Equilibrium melting temperature from small-molecule data

Figure 6.21 Domain order parameter (S) as a function of reduced temperature (Tj being the isotropization temperature, the temperature at which the nematic melt is in equilibrium with the isotropic melt) for a small-molecule- and a side-chain polymer nematic. Drawn after data from Finkelmann and Rehage (1984).

A detailed study of crystals of macromolecules 20,21) and their melting under equilibrium conditions revealed that the entropy of fusion, ASf, is often about 7-12 J/(K mol) per mobile unit or "bead" (22). This entropy is linked mainly to the conformational disorder (A and mobility that is introduced on fusion. Sufficiently below the melting temperature, disorder and thermal motion in crystals is exclusively vibrational. While vibrations are small-amplitude motions that occur about equilibrium positions, conformational, orientational, and translational motions are of large amplittide. These types of large-amplitude motion can be assessed by their contributions to heat capacity (23), entropy (22), and identified by relaxation times of the nuclear magnetization 24), Orientational and positional entropies of fusion ASQ ent trans importance to describe the fusion of small molecules. They can be deriv from the many data on fusion of the appropriate rigid, small molecules of nonspheiical and spherical shapes [nonspheiical molecules Walden s rule (1908), ASf = AS j ent AStrans 20-60 J/(K mol) and spherical molecules Richards rule 0 97), ASf = trans = 2-14 J/(K mol)].. The contributions of ASQ ent melting of... 

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