Flexible spacers, characteristics

Jackson and co-workers reported in 1976 [21,22] (on work carried out in 1971) that poly-p-oxybenzoyl-modified poly(ethylene terephthalate) was found to exhibit thermotropic liquid crystalline characteristics. A variety of thermotropic liquid crystalline polyarylates have since been described, and some of these also contain flexible spacer units [23-30]. 

More recent studies on the effect of flexible spacers now include the use of either poly(ethylene oxide) or polysiloxane segments. Each such spacer brings with it certain unique characteristics to the final polymer, and it is clear that the role of the spacer... 

Both thermotropic and lyotropic liquid crystal polymers exhibit characteristic features with regard to their microstructureJ Anisometrical monomers such as rods or discs are connected to chains in an appropiate manner. These anisometrical monomers are considered to be the mesogens and may be part of main chain LCP, side chain LCP, or of both types together (Fig. 6). Between the mesogens are located flexible spacers of non-mesogenic character. Sufficient flexibility is a prerequisite for liquid crystal formation, with an increase in either temperature or solvent concentration. 

Ratios such as AvilAvg and AvtlAvj i j) are free from the orientational order of the molecular axis Szz and solely depend on the spacer conformation [Eqs. (7.3), (7.6) and (7.7)]. From an analysis of the spectra, these ratios were found to remain nearly invariant with temperature. These observations support the idea that the nematic state is populated with an ensemble of nematic conformers. Since the flexible spacer maintains liquidlike characteristics in this state, the equilibration among the conformers should be affected by the temperature, leading to a small variation in these ratios when the data are collected over a wider temperature range. 

A satisfactory account of the steric interaction characteristic of the dense nematic phase such as those treated here is not yet available. The treatment is largely hampered by the occurrence of the nematic conformation of the flexible spacer in the ordered state [Flory, 1989 Yoon andHory, 1989 Abe and Ballauff, 1991]. 

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