Rigid chain polymers anisotropy

Two approaches to the attainment of the oriented states of polymer solutions and melts can be distinguished. The first one consists in the orientational crystallization of flexible-chain polymers based on the fixation by subsequent crystallization of the chains obtained as a result of melt extension. This procedure ensures the formation of a highly oriented supramolecular structure in the crystallized material. The second approach is based on the use of solutions of rigid-chain polymers in which the transition to the liquid crystalline state occurs, due to a high anisometry of the macromolecules. This state is characterized by high one-dimensional chain orientation and, as a result, by the anisotropy of the main physical properties of the material. Only slight extensions are required to obtain highly oriented films and fibers from such solutions. 

Orientational order appears in the solutions of rigid-chain polymers because a random mutual arrangement of their macromolecules is possible only up to a certain concentration of the solution. To retain a minimal volume (minimal free energy) above a certain critical concentration, asymmetric macromolecules must acquire an ordered mutual arrangement, which corresponds to a transition to the state typical for liquid crystals. In this case the solution becomes anisotropic. The degree of this anisotropy is still less than strict three-dimensional ordering typical of crystalline systems, but at the same time it differs from that of the isotropic state typical of amorphous systems. 

Table 9. Parameters characterizing the equilibrium rigidity and optical anisotropy of some rigid-chain polymer molecules according to the data of flow birefringence in their a luttons... 

The display of intramolecular motion in rigid-chain polymers can also be observed if the kinetics of the Kerr effect is studied for a series of fractions of relatively high molecular we t. In fact, as already mentioned (p. 171), the kinetics of the behavior of a polar chain molecule in the electric fidd is determined by relative values of relaxation times of its orientation tq and deformation tj. Since tq increases with M proportionally to M(rjl, and tj is independent of M, it might be expected that at relatively high M the inequality tq > rj will hold and, hence, the polarization and the anisotropy of the solutmn in the electric field will follow the deformational mechanism. 

It has been shown that the main medianism responsible for EB in solutions of rigid-chain polymers is the rotation of their polar molecules as a whole whereas the anisotropy of the dielectric polarizability of the macromolecules only provides a small contribution to the Kerr effect. Hence, the general theory of the Kerr effect for rigid dipole particles with axial symmetry of the optical polarizability can be... 

In compensation schemes the recording of FB can be conducted either by visual observation or by use of a photoelectric receiver. It has been found that the optical anisotropy of rigid-chain polymer molecules is fairly high. However, when very dilute solutions are studied, the measured excess birefringence can be very low. Hence, to obtain reliable information on the molecular characteristics of polymers, methods ensuring maximum sensitivity of the instrument should be used. 

On the other hand, the peculiarities of the molecular structure of rigid-chain polymers favor the use of EB whereas the application of this method to flexible-chain polymers is much less effective. To a certain extent, this also refers to FB which permits the determination of both the anisotropy and the rigidity of the polymer chain using the molecular weight dependence of the shear optical coefficient when chain rigidity is relatively h. ... 

Rigid chain polymers Same as PPSQ-1 except final equilibration at 100% solids PPSQ-2 High Kuhn segment Dynamo-optical (high negative segmental anisotropy) - (7,8) (9,10)... 

The questions of the Mackian elasticity of rigid-chain polymer systems in isotropic and LC states will be examined in the next section of the chapter. We will report the experimental evidence of the existence of anisotropy of the viscosity and orientational elasticity here. 

Hence, to understand correctly the mechanism of flow birefringence of r d-chain polymers it is necessary to take into account both the asymmetry of the pe of their molecules and their optical anisotropy. Both the properties are the main properties of the molecule and if one of them is absent, flow birefringence in solution of kinetically rigid molecules is impossible. 

The main part of the side group in these macromolecules consists of the alkoxy-benzoic acid moiety. This acid may form thermotropic liquid crystals. The investigation of the hydrodynamic properties of PPhEAA molecules in dilute solutions has revealed that the equilibrium rigidity of their main chains is relatively low (Table 12). Hence, since for all flexible-chain polymers, the shear optical coefficient An/Ar in PPhEAA solutions is independent of molecular weight the segmental anisotropy - tt2 and the anisotropy of the monomer unit Aa may be determined by use ofEq. (67). 

It should be noted that the relative accessibility of the transition into the oriented state observed for polymers of various rigidity under appropriate conditions is due to the internal anisotropy of macromolecules caused by their chain structure (see Sect. 1 of this paper and monographs2 3 ). 

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