Rigid chain polymers concentrated

Hence, Flory s theory offers an objective criterion for chain flexibility and makes possible to divide all the variety of macromolecules into flexible-chain (f > 0.63) and rigid-chain (f < 0.63) ones. In the absence of kinetic hindrance, all rigid-chain polymers must form a thermodynamically stable organized nematic phase at some polymer concentration in solution which increases with f. At f > 0.63, the macromolecules cannot spontaneously adopt a state of parallel order under any conditions. 

Grafting of rigid chain polymers auch as PAN which do not swell in the reaction medium at low concentrations of the initiator may give rise to long-lived grafted macroradicals (occluded macroradicals). 

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. 

Aikawa et al. considered the effect of electric field on the phase transition in solutions of rigid-chain polymers for a PBLG solution in dioxane. Theoretical calculations have shown that the application of an electric field must shift the values of v towards lower concentrations. This conclusion was confirmed in experiments. According to the results obtained by Patel et al the application of electric fields also causes a shift in the temperature of the liquid crystalline transitions. 

Figure 16 shows a schematic diagram of phase transformations for rigid-chain polymers separated from isotropic solutions by introducing a nonsolvent into the system (this is a usual method of obtaining fibres and films) (cf. >). The initial isotropic solution with the polymer concentration V2 and the value of the Huggins-Flory parameter is in the monophase region. The critical concentration of the transition into liquid crystalline state for this system is v. When a nonsolvent is introduced, i.e. when x is increased up to the value >0.5 (x ), two routes of the phase transition... 

Studying molecular properties of rigid-chain polymers by hydrodynamic methods, specific difficulties sometimes arise. Thus, many polymers with aromatic chains that are of great practical importance are molecularly soluble only in very aggressive media such as concentrated sulfuric acid. Hence, experiments in these systems require specific instruments ... 

For rigid-chain polymers An/Ar is also independent of solution concentration. This can be seen in Fig. 12 in which the experimental poonts for solutions of the same polymer at different concentrations fall on the same curve illustrating the dependence of An on At. This coincidence holds for both flexible-chain polyisobutylene (Curve 1) and rigid-chain nitrocellulose (Curve 2). 

As already indicated (Sect. 2.2), many rigid-chain polymers are insoluble in organic solvents k) that their molecular characteristics can only be studied by use of very agressive solvents such as concentrated sulfuric acid. However, sedimentation measurements are not employed in practice and only diffusion studies can provide information about the translational mobility of these polymer molecules in solution. 

As already indicated (p. 98), the interest in rigid-chain polymers in caused to a great extent by the prospects of their practical use. These prospects depend mainly on the possibility of attaining the mesomorphic state in concentrated solutions of these... 

As it is known, the plots of Fig. 39 allow one to determine the values of intrinsic viscosity [rj] by the extrapolation to the polymer zero concentration c. Another method of [r ] evaluation is Shultz-Blashke Eq. (48). In Table 12, the comparison of [q] values, evaluated by two indicated methods, is adduced for polyarylate F-1 at three testing temperatures and polyamidobenzymidazole [94] at two concentrations of solution in sulfuric acid. As it follows from the data of Table 12, the values [q], calculated according to the Eqs. (47) and (48) showed a good correspondence, that allows one to use Shultz-Blashke equation for [q] values of semirigid- and rigid-chain polymers estimation. 

The data in Table 1 suggest that for the appearance of lyotropic mesomorphism in a concentrated solution of a rigid chain polymer, the value of A should be of the order of magnitude of several hundreds of angstroms. 

Liquid Crystaiiine Soiutions. Cellulose esters, when dissolved in the appropriate solvents at the proper concentration, show liquid crystalline characteristics similar to those of other rigid chain polymers (9) because of an ordered arrangement of the polymer molecules in solution. Cellulose triacetate dissolved at 30-40 wt% in trifluoroacetic acid, dichloroacetic acid, and mixtures of trifluoroacetic acid and dichloromethane exhibits brilliant iridescence, high optical rotation, and viscosity-temperature profiles characteristic of typical aniostropic phase-containing liquid crystalline solutions (10). Similar observations have been made for cellulose acetate butyrate (11), cellulose diacetate (12), and other cellulose derivatives (13,14). Wet spinning of these liquid crystalline solutions yields fibers... 

Rigid-chain polymers containing different concentration of laterally attached side rods have been demonstrated in bulk reactions using a Vectra LCP as the base material, as illustrated in Figure 3 (57). These polymers exhibit liquid crystallinity even up to a maximum side-rod concentration of 20 mol%. The crystallinity of the... 

PROPERTIES OF RIGID-CHAIN POLYMERS IN DILUTE AND CONCENTRATED SOLUTIONS... 

It would thus be fcwmally possible to assign the described system, which contains a liquid-crystalline phase, to lyotropic liquid crystals. Actually, this system differs fiom true lyotropic liquid-crystalline systems with respect to the mechanism of the onset of the ordoed state since only a decrease in the temperature of the C-LC transition is involved in the rigid-chain polymer-solvent system due to addition of the solvent, and in this sense, the system does not differ from thomotropic liquid-crystalline systems formed in the pure polym with an increase in the temperature. In addition, let us examine the behavior of a system with a fixed concentration of solvent cwresponding to a composition of the system V2 (ho% and below, subscripts 1 and 2 in the volume and weight compositimis of the system refer to the solvent and polymer, respectively). An increase in the temperature to Tj results in the complete transition of the system into the liquid-crystalline state, which corresponds to the usual thermotropic transition, and the solvent does not play any specific role here except for decreasing the melting point of the crystalline phase. With a further increase in the tempaature, the same ttansitions LC -> LC +1 (at and LC +1 -> I (at as in ordinary thermotropic liquid-crystalline systems take place. 

Fig. 2.4. Dependence of the critical concentration of the transition of a solution of a rigid-chain polymer into the anisotropic state (v ) on axial ratio x solid line theoretical curve O experimental points for PBA-DMAA + Ua [21] forPPTA-H SO [22] A forPBA-DMAA + Lia[23].

The case of a mixture of rods (rigid-chain polymers) with random coils has been particularly examined in theoretical studies. It has been shown that the coils are primarily (or even totally) concentrated in the isotropic phase. The unpleasanmess of a nematic phase of random coils demonstrates a major characteristic of crystals in general, as Flory notes, thus confirming the term liquid crystal. Nevertheless, the presence of random coils affects the composition of the anisotropic phase (its concentration) due to a change in the chemical potential of the solvent... 

Special attention should be turned to the sharp transition from a narrow concentration corridor to a broad heterophase region, mentioned above, which takes place for low positive values of parameter x- It is int esting to compare the appearance of this broad region with the phenomenon of decomposition of solutions of flexible-chain polymers into two phases with the formation of two liquid (amorphous) phases with values of x in the limit (with infinitely high molecular weight of the polymer) of 0.5. TTie phase equilibrium diagrams (in coordinates v-x) for a rigid-chain polymer with an axial ratio of x = 150 and 350... 

In studying the viscous properties of dilute solutions of rigid-chain polymers [10-16], it was shown that the Martin equation [9] applies to them, as to solutions of flexible-chain polymers, for low concentrations c ... 

In going to semidilute solutions of both flexible-chain and rigid-chain polymers, it is thus necessary to find a new parameter which describes the properties of such a network. The ratio (c/c ) can be used as such a parameter [18, 19], where is the concentration corresponding to the formation of a system of intermolecular contacts, and P is the exponent in the equation i] c. In view of this, Martin s equation should be updated ... 

Fig. 9.1. Generalized concentration dependence of the viscosity of solutions of polymers (the different points correspond to different polymers, where the most flexible-chain polymer is polycaproamide and the most rigid-chain polymer is poly-p-benzamide).

DEPENDENCE OF THE VISCOSITY OF SOLUTIONS OF RIGID-CHAIN POLYMERS ON THE CONCENTRATION... 

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