Molecular Engineering of Liquid Crystalline Polymers

Actual molecules always have certain flexible elements and are not as rigid as a true rod. Taking the rod-like poly(l,4-phenylene) as one example, 

438 °C, Ti = 565 °C. Table 3.1). Thus, PHA can have a higher degree of polymerization than poly(l,4-phenylene) before the transition temperatures become infinite. However, studies on PHA have shown that its melting point is already higher than its decomposition temperature if the degree of polymerization reaches only such a low value as 13 (Table 3.2). 

It is worthwhile pointing out that the critical axial ratio 6.4 given by Flory s theory is based on assumptions that include zero free volume and zero net interaction energy between the rods (Chapter 2). By increasing the free volume, for example, the ratio will be increased accordingly. It is thus also understandable that the critical axial ratio will become larger when the temperature is increased. On the other hand, no actual system will meet these assumptions to any perfection. In addition, the flexibility of an actual molecule such as poly(l,4-phenylene) and PHA will increase with increasing temperature. In other words, the axial ratio of a molecule is not a constant, 

Unfortunately, because of the very low mixing entropy, not many of the rigid rod-like high polymers have sufficient solubility and form solvates. For example, among many others, poly(l,4-phenylene) and 

Side-Group Type with Side-on Attachment (Mesogen-Jacketed Liquid Crystal Polymers) 

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Zhou and Lenz (1983) and Zhou et al. (1985) discussed the substitution effect from the point of view of steric and polar effects. However, from above discussions one sees a more intricate picture. A full understanding of the effect demands further study. Nevertheless, substitution is so effective that a lot of studies have used this concept in the molecular engineering of liquid crystalline polymers in order to have desired phase properties. 

MOLECULAR ENGINEERING OF LIQUID CRYSTALLINE POLYMERS BY LIVING POLYMERIZATION... 

V. Percec and C. Pugh, in "Side Chain Liquid Crystal Polymers", ed. C. B. McArdle, Chapman and Hall, New York, (1989), p. 30 V. Percec and D. Tomazos, Molecular Engineering of Liquid Crystalline Polymers, in "Comprehensive Polymer Science", Supplement 1, Sir G. Allen and J. C. Bevington Eds., Pergamon Press, Oxford, (1992), in press... 

Percec Virgil, and Tomazos Dimitris. Molecular engineering of liquid crystalline polymers. In Comprehensive polymer science, eds. Allen G, Bevington J.C., pp. 1-53. Oxford Pergamon Press, 1992. 

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