Liquid crystal polymers conformation

Very little work has appeared in the literature which deals with blends in which the component materials can cocrystallize. It is generally believed (16.17) that a requirement for cocrystallization is that there must be a close matching of the polymer chain conformations and of crystalline dimensions. Also, some level of miscibility should exist between the two polymers and the crstallization kinetics cannot be very different. Certainly, in the case of liquid crystalline polymers, in general, these requirements would be expected to be met. Some of our recent work (8) has suggested, however, that not all liquid crystal polymers do cocrystallize. The present work suggests that in certain cases it may be possible to achieve this effect. 

Comparison of the values of C for the polymers with a flexible C-C or Si-O-Si backbone (as occurs in siloxane polymers) of about 6-10 with the value for the rigid-rod polymer of 125 demonstrates the fundamental difference in the solution properties of the latter polymer which has a highly extended conformation characteristic of liquid-crystal polymers. Equation (1.5) also shows that for a real chain the value of Rq would be expected to increase as the half power of the number of repeat units, i.e. the degree of polymerization, DP. ... 

Assume a side chain liquid crystal polymer, consisting of a flexible backbone and mesogenic side groups, is able to form the Sc phase and its glass transition is below the ambient temperature, such a side chain liquid crystal polymer will be expected to show ferroelectricity. The side groups are packed in a way similar to small molecular mass ferroelectric liquid crystals. The backbone is suppressed between layers, occasionally penetrating into them. The conformation of the backbone is discus-like. 

Ordered behavior is also observed in solutions of some liquid crystal polymers (lyotropic LCPs). Unlike flexible polymers that assume a random coil conformation in solution, the rigid polymers being... 

As has already been discussed earlier in this chapter, one of the most common ways to achieve miscibility in high temperature polymer blends is through the introduction of specific interactions between the two components in the mixture. This is not surprising, as this is also an often-used approach to develop miscibility in other types of blend systems as well. The one exception to this pattern is the case of blends of two liquid crystal polymers in which miscibility has been determined in systems in which there is no well-defined specific interaction. In those systems, miscibility appears to occur based primarily on similarities in molecular conformation between the two blend components. These results suggest that entropic effects play a significant role in defining the phase behavior of mixtures that contain high performance polymers. 

For liquid crystal polymers comprised of completely rigid units with no conformational freedom, the calculation of /S,(s) using data from model compounds etc would present no problems. However, no liquid crystal polymer falls into that category, even the so-called rigid-chain polymer systems having considerable flexibility. Thus to proceed we need a method of evaluating the chain structure. 

Keywords Chain conformation Cholesteric Elastomer Liquid crystal Liquid crystal polymer Nematic Polymer networks Smectic... 

Noirez L, Keller P, Cotton JP (1995) On the structure and the chain conformation of side-chain liquid-crystal polymers. Liq Cryst 18(1) 129-148. doi 10.1080/02678299508036602... 

Pepy G, Noirez L, Keller P, Lambert M, Moussa F, Cotton JP, Strazielle C, Lapp A, Hardouin F, Mauzac M, Richard H (1990) Observatimi of the conformation and structiue of some liquid-crystal polymers by small-angle neutron-scattering. Makromol Chem Macromol Chem Phys 191(6) 1383-1392. doi 10.1002/macp.l990.021910617... 

The basic chemistry of liquid crystal elastomers is very similar to the chemistry of linear liquid crystal polymers. One can distinguish between main- and side-chain elastomers where the mesogenic units are either segments or side groups of the monomer imits of the macromolecular chain. The elastomers only differ from linear liquid crystal polymers by conventional crosslinking reactions. New aspects for chemistry appear when liquid crystal networks are required that spontaneously, and without any external fields, exhibit a macroscopically uniform orientation of the director. For these conditions, networks have to be sythesized where the chain conformation is consistent a priori with the uniform liquid crystal order [5]. 

Liquid crystal polymers containing azobenzene groups in the side chains have also been reported. Polymers 18 and 19 are signihcant examples in which light-triggered conformational changes of the azobenzene units resulted in nematic/smectic-isotropic phase transitions. These systems will be discussed in detail in Section 4.3. Isomerization of spiropyran moieties (20 in Fig. 4.4) introduced in the side chain have also been used to modulate polymer solubility in an aqueous environment and we will further discuss this below. 

Further examples of characteristic dynamical processes in liquid crystal polymers, discussed in detail elsewhere [19,31], include the phenyl ring flip in side chain liquid crystal polymers [100] and the conformational dynamics of the spacer in a main chain liquid crystal polymer [101]. 

The antiferroelectric SmC structure (see Fig. 17) can also occur in racemates [94] or in nonchiral compounds such as symmetric dimers [136, 137], nonsymmetric dimers [133], and main chain liquid crystal polymers [138], where its formation is driven by steric and/or conformational effects. Antiferroelectric ordering has been shown to increase the smectic order parameters in ferroelectric liquid crystals [94, 95]. 

Similar measurements on main chain liquid crystal polymers have been used to determine the persistence length of the chains in the nematic phase. Again, there are theoretical predictions for the temperature behavior. It has been suggested that as the temperature increases the number of hairpin bends of the chain will increase and the conformation of the chain will become more isotropic [26]. This has been confirmed qualitatively, but since most of the materials have been polyesters and have had nematic phases at quite high temperatures the effects of transesterification have interfered [27-30]. However, similar results have been found in a polyether [31] where the chains were found to contain one hairpin defect on average. 

In the entire field of liquid-crystal polymers, an important place has been assigned to the analysis of the molecular structure of polymers capable of exhibiting lyotropic or thmnotropic mesomorphism. The study of the conformation of the macromolecules, the features of their structure determined by the structure of the monomeric unit and the molecular weight, and the degree of flexibility of the polymer molecule showed that the nature of polymeric mesomorphism is determined on the molecular level [1, 2]. 

The data on the conformational composition (trans/gauche) of the aliphatic spacer in comb-shaped liquid-crystal polymer I with a different degree of polymerization (DP) in the starting and oriented states for T 20 C are... 

Conformational disordering It appears during the formation of more than one conformation or configurations in the liquid crystal polymer structures. It appears due to rotatiOTi about a single bond. 

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