Spacers substituted mesogens

With such a varied menu of comb LCP components to choose from, it is important for synthetic chemists to appreciate how decisions about molecular architecture might influence the properties of polymer products. While the spacer, the mesogen and the constitution of the backbone are fixed by preselection of reactants, the average molecular mass, the polydispersity, the degree of substitution along the backbone, and the purity are variable. Systematic syntheses with these latter variables under careful control have been performed only quite recently. We shall review some of the results once we have covered the actual constitution of the backbone. A good deal is known about the effects of variations to the spacer and mesogen components, and the major correlations are listed in Sections 7.3.2 and 7.3.3. 

Properties have been determined for a series of block copolymers based on poly[3,3-bis(ethoxymethyl)oxetane] and poly [3,3-bis(methoxymethyl)oxetane]- (9-tetrahydrofuran. The block copolymers had properties suggestive of a thermoplastic elastomer (308). POX was a good main chain for a weU-developed smectic Hquid crystalline state when cyano- or fluorine-substituted biphenyls were used as mesogenic groups attached through a four-methylene spacer (309,310). Other side-chain Hquid crystalline polyoxetanes were observed with a spacer-separated azo moiety (311) and with laterally attached mesogenic groups (312). 

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 This conclusion is supported by additional unpublished experiments performed in our laboratory. 

Summary Cyclic siloxanes substituted with mesogenic groups, which are connected to the backbone by aliphatic spacers, exhibit liquid crystalline (LC) phases as the classic calamitic liquid crystals. 

FIGURE 7.10 Typical examples of the H-NMR spectra of CBA-w in the nematic LC state and the definition of the molecular axis (see the text) (a) n = 9 (b) n = 10. In the upper diagrams, the quadrupolar splittings (Av ) are assigned to the individual C-D bonds of the spacer. The dipolar ( >hd) quadrupolar splittings (AVo) of the mesogenic core (lower diagrams) are due to the deuteron substituted at the ortho position. (From Abe and Furuya [1988].)... 

Homopolymers. Phenylesters of benzoic acid were chosen as mesogenic groups for the synthesis of suitable monomers. The acryloyl- and the methacryloyl moieties were used as polymerizable groups. The flexible spacer was an alkyl chain or an alkyloxy chain of varying length. Thus a homologous series of p-"(2-methylpropenoyloxyalkyoxy) benzoic acid -p substituted phenyl esters 1 was prepared. The monomers were synthesized by the standard methods as follows ... 

The NMR technique has provided useful information regarding the orientational characteristics of nematic liquid crystals [96]. In many examples, the order parameters of the mesogenic core comprising a linear array of aromatic nuclei have been accurately determined from the observed dipolar (Dhd) and quadrupolar (Av) splittings by using deuterium-substituted samples. An attempt was made to elucidate the nematic conformation of the polymethylene-type spacer involved in dimer compounds, o, ct)-bis(4-cyanobiphenyl-4 -yloxy)alkanes (CBA- , with = 9,10), by the combined use of NMR and RIS analysis. The chemical structures and the phase transition data obtained by differential scanning calorimetry (DSC) are shown in... 

Giomez, M. A., Roman, R, Marco, C., Del Pino, J., Fatou, J. G. (1997). Relaxations in Poly(tetramethylene terephtaloy l-bis-4-oxybenzoate) Effect of Substitution in the Mesogenic Unit and in the Flexible Spacer. Polymer, 38(21), 5307-5311. 

The liquid crystalline side chain polymers investigated have either a polysiloxane (compounds SiCl and SIGN) or a polyacrylic (compound AcCN) polymer chain. These are coupled via flexible spacers with the mesogenic p-substituted benzoic acid phenylester groups. SiCl and SiCN are copolymers with p-methoxy components in excess. 

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