Calorimetric Measurements in Nematics

Discussed in this section are the effects of fluctuations and of external fields on the phase transition. Experimentally, the NA transition is usually indistinguishable from second order [48] however, for materials with a small nematic range, calorimetric measurements have detected a small latent heat associated with the phase change, which is interpreted as a mean-field, second-order phase transition that is driven first order by the coupling to a second, strongly fluctuating order parameter. The second order parameter is associated with a nearby transition, between the isotropic and nematic phases (IN). 

In the p-alkoxyphenyl esters of cyclohexanedicarboxylic acid, the series was prepared and thermally examined in which the alkoxy group was varied from methoxy through hexyloxy The thermal properties and analytical results for this series are listed in Table 1. The first two members of the series have only nematic mesophases, while the propoxy through the hexyloxy have both nematic and smectic phases As the length of the alkyl tail increases, so does the complexity of the transitions There are four distinct solid polymorphs in the pentyloxy and three in the hexyloxy compound The hexyloxy also has three smectic phases as well as the nematic These three transitions were identified as smectic by microscopy, however no attempt was made to assign them to a particular class The heats of two of these transitions were too small to measure with any degree of accuracy and only the temperatures were recorded Solid polymorphism, such as encountered in these compounds where the transitions occur at very close temperatures, prevents the application of the differential scanning calorimetric method for determination of purity ... 

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