Poly , liquid-crystalline behavior

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. 

Block copolymers with PS and a polymethacrylate block carrying a liquid crystalline group, PS-b-poly 6-[4-(cyanophenylazo)phenoxy]hexyl methacrylate, were successfully prepared in quantitative yields and with relatively narrow molecular weight distributions (Scheme 5) [18]. The thermotropic liquid crystalline behavior of the copolymers was studied by differential scanning calorimetry. 

Whereas poly(9,9-dihexylfluorene) (PDHF, 195) is generally considered as amorphous, PF with longer octyl side chains, PFO 196, is crystalline material. Many PFs — dioctyl (PFO 196 [228,230,231]) or 2-ethyl hexyl) (197 [232]) as well as some fluorene copolymers [233] exhibit liquid crystalline behavior, opening a possibility to fabricate polarized LEDs [224,234,235] (Chart 2.45). 

Pang, Li, and Barton indicated, based on viscosity data, that PAEs had a stiff structure in solution [43]. A PAE shows a Mark-Houwink constant as high as a= 1.92 [43], revealing that it has a very stiff structure, similar to rodlike poly-(pyridine-2,5-diyl) [65,66]. Liquid crystalline behavior of PAEs with a meso-gen has been reported [45]. The chemical properties of PAEs have also been investigated with oligomeric model compounds [67-76]. 

Scheme 6.6. Procedures for the preparation of poly(aryl amide) polymers. The carboxylic acid terminated polymers exhibited lyotropic liquid crystalline behavior.

Lor polymers in which the mesogen is separated by a spacer of six methylenes units from the polymer backbone, it is obvious that the more rigid poly(norbornene)s favor nematic liquid crystalline phases. Poly-(VI-6) with the rigid and bulky 2,5-dimethine oxacyclopentane-3,4-dicarboximide unit in the main chain does not show liquid crystalline behavior (Table 6, entry 12). The more flexible poly-(II-6) backbone allowed the formation of a nematic mesophase. If the mesogen density was increased, as realized in poly-(IV-6), the isotropization temperature was found to be 26 °C higher than that for poly-... 

Stabilization of the mesophase was observed as the degree of polymerization was increased. The Tg values of the poly(norbornene)-polymers were about 30 °C higher than those of the poly(butadiene) polymers. Both polymers showed similar isotropization temperatures, but they differed substantially in their liquid crystalline behaviors. Poly-(IX-n)s with a poly(norbornene) backbone exhibited textures typical of nematic mesophases, whereas the poly-(butadiene)-based polymers poly-(X-n) displayed textures representative of smectic A mesophases. The more flexible backbone of poly(butadiene) allowed a higher order of alignment of the mesogenic units, resulting in the more ordered liquid crystalline smectic A phase. 

A multistep reaction pathway leads to polymers 43 and 44 with phosphatidylcholine moieties in the main chain and long alkyl groups in the side chain [122]. These polymers exhibit thermotropic liquid-crystalline behavior. Polyamides 45 were obtained by interfacial polycondensation they are insoluble in any normal solvent [123]. Poly-MPC capped with cholesteryl moieties at one or both polymer ends was prepared by the radical polymerization of MFC initiated with 4,4 -azobis[(3-cholesteryl)-4-cyanopentanoate] in the presence of a chain transfer agent [124]. The self-organization of these polymers was analyzed with fluorescence and NMR measurements. 

Although the technical applications of low molar mass liquid crystals (LC) and liquid crystalline polymers (LCP) are relatively recent developments, liquid crystalline behavior has been known since 1888 when Reinitzer (1) observed that cholesteryl benzoate melted to form a turbid melt that eventually cleared at a higher temperature. The term liquid crystal was coined by Lehmann (2) to describe these materials. The first reference to a polymeric mesophase was in 1937 when Bawden and Pirie (2) observed that above a critical concentration, a solution of tobacco mosaic virus formed two phases, one of which was bireffingent. A liquid crystalline phase for a solution of a synthetic polymer, poly(7-benzyl-L-glutamate), was reported by Elliot and Ambrose (4) in 1950. 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

Kinoshita, I. Kijima, M. Shirakawa, H. Liquid crystalline behavior of poly(m-phcnylcncbutadiynylene)s having a mesogenic substituent. Mol. Cryst. Liq. Cryst. 2001, 365, 1193-1202. 

One of the basic problems confronting molecular composites is the difficulty of finding miscible combinations of rigid rod polymers with flexible chain polymers. Poly(p-phenylene benzobisthiazole)/poly(-2,5(6)-benzimidazole block copolymers have been reported by Tsai et al. [1985] and are noted to exhibit better processability and mechanical properties than the simple blends of the block copolymer constituents. Chang and Lee [1993] prepared poly(p-benzamide)/Pl block copolymers and reported on the liquid crystalline behavior. Such approaches would appear to have future implications. As an example PA e.g., PA-66) block copolymers with rigid rod polyamides could be prepared and used in blends with PA-66 to yield the desired molecular composite. 

A selection of monosubstituted and disub-stituted poly-(/ -phenylene-terephthalate)s is compared in Table 2. Poly(p-phenylene-terephthalate)s with methyl, methoxy, chlo-ro, or bromo substituents on either the hy-droquinone or the terephthalic acid moiety exhibit melting temperatures of 350 °C or higher. Thermotropic liquid crystalline behavior is observed in these samples, although it is in the range of thermal decomposition. A comparison of the mono- and diphenyl substituted polyesters reveals an important trend. The monosubstituted poly-(p-phenylene-terephthalate) with the phenyl substituent in the hydroquinone moiety melts at 346 °C, also forming a nematic melt up to a clearing temperature of... 

It should also be mentioned that a combination of ester and amide linkages was frequently used to prepare thermotropic LC polyesteramides. Frequently, the inexpensive p-aminophenol is incorporated. For example, the Vectra B series is a poly(naph-thoate-aminophenolterephthalate) derived from 6-hydroxy-2-naphthoic acid (HNA), p-aminophenol, and terephthalic acid [14]. It is obvious that only a limited amount of p-aminophenol can be incorporated in order to maintain thermotropic liquid crystalline behavior. The amide linkage enhances the intermolecular interactions via hydrogen bridges and improves the solid state properties of the material. 

Miteva, T., et al. 2000. Interplay of thermochromicity and liquid crystalline behavior in poly(p-phenyleneethynylenejs tt-tt interactions or planarization of the conjugated backbone Macromolecules 33 652. 

An additional poly[(lif,li7,2i7,2i7-heptadecafluorodecyl acrylate (ln))-co-(stearyl methacrylate (lo))] was successfully obtained in water in the presence of RAMEB. The copoljuner shows two broad endothermic transitions at 32°C and 50° C, which indicates a liquid-crystalline behavior. The results of X-ray diffraction of the copol5uner poly(ln-co-lo) at 20°C indicate highly ordered lamella structure (18). 

Polyisocyanates are comprised of only amide bonding in the main chain and maintain their helical rod structures, so they normally show liquid crystallinity or high crystallinity. For instance, the helix conformation (12/5) of poly(hexylisocyanate) (PHIC) and its high crystallinity in the solid state are well observed by X-ray scattering (Wu et al., 1992). PHIC displays rod structures with a very long persistence length of 50-60 nm in the solution state (Bur and Fetters, 1976). It also shows lyotropic liquid-crystalline behavior at high concentrations (Aharomi, 1979 Aharoni and Walsh, 1979). 

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