Poly -liquid-crystalline effect

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

Kricheldorf [17] studied liquid-crystalline cholesteric copoly(ester-imide)s based on 1 or 2. The comonomers to obtain these chiral thermotropic polymers were N-(4-carboxyphenyl)trimellitimide, 4-aminobenzoic trimellitimide, 4-aminocinnamic acid trimellitimide, adipic acid, 1,6-hexanediol, and 1,6-bis(4-carboxyphenoxyl) hexane. Apparently the poly (ester imide) chains are so stiff that the twisting power of the sugar diol has little effect. 

Most interesting are the effects of salt complexation on the mesomorphic behavior of liquid crystalline crown ethers and liquid crystalline crown ether polymers. Sodium triflate was added to poly(17) [34] and poly(25) (Scheme 14) [39]. The enantiotropic nematic and smectic phases of poly(17) were changed dramatically [40]. With increasing amounts of salt, the clearing temperatures are shifted to higher values while the melting transition increases only slightly. 

A mixture of molybdenum hexacarbonyl and 4-chlorophenol is effective in performing al-kyne metathesis of dipropynylated dialkylbenzenes. Alkyne metathesis of these precursors leads to the clean formation of dialkyl poly( paraphenyleneethynylene)s (PPEs) in high yield and with high molecular weights. This facile yet effective access to the PPEs has allowed study of their spectroscopic, structural, and thermal properties. While PPEs have been made before, the dialkyl-PPEs turned out to have particularly interesting optical and liquid-crystalline properties that can be explained in terms of the conformation of the main chains. The PPEs have also been utilized to construct light-emitting diodes and other semiconductor devices. This chapter discusses the interplay of structure, chromicity, and electronic properties of the dialkyl-PPEs. 

The concept of a side chain liquid crystal polymer has been demonstrated in a number of laboratories and is well documented in the literature (J ). Most of the side chain liquid crystalline polymers reported to date contain polysiloxane, polyacrylate or polymethacrylate main chains. More recent studies on the effect of backbone flexibility now include the use of flexible poly(ethylene oxide) or more rigid poly( a-chloroacrylate) chains. 

In field-effect transistors based on the thin films of poly(3-alkyl)thiophene derivatives, high field-effect mobility has been observed because of the formation of crystal-like grains [116]. The mobility depends strongly upon the crystallinity of the thin films. Introduction of liquid crystallinity into conjugated polymers is expected to be effective for enhancing microscopic crystallization of conjugated polymers to increase their carrier mobilities. It can also cause anisotropy in various physical properties if macroscopic molecular alignment is achieved. 

Besides the above conventional effects, Chapter 3 summarizes data suggesting the ability of some gases to sorb and diffuse inside the actual crystals of poly(4-methyl-l-pentene) (68,69). Finally, Chapter 3 considers liquid crystalline polymers, which seem to form a new class of materials in terms of barrier responses(57). The high barrier nature of liquid crystal polymers appears to be largely due to their unusually low solubility coefficients for typical penetrants. This is quite different from the case for most high barriers like EVOH, and polyacrylonitrile that typically function due to the unusually low mobilities of penetrants in their matrices (70). ... 

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