Esters substituted mesogens

The ester models, as stated above, were all LC, but two of the ether model compounds, the methoxy- and ethoxy-substituted derivatives, were not, despite the fact that each of these mesogens yielded some liquid-crystallinity in the poly(ether) form. Therefore it seems that the polymers in this system tend to be "more liquid crystalline" than the related small molecules. This hypothesis is supported by the fact that Memeger(H) found liquid crystallinity in allhydrocarbon polymers incorporating the distyrylbenzene mesogen, even in cases where the cis/trans ratio of the unsaturations was as large as 0.3, while Campbell and McDonald (10) noted that iodine isomerization to the all-trans form was essential for the observation of an LC phase in the small-molecule derivatives which they prepared. 

In series 7 both substituents form a mesophase (MI>10) when the hydroqui-none is copolymerised with N-(4-carboxyphenyl) trimellitimide (Table 25). However, when the substituted hydroquinone in polymer 6 is replaced by aliphatic diols in polymers 8 und 9, then isotropic melt properties are obtained (MI=8). 4-Carboxy phenyletherphthalimide is a poor mesogen and relies on a predominance of ester mesogen in the PEI to achieve a mesophase. By comparison, N-4-carboxyphenyl) trimellitimide is a better mesogen. In PEIs with phe-nylhydroquinone and isophthalic acid (Table 25), it has a lower critical MI>9.5. 

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 ... 

Since the cyanobiphenyl derivatives had proved to be excellent materials for electro-optic displays, equivalent homopolymer poly-siloxane liquid crystals, based on these low molar mass mesogens, were synthesized. Their properties are summarized in Table 1. Since the homopolymers only exhibited smectic phases, copolymers were prepared using destabilizing 2-methyl-substituted ester side-groups. As shown in Table 2, at sufficiently high ester-group concentrations, a... 

So how do these common, useful polymers relate to liquid ciystal polymers (LCPs) Well if the alkene monomer (1) is substituted by a moiety conducive to the generation of liquid crystal phases, then the resulting polymer may exhibit liquid crystalline phases (see below). Similarly, if the methyl ester unit in monomer 3 is replaced by a long, rod-like moiety similar to the liquid crystal stractures shown in Chapter 3, then on polymerisation a liquid crystalline methacrylate polymer may result. In each case the mesogenic units are attached to the side of the polymer backbone and such systems are denoted side chain polymers. If either or both of the bifunctional units involved in a condensation polymerisation e.g., 8) are of a mesogenic nature e.g., a biphenyl or terphenyl unit), then a liquid crystalline polymer may result. In this case the mesogenic units are a part of the backbone stmcture and such systems are denoted main chain polymers. 

Other examples of functionalized monomers are polyoxyethylene (POE) chains linked to the thiirane with monothioacetal or ester functions (macromonomers), POE chains containing a thiirane group at both chain ends (bis-macromonomer), and stilbene mesogen and other side chain liquid crystalline-substituted thiiranes. ... 

Fig. 2.1. Typical molecular structures, (a) Rod-like mesogens (i) p-azoxyanisole (ii) p -n-hexyloxybenzylidene-p-n-hexyl oxyaniline. (b) Discshaped mesogens (i) hexa-substituted esters of benzene (ii) hexa-substituted esters or ethers of triphenylene " (iii) hexa-n-alkanoates of truxene. (c) Phasm-like mesogens bis-(3,4,5-trialkoxybenzoyloxy)4 -phenyl terephthalates. (d) Pyramid-like mesogens (i) hexa-substituted tribenzocyclononanef (ii) octa-substituted tetrabenzocyclododecatetraene. ...

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