Intramolecular orientational order

A more serious factor that has direct regard to initial steps of mesophase formation in polymer solutions has to be assessed. That is the interaction between the molecules of LC polymer and of the solvent. The conformation of the macromolecule appears to be sensitive to the thermodynamic quality of the solvent, and this has a very pronounced effect on the mode of intramolecular structure formation. For instance, the folding of the chain in a bad solvent leads to a sharp rise in intramolecular orientational ordering of the side branches. This is manifest as an increase of optical... 

Appendix. Intramolecular Orientational Ordering in the Isotropic Phase. . 95... 

Intramolecular orientational order in polymer molecules was investigated by a comprehensive study of the conformation of these molecules in the unassociated state... 

The study of the molecular organization of lyotropic and some thermotropic comb-shaped polymers was of primary importance in understanding the mechanism of formation of intramolecular orientational order [9] these concepts subsequently served as the basis for notions on ordering of liquid-crystal polymers on the molecular level. 

Many conformations were sampled by the usual MC procedure. The result is of course that there is no preferred orientation of the molecule. Each conformation can, however, be characterised by an instantaneous main axis this is the average direction of the chain. Then this axis is defined as a director . This director is used to subsequently determine the orientational order parameter along the chain. The order is obviously low at the chain ends, and relatively high in the middle of the chain. It was found that the order profile going from the centre of the molecules towards the tails fell off very similarly to corresponding chains (with half the chain length) in the bilayer membrane. As an example, we reproduce here the results for saturated acyl chains, in Figure 10. The conclusion is that the order of the chains found for acyl tails in the bilayer is dominated by intramolecular interactions. The intermolecular interactions due to the presence of other chains that are densely packed around such a chain,... 

However, it must be recalled that the Lifshitz theory was originally formulated23 25 for the model of beads (see Fig. 7 a). In this model, each monomer is represented as a material point thus, this model cannot be used for the description of the intramolecular liquid-crystalline phase. The description of the orientational ordering, requires the generalization of the Lifshitz consideration for the models, in which the state of an elementary monomer is defined not only by its spatial position but also by its orientation (see, for example, the models of Fig. 7 b-db Such a generalization will be our first aim in this section. 

Inspection of Table 7 reveals that the dimers 7 are good model compounds for the polymers 4. This result can be rationalized by the presence of two mesogenic units forcing the flexible spacer in a more extended configuration. Similarly, the increase in orientational order from the monomers 8 to the dimers 7 can be understood by intramolecular order transfer via the extended spacer. 

Molecular order of LC main chain polymers is characterized by a high degree of conformational and orientational order of the polymer chains. The observed order parameters exceed those of conventional LCs by a considerable amount. The pronounced increase in orientational order from the monomers to the polymers can be rationalized by an intramolecular order transfer via highly extended spacers. 

Table 2 gives the experimental data on the degree of Intramolecular orientational axial order S for a series... 

Table 2. Intramolecular Orientational-Axial Order Degree S for Some Mesogenic Polymers. 

It may be assumed that the decisive part in the appearance of lyotropic mesomorphism in a concentrated polymer solution is played by the parameter A which determines the degree of its intramolecular orientational axial order, Experimental data show that in contrast to low molecular weight liquid crystals, in this case, the mesomorphism is determined by the length of the part that is of rodlike shape rather than by the chain length as a whole. 

In the nematic phase, the same timescales are operative. Although conformer probabilities are slightly shifted from the distribution in the isotropic liquid (more anisometric conformers are favored in the nematic), intramolecular isomerization rates are not influenced by the long-range orientational order. Incoherent, quasielastic neutron scattering gives the typically fast rotational diffusion about the principal axis 1, r 10 °-10 s. Reorientational flipping of the I axis is itself... 

NMR relaxation and small-angle X-ray scattering indicated that the intramolecular orientation is only slightly affected by the presence of other chains in the glassy amorphous state and that axial correlation is of the order of 1 nm or less. 

The minute details of the local intramolecular structure, especially in terms of side-chain and main-chain orientational ordering, remain poorly defined. By their very nature conducting polymers exhibit an intimate coupling between the electronic states and the local structural degrees of freedom. In many instances direct quantitative measures of these attributes would contribute immensely to the level of understanding. 

The optical characteristics of polymer 5 (Table 3.7) in comparison to the properties of polymers 15 and 18 (Table 3.5) also confirm the role of the combshaped structure of thermotropic mesogenic molecules in the formation of intramolecular axial orientational order. The values of [ ]/[ n] and aj - ot2 of polymer 5 are 2-3 times lower than the corresponding paramet for polymers 15 and 18. This difference is not determined by the simple additive contribution of the optical anisotropy of the -CjgH33 aliphatic group, which is confirmed by the values of [n]/[ti] and aj - ot2 of PCMA (polymer 5, Table 3.5). 

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