Semirigid chains

Inasmuch as semiflexibihty, or semirigidity, can be realized in diverse ways, it is scarcely to be expected that a single theory will comprehend all polymers of this kind. 

With the chain diameter given by d = (M /NAQl y/2 (see footnote 4, one obtains) 

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The wide variety of ketomethylene and amino ketone monomers that could be synthesized, and the abiUty of the quinoline-forming reaction to generate high molar mass polymers under relatively mild conditions, allow the synthesis of a series of polyquinolines with a wide stmctural variety. Thus polyquinolines with a range of chain stiffness from a semirigid chain to rod-like macromolecules have been synthesized. Polyquinolines are most often prepared by solution polymerization of bis(i9-amino aryl ketone) and bis (ketomethylene) monomers, where R = H or C H, in y -cresol with di-y -cresyl phosphate at 135—140°C for a period of 24—48 h (92). 

FIG. 14 Semi-log plot of mean chain length L vs width of the open slit D at various temperatures in 3d. Full symbols denote flexible chains and empty symbols semirigid chains with activation energy a = 0.5 [61]. 

Volume fractions of polymer at mesophase separation as function of axial ratio of rod-like or semirigid chain, (fi and a, the volume fractions of polymer in coexisting isotropic and anisotropic phases, respectively, are calculated on the basis of Odijk s theory with q = 1200 A and d = 18 A. The metastable volume fraction, Vn, calculated on the basis of X = L/d from Flory s lattice model is also shown for comparison. Filled circles are experimental data points for PBG in nitrobenzene. The vertical bar on the abscissa is the axial ratio at which L q for this polymer in this solvent. 

We shall not attempt to review and compare critically various theories of liquid crystallinity in this chapter. Inasmuch as theory based on a lattice model has proved most successful in the treatment of liquid crystallinity in polymeric systems, we shall present an abbreviated account of that theory confined to its essential aspects. The versatility of this theory has permitted its extension to polydisperse systems, to mixtures of rodlike polymers with random coils and to some of the many kinds of semirigid chains. These ramifications of the theory will be discussed in this chapter... 

Fig. 6. The effect of the variation of the flexibility parameter for a semirigid-chain polymer (at X = 100) on the composition of coexisting phases (according to...

Miller et al. have shown that the variation of/for semirigid-chain polymers does not change the general form of the diagram. However, with increasing of flexibility a narrow heterophase region is shifted toward higher concentrations and considerably broadened. 

Valenti et al. noted a transition to the mesophase under the effect of the applied external field for those semirigid-chain polymers which do not exhibit the liquid crystalline transition under normal conditions. A moderately concentrated solution of polyterephthalamide of p-aminobenzylhydrazide (X-500) in dimethyl sulphoxide does not exhibit the transition into the liquid crystalline state, which can be explained by a relatively low axial ratio for the macromolecules. The persistence length for this polymer is estimated being equal to 50 A. At the same time, according to the data from the literature, high-modulus fibres have been obtained from the solutions of this polymer, which can be connected only with the appearance of the liquid crystalline state in the process of fibre formation. The authors of believe that this... 

The general scheme of the transition of semirigid-chain polymers from the non-equilibriimi amorphous state to the liquid crystalline or crystalline state is given... 

The rapid growth of the number of publications concerning polymeric liquid crystals indicates that we should expect the appearance of new fundamental studies on the transition of rigid- and semirigid-chain polymers into this state. The range of moderately concentrated solutions for these polymers is studied sufficiently well, while the development of the methods of establishing the liquid. crystalline state in superconcentrated systems and in pure polymers with semirigid chains, as well as the analysis of kinetics of phase transitions, are the subject for further theoretical and experimental works. 

PROPERTIES OF SPECIAL INTEREST The Structure of polyquinolines can be altered from a semirigid chain to a rigid one during the synthesis. Although largely an amorphous polymer, some substituted rigid-rod members exhibit crystallinity in low amounts. Polyquinolines are found to posses excellent thermal and oxidative stability, good mechanical properties, low dielectric constants, low values of moisture absorption, and low thermal expansion coefficients. 

It is also possible to model the semirigid chain in a continuous way. This wormlike model is shown in Figure 4. The overall length or its contour length is againZ. The unit tangential vector at location r (5) of the chain is defined by ... 

Thermal Behavior of Semirigid Chain Polymers Glass Transition Temperature versus Crystallinity 238... 

Semicrystalline Structure of Semirigid Chain Polymers and Its Evolution during Crystallization and ... 

Similarly to the didectric analysis, the modification of the a-rdaxation spectrum during crystallization of a semirigid chain polymer can also be monitored by dynamic mechanical thermal analysis (DMTA) In this case, due to a more restricted frequency... 

Thermal Behavior of Semirigid Chain Poiymers Giass Transition Temperature versus Crystaiiinity... 

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