Entropy isotropic fluids

Consider an isotropic fluid in which viscous phenomena are neglected. Concentrations and temperature are non-unifonn in this system. The rate of entropy production may be written... 

Referring to the plot of G vs. T for the isotropic and nematic fluids, a uni-molecular isomerization process would show the same characteristics if one of the isomers had a higher entropy of formation than the other. In the molecular case, however, the equilibrium constant at a given temperature would derive from the free energies on a per molecule basis, while for the phases this free energy is per collective volume of molecules. The similarities to a molecular isomerization, however, are more important than the differences for the purposes of this discussion. The transition from isotropic fluid to nematic LC can be considered a temperature-driven, or thermotropic, isomerization. 

The conformational analyses of mainchain LCs have been reported from various laboratories. Although the results seem to vary somewhat depending on the models adopted in the treatment of experimental data, all suggest that flexible spacers prefer to take extended conformations in the nematic state. Efforts to formulate molecular theories to describe the N1 transition characteristics of the mainchain LCs in terms of the molecular parameters have also been reported [7,43,44]. In an ideal crystalline state, molecules are aligned in a perfect order, often only the most preferred conformation being permitted for the spacer [45]. The structural characteristics of chain molecules in the crystalline, liquid crystalline, and isotropic fluid states must manifest themselves in the conformational entropy of the system upon phase transitions. As the DP of the mainchain LC sample increases, however, the degree of crystallinity tends to be lower, and accordingly the CN transition becomes less sharp [11]. 

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