Polymers, liquid crystalline mesogenic units

Several different structural factors influence the properties of the mesophase in these polymers, including dipolar effects, the planarity and rigidity of the mesogenic unit, and its length-to-width ratio among others. These factors are difficult to quantify, either absolutely or relatively, but some idea of their influences can be obtained by comparing the properties of polymers with different mesogenic units when combined with the same flexible spacer. This comparison has already been made for the dyad and triad esters in Table 2, and in this section it will be extended to other types of liquid crystalline polymers which contain a common decamethylene spacer. 

Rigid polymers are not the only class of polymers which can show liquid crystal phases. These phases are also found in the melts of polymers in which mesogenic units are incorporated in the backbone (main chain polymers) or attached as pendants to the backbone (side chain polymers). These liquid crystalline polymers will be discussed in later sections. 

In addition to barotropic liquid crystals of flexible polymers, there are also a few reports on the observation of the liquid crystalline phase of polymers containing no mesogenic units of rigid rods. Aharoni (1988), for example, was very lucky to have found a series of the following polymers that form a thermotropic liquid crystal phase. 

Introduce the chiral center into one end of the side groups and one may obtain a cholesteric liquid crystalline polymer. If both mesogenic units... 

Keith, C. Reddy, R. A. Tschierske, C. The first example of a liquid crystalline side-chain polymer with bentcore mesogenic units ferroelectric switching and spontaneous achiral symmetry breaking in an achiral polymer. Chem. Commun. 2005, 871-873. 

A second example for the synthesis of a functional surface-attached polymer brush is the preparation of monolayers of a liquid-crystalline polymer (TCP) with mesogenic units in the sidechain [43]. Such a system could be of interest for the preparation of alignment layers for liquid-crystal displays (LCD) [44,45]. Alignment layers are key components for the production of LCDs [46-50]. As the orientation of the nematic director of the LC in contact with a surface is energetically degenerate, domains are formed in the LC layer that lower the contrast of the display. Thin films of polymers such as rubbed polyimide film ( alignment layers ) are frequently used to align the liquid-crystalline molecules in order to form extended monodomains. 

Macromolecules dendritic, 285 hyperbranched, 284-286 Main-chain liquid crystalline polymers, 49 Main-chain polyester LCP mesogenic units, 50... 

Poly(epichlorohydrin) (PECH) and poly(2,6 - dimethyl-1,4-phenylene oxide) (PPO) containing pendant mesogenic units separated form the main chain through spacers of zero to ten methylene units were synthesized and characterized in order to test the "spacer concept." Both polymers were modified by phase transfer catalyzed esterifications of the chloromethyl groups (PECH) or the bromobenzyl groups (brominated PPO) with potassium co -(4-oxybiphenyl) alkanoates and potassium u-(4-methoxy-4-oxybiphenyl)-alk.an oates. While PPO required ten methylene units as a spacer and 4,4 -methoxybiphenyl as mesogen to present thermotropic liquid crystalline mesomorphism,... 

In order to determine the necessity and/or the length of the spacer that is required to achieve liquid crystalline behavior from flexible vs. rigid polymers, we have introduced mesogenic units to the backbones of a rigid [poly(2,6-dimethyl-l,4-phenylene oxide) (PPO)] and a flexible [poly(epichlorohydrin) (PECH)] polymer through spacers of from 0 to 10 methylene groups via polymer analogous reactions. 

The attachment of mesogenic units to a polymer backbone via polymer analogous reactions is not a new concept, although they are much less frequently used than the polymerization of mesogen containing monomers. Liquid crystalline polyacrylates,... 

In contrast to the substituted PPO s, It Is theoretically possible to obtain the same substituted PECH s by homopolymerization of the corresponding mesogenic oxirane, or by its copolymerization with epichlorohydrin. We have attempted these polymerizations in order to better interpret the thermal behavior of the more complicated copolymers that we have obtained by polymer analogous reactions. Homopolymerization would be instructive because the incorporation of nonmesogenic units into liquid crystalline homopolymers doesn t as a rule change the type of mesophase obtained (5). 

Thermotropic liquid crystalline polymers, like polyesters containing mesogenic units on the main chain, may not be described by the wormlike chain model (cf. Sect. 1.2). The present article does not consider this type of polymers. 

---