Effects of Structure on Mesophase Thermal Stability

Of the transitions observed upon melting a liquid crystalline material, the melting point is least correlated with molecular structure. This is so because the thermal stability of the solid state is principally influenced by attractive forces in the crystal lattice. Since these short-range associations are only secondarily related to molecular structure, the melting point is not usually influenced in an obvious manner by molecular structure. 

The forces responsible for the mesophase, however, are primarily dipole-dipole (both permanent and induced) and dispersion forces, which, since the molecules in the mesophase or melt are free to rotate, 

For a mesophase to obtain, it is generally necessary that the terminal groups of the molecules contain permanent dipoles. For example, the N-I transition of 4-octyloxybenzoic acid is some 40° higher than that of 4-nonylbenzoic acid despite the essentially equivalent size of the two molecules. Similarly, the 4-alkoxy-4 -cyanobiphenyls form substantially more stable mesophases than 4-alkyl-4 -cyanobiphenyls. Polarity of the termini, however, frequently gives rise to very strong intermolecular attractions. In such cases, the melting points of the compound may be raised so high that the mesophase cannot survive. 

For example, the end-group polarity of 6-hydroxy-2-naphthoic acid (C—1=250°) is certainly greater than that of 6-methoxy-2-naphthoic acid (C—N=206° N—1=219°). In this case, hydrogen bonding is thought to be responsible for the nonmesogenic property of the hydroxy acid. 

The polarity and polarizability of the central groups of mesogenic compounds are more clearly correlated with thermal stability. Thus, 4-alkoxy-4 -biphenyl carboxylic acids, form more stable mesophases than corresponding 4-alkoxybenzoic acids. Similarly, esters of cholesterol form generally more thermally stable mesophases than esters of cholestanol. These differ only by the presence of a double bond, as shown in Fig. 5. Again, certain conjugated unsaturated aliphatic carboxylic acids show liquid crystalline phases, while normal aliphatic 

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