Compounds with highly polar end groups

Let us consider smectic A phases in which the molecules form a regular stack of infinite 2D liquid-like ordered layers with their director perpendicular to the layer the presence of longitudinal dipoles raises the question of their orientation within each layer. Over the past few years five Sa modifications have been defined which involve different overlapping structures of the molecules e.g. A (monolayer), A2 (bilayer), A (interdigitated bilayer) and A and A (antiphase and crenelated phase, in-plane modulated structuring) (Fig. 2.12). In the A phase the dipoles are randomly oriented in each layer, the period is then equal to the molecular length. In the Sa phase, the dipoles are all in the same 

Normally in phase transitions the higher-temperature phase is less well ordered than the lower-temperature phase. This is not a law of thermodynamics, however, and it has been found that certain cyano- 

The lower-temperature nematic phase was referred to as re-entrant nematic phase (Nr). Examples of more complex re-entrant behaviour are now known for pure compounds.  

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