Nematic copolymers

While the possibility of this dispersion of micro-domains of the nematic phase in an isotropic phase cannot be dismissed, concrete evidence for morphologies of this kind in nematogenic copolymers is not prominently in evidence. The longer sequences of rigid units undoubtedly are responsible for promotion of liquid crystallinity but, as theory suggests they appear to be uniformly dispersed Sequences of units that include many flexible members and hence are not rodlike may assume a role analogous to that of the solvent in a lyotropic system The nematic copolymer should, on this basis, consist of a single phase. 

As the previous sections have shown, nematic polymer liquid crystals may be oriented by surface forces and in electric fields. It has been shown recently that such field-induced changes in orientation may also be used to orient pleochroic dyes through the guest-host effect. In such an effect either guest dyes dissolved in a nematic polymer " host or side-chain dye moieties in a nematic copolymer system (where A is a nematic moiety and B is a dye in Fig. 2b) undergo a cooperative realignment as the nematic director responds to the applied field. Since the pleochroic dye has its absorption transition... 

The authors proposed a different mechanism for the formation of the above morphologies. During toluene casting the solution seems to separate into a sol-vent-rich phase and a nematic copolymer-rich phase. After further evaporation, a second transition to a smectic phase appears to occur and because toluene is a better solvent for the polystyrene block, the coil part can stretch and the zigzag morphology can build by tilted assembly of the rods and eventual locking of... 

LC polymers with paired mesogens in the side branches (cf. Fig. 6.3c) form a peculiar type of nematic copolymers. If the lengths of the aliphatic spacers which join the mesogenic groups to the main chain differ, the nematic mesophase appears, as seen from the comparison of two types of polysiloxanes ... 

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Polymers formed between a and c, d and e all failed to show any liquid-crystalline behaviour. However, for all a examined (m = 2,4,6 and 8), nematic phases were observed with b-4 (all monotropic) - a further monotropic nematic material was the copolymer of a-6 and b-3. Unidentified crystal smectic mesophases were reported for a further three examples. 

The polyamides are soluble in high sirength sulfuric acid or in mixtures of hexamelhylphosphoiamide. AMV-diinethylaeciamidc. and l.ifT. The liquid-crystal phase is optically anisotropic and the texture is nematic. The nematic texture can he transformed lo a chiral nematic texture by adding chiral species us a dopant or incorporating a chiral unit in the main chain as a copolymer. 

The article covers synthesis, structure and properties of thermotropic liquid-crystalline (LC) polymers with mesogenic side groups. Approaches towards the synthesis of such systems and the conditions for their realization in the LC state are presented, as well as the data revealing the relationship between the molecular structure of an LC polymer and the type of mesophase formed. Specific features of thermotropic LC polymers and copolymers of nematic, smectic and cholesteric types are considered. 

The case is quite different for copolymers 5-5. Here, the transformation from a nematic mesophase to a smectic one is a result of gradual decrease in flexibility of the main chain. As seen from Fig. 22a, b, the plots of Tcl and AHC, on composition have a minimum. This could be the consequence of a more defective packing of side groups due to incorporation within the backbone of units of different chemical nature. A sharp rise in AHC, indicating transformation to a smectic mesophase type, begins only at... 

The first success was achieved when optically active (chiral) monomeric units were combined with a nematic LC polymer 105,123,143,144). The approach was based on the idea that a cholesteric mesophase may actually be realized as a helical nematic structure. Then by chemical binding of chiral and mesogenic units into a chain, accomplished by copolymerization or copolycondensation (in case of linear polymers) of nematogenic and optically active compounds, it was found feasible to twist a nematic mesophase and obtain copolymers of cholesteric type (Table 13). 

The main feature identifying a cholesteric mesophase in polymers is the presence of optical texture with selective circularly-polarized light reflection. This indicates the formation of 1-helical cholesteric structure in LC copolymers. The X-ray patterns of actually all cholesteric copolymers described (with the exclusion of polymers 3.1 and 4.1, Table 13) correspond to those of nematic and cholesteric low-molecular liquid crystals, which is manifested in a single diffuse reflex at wide scattering angles. At the same time, for copolymers 3.1 and 4.1 (Table 13) small angle reflexes were observed 123), that are usually missing in low-molecular cholesterics. 

The example of the above mentioned specific features of low-molecular liquid crystals, capable of orientation in an electric field, has led recently to the synthesis of a series of acrylic, methacrylic and siloxane polymers and copolymers of nematic and smectic types with nitrile end groups in the side branches (see Tables 3, 4, 9, 12) ... 

Preceding the discussion of orientational effects in LC polymers, it is worth mentioning that for a nematic and a smectic phase A of LC polymers only the S-effect was discovered and investigated. This started with works U9,124, 137 138), that demonstrated the ability of LC polymers to orient in permanent and alternating electric fields. The structural formulas of some of the polymers and copolymers investigated are given below ... 

In view of the effect of molecular mass on orientational phenomena the results of151) seem to be more explicable. In this work surprisingly low values for threshold voltage (U 8-40 V) and rise and decay times (x a 200 msec) were observed for an array of nematic polymers and copolymers. They are close to the corresponding values for low-molecular liquid crystals, which implies presumably that the polymers investigated were of low degrees of polymerization or had a very wide molecular mass distribution. 

Similar considerations of symmetry apply in other systems, for example nematic liquid crystals and aligned short fibre composites have symmetry D h, smectic A liquid crystals D , while in copolymers and certain fibre composites examples of hexagonal symmetry may be found and translational symmetry may also be present, which is not found in petrology. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

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