Smectic phase stability

The cyclohexyldecalin material (compound 37) has a completely saturated core that may be expected to pack well in a lamellar fashion and indeed an SA phase is exhibited. However, the broad molecular structure reduces smectic phase stability and allows a large nematic range to be generated. When this broad core structure comprises saturated and aromatic regions (compound 38), the smectic phase is completely ehminated to give a nematogen of reduced phase stability. 

Compound 46 shows the effect of the polar carbonyl unit in a terminal chain (in fact the carbonyl group can be, and often is, referred to as a linking group between the chain and the core). As can be seen, in comparison with compound 32 the smectic phase stability is much greater for compound 46 despite the steric effect of the carbonyl units which tends to disrupt lamellar attractions. Similarly, the use of a polar branching substituent e.g., F, Cl or CN) in a terminal chain (usually chiral) tends to give smectic phases and will be discussed in Chapter 6. 

The parent material (79) has a high melting point and has a high smectic phase stability. The lateral fluoro substituent dismpts the lamellar packing and reduces the smectic phase stability by far more than the T j, which when combined with the reduced... 

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]. 

However, if the branch unit is polar (e.g. fluoro), then smectic phases are often supported to the exclusion of the nematic phase due to enhanced lateral attractions (e.g. compound 31 [33] compared with compound 15). In fact, superfluOTO and perfluoro chains are voy stiff which fiicilitates excellent lamellar packing and so excq)tionally high smectic phase stability is goi ted (e.g. compound 32) [34]. 

The use of latoal substituents in liquid crystals has proved to be very important, initially in nematic material and lata- in smectic C matoials. Clearly, anything that sticks off the side of a rod-like molecule will tend to reduce the liquid crystal phase stability, and genmlly the larg the lateral substituent the greater the reduction in liquid crystal phase stability. Usually, the smectic phase stability is much mmre affected than that of the nematic phas especially by larger substitumts because of the obvious reduction in lateral attractions, but increased lateral attractions associated with polar substituents cause a smaller reduction in smectic phase stability (see compounds 52-57) [46]. 

Lateral fluoro substitution in an aromatic environment tends to cause large reductions in melting points and/or smectic phase stability to create compounds with wide nematic ranges (compare compound 76 with compound 6) [54]. Much less reduction of smectic character is caused by the corresponding lat l fluoro substitution in an alicyclic environment (compound 77), but the nematic phase is generated [12]. 

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