Structure 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). 

The situation looks more complicated for polymers with mesogenic groups in the side chain, but in fact it is rather simple too. All these polymers can be represented by a general structural formula structure I in Fig. 6.17. This structure can immediately be derived from the general structure of comb-polymers (structure II in Fig. 6.17) the SCLCP is a comb-polymer with an... 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

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. 

Fig. 8 Polymer backbone structures of SCLCPs based on cyanobiphenyl mesogens...

Beside classical SCLCPs, attaching dendritic side chains to poly(norborn-enes) and poly(7-oxanorbornenes) leads to highly-ordered columnar mesophases (Sect. 2.5). In these polymers, the dendritic side chains force the polymer to adopt a rod-like structure. 

The elementary synthetic and structural principles of SCLCPs are now being elucidated using living polymerizations in which the effect of a single structural feature can be isolated, while other structural variables remain constant. In particular, living polymerizations are used to determine the molecular weight at which the thermotropic be-... 

Although many controlled cationic polymerizations that have been developed [120], only mesogenic vinyl ethers have been used in an attempt to prepare well-defined SCLCPs by a cationic addition mechanism [121]. Nevertheless, these polymerizations (and copolymerizations) provide the most complete series of SCLCPs with the widest range of structural variables. As shown in Eq. (22), most of these monomers, includ-... 

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... 

One of the most fundamental questions still open regarding the structure/property relationships of SCLCPs is the effect of polydispersity. In some cases, the temperature and nature of the mesophase(s) depend not... 

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. 

Chemical modification of the polymer structure allows the obtention of nematic and smectic phases [4, 5]. If the side group and/or the chain are chiral, then cholesteric or chiral smectic C (SmC) phases can be obtained. These can also be obtained by mixing a chiral compound with the SCLCP. SmC SCLCPs are of particular interest and their behavior is described in Sec. 2 of this Chapter. 

In addition to this SCLCP host, MCLCPs have also been used. Li et al. [ 168] dissolved molecules of an oligomeric fragment of po-ly(p-phenylenebenzobisthiazole) 10, which was terminated by opposing nitro and di-methylamino groups, in the polymer itself. The resultant material has a value of 6x10 esu. Stupp et al. [169, 170] carried out a careful analysis of the system of Disperse Red 1 (11) dissolved in a chemically aperiodic nematic polymer containing the following three structural units (12). 

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 ... 

In order to achieve amorphous polar solids, some research groups have explored ferroelectric SCLCPs, and have shown that the basic rules governing the relationships between molecular structure and macroscopic ferroelectric LC properties are the same [225-228]. The main difference between low molar mass and polymer ferroelectric LCs, however, is the existence of stable glassy phases in most of the latter. 

The techniques used to obtain the untwisted SmC phase structure in low molar mass LCs and SCLCPs are limited to thin layers. In contrast to this, LC elastomers can be macroscopically uniformly oriented by mechanical deformations [230], and this orientation process is not limited to thin samples or suitable dielectric anisotropy of the material. Furthermore, for LC elastomers the oriented structure can be chemically locked in by crosslinking, resulting in the so-called liquid single crystal elastomers [231],... 

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,... 

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