Transition smectogens

Detailed x-ray diffraction studies on polar liquid crystals have demonstrated tire existence of multiple smectic A and smectic C phases [M, 15 and 16]. The first evidence for a smectic A-smectic A phase transition was provided by tire optical microscopy observations of Sigaud etal [17] on binary mixtures of two smectogens. Different stmctures exist due to tire competing effects of dipolar interactions (which can lead to alternating head-tail or interdigitated stmctures) and steric effects (which lead to a layer period equal to tire molecular lengtli). These... 

FIGURE 1.4. Some examples of smectogens 4-n-pentylbenzene-thio-4 -n- decyl-oxybenzoate (10S5), p, p -dihetyloxyazoxybenzene (HOAB), and 4-n-pentyloxy-benzylidene-4 -n-heptylaniline (50.7). Transition temperatures are in °C. 

Figure 5.3 Examples of smectogens, together with phase transition temperatures...

The measurement of elastic constants is of particular interest near the nematic-smee-tic A (Tn-sitia) transition. The bend and twist elastic constants contain terms that diverge at TiM.smA according to a power law in reduced temperature [329] KjjCritical exponents have been measured for the twist [77-79, 81-87, 171] and bend [77-79,81,82,85,88-91,93-97,168,171, 172] constants. X-ray diffraction measurements made by Bradshaw et al. [98] indicate a correlation between short-range smecticlike order and the TTjg/A i, ratio, even for non-smectogenic substances. 

Copolymerization of nematogenic and smectogenic monomers is one method of obtaining nematic polymers in this case, it is possible to cause the transition from smectic polymers to copolymers of the nematic type by varying the ratio of the monomeric units in the copolymer [14,40]. 

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