Smectic phase nematic mesophase compared

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. 

The effect of the diol groups at the end of the alkyl chains has been compared for compounds 75 and 76 [160], Compound 75 has a structure that incorporates diol moieties into its parent compound 76, Compound 76 melts into a nematic phase up to 79 °C, while compound 75 shows smectic phases to 165 °C, The mesophase is thermally stabilized at 86 °C by the incorporation of the diol groups, which form H-bonds,... 

Many other structural possibilities for the terminal chains can have a great influence on melting point, transition temperatures and mesophase morphology. For example, branched chains are common, usually for the purpose of generating a chiral centre for chiral nematic or chiral smectic C liquid ciystals. The effect of the branch is to broaden the molecules and hence the transition temperatures are usually depressed significantly, often with the largest reduction in the smectic phase stability, but melting points are often not so much affected (compare compounds 29 and 30) [32]. 

Using nitromethane-trispropanol, pentaerythritol or dipentaerythritol as the central unit and typical rod-like units with no spacers, Wilson [240] produced three-, four-and six-armed star-like molecules that showed smectic (mostly type A) and nematic phases. In order to explain the existence of the mesophases, Wilson compared the molecules to small sections of side-group polymers, where the flexibility of the central unit is emphasized. 

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