Transition substituted mesogens

Wittmann JC, Lotz B (1985) Polymer decoration the orientation of polymer folds as revealed by the crystallization of polymer vapors. J Polym Sci, Polym Phys Ed 23 205-211 Wunderlich B, Grebo wig J (1984) Thermotropic mesophases and mesophase transitions of linear, flexible macromolecules. Adv Polym Sci 60/61 1-59 Zhou QF, Li HM, Feng XD (1987) Synthesis of liquid-crystalline polyacrylates with laterally substituted mesogens. Macromolecules 20 233-234... 

The synthesis and some thermal properties of three series of block copolymers comprising both main-chain and side-chain liquid-crystalline (LC) blocks in the same macromolecular structure are described. The former block is a semiflexible LC polyester (block B), and the latter is an LC polymethacrylate (block A) containing a variously substituted mesogenic unit. The two structurally different blocks were partly phase-separated within the glassy and LC states and underwent distinct phase transitions. Significant deviations of the transition enthalpies relative to those of the corresponding homopolymers suggest the occurrence of a more or less diffuse interphase which may depend on the nature of the mesophase formed. 

Lastly, it was demonstrated with PPO substituted with a series of alkyl side-chains as we have here, that the glass transition temperature decreases with an increase in the side-chain length (28). At the same time, the Tg s of the more flexible side-chain liquid crystalline polymers investigated to date are always much higher than those of the corresponding polymers without the mesogenic side-chains (3). Therefore, it is quite likely that we may obtain side-chain liquid crystalline polymers of approximately the same Tg from PPO and PECH. 

Comparison ot Tables V and VI demonstrates that the thermal behavior of MelOCOO-PPO and MelOCOO-PECH are very similar, with the glass transition temperatures converging with substitution. Therefore, it appears that when very long spacers are used with the same mesogen, the polymer backbone has little effect on the thermotropic phases formed. This conclusion is supported by additional unpublished experiments performed in our laboratory. 

For polymers which, on heating, yield Mesophases (liquid crystal melts), the so-called mesogenic polymers or liquid crystal polymers (LCPs), the situation of phase transitions is much more complex. In this case the simple Volume-Temperature diagram, given in Fig. 4.2 is not valid anymore and has to be substituted by a more complicated one, which is shown in Fig. 6.12. 

We see in these examples that great effect can be created by substitutions on the two ends of the mesogenic cores. The consequence includes not only the formability of liquid crystal phases and transition temperatures, but also the types of the liquid crystalline phase. It is well known that substitution on the end with longer linear alkyls and alkoxyls favor the formation of smectic phases. One example for this remark has been given in Table 3.3. In the design of liquid crystal polymers, especially in the cases of side-group type and molecular shish-kebabs, the concept of end-substitution has been fully utilized. 

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