Ester linkages, nematics

The behavior of Polymers b3 and b4 of Table 2 is rather interesting. Both are of the same structure, the only difference being that in Polymer 3 the central ester linkage is placed in the chain in an alternating fashion. This small regularity in structure causes a loss of the mesophase while the compositionally identical polymer with a random placement of the ester link has a monotropic nematic phase. 

Esters are very common liquid crystal compounds, and lateral fluoro substitution has provided some interesting modifications to melting points, transition temperatures, mesophase morpholo and physical properties. Compounds such as 66 [26] allow for the gen ation of high positive dielectric anisotropy, but the fluoro substituent has caused a large reduction in the nematic phase stability when compared with compound 20, due to the reduction in antiparallel correlations. However, the fluoro substituent in compound 67 [26] is somewhat shielded by the zig-zag nature of the ester linkage hence the nematic-isotropic transition temperature is identical to that of compound 20. The lateral fluoro substituent in compound 68 [51] is not as shielded by the zig-zag structure and the nematic-isotropic transition temperature is much reduced, but not to the same extent as for compound 66. 

In the ordered smectic or nematic phase, the rigid rods are arranged in parallel arrays that allow for close packing. The nematic phase is the most common type found with synthetic polymer molecules. The molecules long axes are parallel, but there is no layering. Aromatic polymer chains that have stiff ester or amide linkages are ideal. 

In 1994 Kricheldorf reviewed the field and showed that it was difficult to obtain LC-polyimides free of ester groups. Only three classes were then cited, with ether or amide supplementary linkages, which contained either aliphatic spacers in the main chain with a high tendency to form layer structures in the solid state, or a fully aromatic chain sufficiently substituted to stabilise the formation of nematic phases [41]. 

Cycloaliphatic LC PEAs based on commercially poly(l,4-cyclohexanedimelhylene terephthalate) (PCX) have also been synthesized with two cycloaliphatic diamines and a linear counterpart (1,6-hexamethylenediamine) (Figure 8.12) [55]. The compositions of the ester/amide units in the copolymers were varied up to 50% by the adjustment of the amounts of the diol and diamine in the feed. The introduction of amide linkages was found to induce nematic LC properties into the polyester backbone, which in turn increased the polymer chain alignment. Interestingly, the introduction of nematic LC phases into PCT was only possible when a low ratio of amide units (i.e., less than 25 mol%) was incorporated into the polymer backbone. 

A vast number of liquid crystals showing nematic or other mesophases are obtained using benzene, biphenyl or terphenyl units as core systems, often with the rings connected by polarizable linkages such as ester or alkynyl functions or direct-... 

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