table I. Chemical Structure and Transition Temperatures of the polymers...

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