Thermotropic melt

In their original theory, Maier and Saupe supposed that the molecular interactions responsible for the nematic state are anisotropic van der Waals interactions (discussed in Section 2.3), in which case mms should be temperature-independent. However, it is now recognized that shape anisotropy is also important, even for small-molecule thermotropic nematics. By making mms temperature-dependent, the Maier-Saupe potential can, in principle, accommodate both energetic and entropic effects. In fact, if the function sin(u, u) in the purely entropic Onsager potential Eq. (2-5) is approximated by the expansion 1 — V2 cos (u, u)+. . ., then to lowest order the Maier-Saupe potential (2-7) is obtained with C/ms — Uo bT/S, where we have defined the dimensionless Maier-Saupe energy constant by Uus = ums/ksT, Thus, the Maier-Saupe potential can be used as an approximation to describe orientational order in either lyotropic (solvent-based) or thermotropic nematics. For a thermotropic melt, the Maier-Saupe theory predicts a first-order transition from the isotropic to the nematic phase when mms/ bT = U s — t i.MS = 4.55, and at this transition the scalar order parameter S jumps from zero to 0.43. S increases toward unity with further increases in Uus- The spinodal point at which the isotropic phase is unstable to even small orientational perturbations occurs atU — = 5 for the Maier-... 

Liquid crystalline polymers have been discussed in many texts and review papers [65, 400-413] during the last decade, in which the synthesis, processing, morphology, orientation and structure-property relations are described. The major applications of these materials have been as high modulus fibers and films, with unique properties due to the formation of ordered lyotropic solutions or thermotropic melts which transform easily into highly oriented, extended chain structures in the solid state. 

Fig. 5.85 Thick and thin regions of a thermotropic melt structure in polarized light. In a thick region (A) the fine structure is not too clear but the onset of decomposition is shown by the round bubbles. A thinner region (B) shows thread-like detail and a nematic texture with four brushes.

Notes Theoretical in bulk, I Isotropic solution, A LC solution, T Thermotropic melt. 

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