Spacers SCLCP

All Liquid Crystal Polymers are characterised by the fact that they contain stiff meso-genic groups, often inserted in flexible chain systems (so called "spacers") and connected to them by linking functional groups the mesogenic unit is inserted either in the main chain or in the side chains or (in exceptional cases) in both. We shall discuss MCLCPs and SCLCPs. A schematic representation of common structures of LCPs is displayed in Fig. 6.14 (Jansen, 1996). An example of a SCLCP with disc-like mesogens is displayed in Fig. 6.15 (Franse et al., 2002, 2004). 

SCLCPs combine liquid crystalline properties and polymeric behavior in one material. If the mesogenic unit is fixed directly to the polymer main chain, the motion of the liquid crystalline side chain is coupled with the motion of the polymer backbone, preventing the formation of a LC mesophase. Therefore, Finkelmann and Ringsdorf proposed that the introduction of a flexible spacer between the main chain and the mesogenic unit would decouple their motions, allowing the mesogenic moiety to build up an orientational order [29,30]. 

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. 

The extensive literature on low molar mass liquid crystals demonstrates that specific mesogens (specific chemical structures) tend to form specific mesophases, which vary somewhat with the length of the flexible substituent. We therefore expect that the type of mesogen, including the terminal substituent(s) and the length of the spacer should be the primary factors determining the specific mesophase(s) exhibited by a given SCLCP. The nature of the polymer... 

Increasing the spacer length has much the same effect on the thermotropic behavior of SCLCPs as increasing the length of the flexible substituent has on that of low molar mass liquid crystals. That is, it destabilizes some phases and stabilizes others. For example, just as increasing the length of the flexible substituent depresses the melting... 

The temperature of crystalline melting of tactic SCLCPs also decreases as the spacer length increases (Fig. 15), albeit in an odd-even alternation as in low molar mass liquid crystals as a function of the length of the... 

Many reviews [1-3] of SCLCPs describe the structure of the backbone (main chain), the spacer (flexible linkage), and the side group (mesogenic unit) of the SCLCP. For example, the most widely used backbones include polyacrylates or polymethacrylates, polysiloxanes, and polyphosphazenes po-ly-ct-chloroacrylates, itaconates, and ethylene oxides have also been reported. 

The backbones that have been most commonly employed are those of the acrylate [177, 190, 194, 196], methacrylate [152, 171,196,198-200], and siloxane [152,177, 197] types. Polyethers [207-209], polyesters [182,191, 192], and polystyrenes [177, 189, 195] have also been reported. Typical spacer groups consist of between 3 and 12 methylene units. The phase transitions of a number of SCLCPs containing NLO meso-genic groups are collected in Table 19. Unfortunately, the molecular masses of many of these polymers have not been determined, and the influence of the polymer structure on the phase transitions can not therefore be quantitatively discussed. However, the general points to emerge from these data are as follows ... 

Side chain liquid crystal polymers (SCLCPs) consist of mesogenic structural moieties appended from a polymer backbone (Figure 5.2). The mesogenic units that have been used parallel those previously used for low molar mass liquid crystals, and the stmctural nature of the polymer backbone is widely variable. The mesogenic units (usually calamitic but many discotic types exist) are invariably separated from the polymer backbone by fairly long spacer units which are usually several methylene (-CH -) units,... 

The reasons for employing a flexible spacer that links the mesogenic nnit to the polymer backbone are discussed above. The influence of the flexible spacer that is normally essential for the generation of mesophases in SCLCP is of great interest Figure 5.5 shows some examples of simple flexible spacers where the actual size of the spacer may be smaller or larger. 

The resemblance in Fignre 5.6 to the mesogenic stractures of low molar mass Uquid ciystals is obvions however, mesogenic nnits in SCLCPs can also be attached to the spacer at a lateral position of the calamitic unit or be of the discotic type. 

However, there is an enormous number of possible polymer backbones that can be used for generating SCLCPs, but only a small selection have been widely used in the synthesis and evaluation of SCLCPs. In general, poly(siloxanes) and poly(aciylates) constitute the vast majority of SCLCPs. The reported role of the spacer group is to decouple the effects of the backbone from those of the mesogenic side chains. Accordingly, the backbone should not influence the mesomorphic properties of SCLCPs. However, decoupling is... 

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