Mesogens, density

A double mesogen density is easily achieved by using disubstituted norbornene monomers of type IV-n and V-n (Fig. 4). 

In this chapter we want to discuss the correlation of the mesophase behavior of a cyanobiphenyl-based SCLCP with its backbone structure. As shown before, the backbone structure, the spacer lengths, and the mesogen density per repeat unit have great influence on the LC mesophase evolved. Ligure 8 shows some examples of backbone structures bearing the cyanobiphenyl-moiety that have been reported in literature. The above-mentioned ROMP-derived polymers poly-(II-n) [39],poly-(IV-n) [42,47],poly-(VI-n) [41],andpoly-(VII-n) [53] will be compared with each other and with acrylate-based [56-59], siloxane-based [60] and vinylcyclopropane-based systems [61]. The detected mesophases and their transition temperatures are summarized in Table 6. 

Lor polymers in which the mesogen is separated by a spacer of six methylenes units from the polymer backbone, it is obvious that the more rigid poly(norbornene)s favor nematic liquid crystalline phases. Poly-(VI-6) with the rigid and bulky 2,5-dimethine oxacyclopentane-3,4-dicarboximide unit in the main chain does not show liquid crystalline behavior (Table 6, entry 12). The more flexible poly-(II-6) backbone allowed the formation of a nematic mesophase. If the mesogen density was increased, as realized in poly-(IV-6), the isotropization temperature was found to be 26 °C higher than that for poly-... 

For a spacer length of 11 methylene units, all polymers showed smectic mesophases except for poly-(II-ll) and poly-(VII). Even the flexible poly-(cy-clooctene) main chain prevented a smectic mesophase. Compared to all of the other polymer architectures, poly-(II-ll) and poly-VII present the lowest ratios of mesogens to atoms in the main chain. It can therefore be assumed that smectic phases will only be formed when there is sufficient mesogen density. For the norbornene chain, it is notable that a high Z/E-ratio and a high tactic-ity increased the stability of the smectic A phase. 

A great number of initiators and monomers are now available, allowing almost perfect control over most of the important parameters of LCPs main chain stiffness tacticity glass transition temperatures processabiUty from solution or melt mesogen density along the main chain combination with... 

Further comparisons to appropriate model compounds and the effect of increased mesogen density are discussed in the next sections. 

It is straightforward to measure the birefringence as function of temperature [8] as well as strain [9]. One experimental consideration to take into account is the change in sample thickness due to strain. The response can be readily xmderstood in terms of the simple model described in the Introduction. LCEs have been swollen with both isotropic and anisotropic solvents [8, 10, 11]. Isotropic solvents reduce the number density of mesogens and, hence, the critical temperature = pU/5k. They also reduce the crosslink density and the influence of the network. Nematic solvents, on the other hand, up to first order do not reduce the mesogen density, but... 

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