Rigidity of the polymer chains

On the other hand, the moduli of as-spun fibers of polyarylates from Cl-HQ and l,2-bis(2-chlorophenoxy)ethane-4,4/-dicarboxylic acid (Cl-PEC) (Cl-HQ/ Cl-PEC), and Me-HQ and Cl-PEC (Me-HQ/Cl-PEC), were higher than those of Cl-HQ/PEC and Me-HQ/PEC. The reason for the higher moduli seemed to be the increased rigidity of the polymer chain caused by the restricted rotation of the ether linkage of Cl-PEC as a result of the steric hindrance of the Cl atoms. However, the moduli of polyarylates from fBu-HQ and Cl-PEC (fBu-HQ/Cl-PEC), and Ph-HQ and Cl-PEC (Ph-HQ/Cl-PEC), were lower than those of fBu-HQ/PEC and Ph-HQ/PEC, because the Cl atoms on the PEC units prevent... 

Tensile-fractured as-spun fibers of /Bu-HQ/Cl-PEC and Ph-HQ/Cl-PEC no longer exhibited fibrils, with such phenomena being different to those of the fBu-HQ/PEC and Ph-HQ/PEC systems. Thus, in order to obtain high-modulus as-spun fibers, the stability of the liquid crystalline state and the rigidity of the polymer chain are both assumed to be influential factors. 

However, in spite of the higher rigidity of the polymer chains, many pol-yarylates derived from substituted HQs and BB exhibited liquid crystallinity and lower Tms when compared to the polyarylates derived from substituted HQs and TA. 

Although as-spun fibers of Me-HQ/BB exhibited a lower modulus than those of Me-HQ/Cl-PEC, injection molded specimens of Me-HQ/BB exhibited a higher flexural modulus than those of Me-HQ/Cl-PEC due to the higher rigidity of the polymer chain, in spite of the lower / -value. Flexural-fractured injection molded specimens of Me-HQ/BB exhibited fewer fibrils than Me-HQ/Cl-PEC due to the lower / "-value, as shown in Figure 19.7. Thus, there seemed to be no relationship between the moduli of injection molded specimens and the E-values. 

As-spun fibers of Ph-HQ/HQ/BB with m/n = 50/502 exhibited a much higher modulus (100 GPa) than Ph-HQ/BB (40GPa) due to the increased F-value and rigidity of the polymer chain. Injection molded specimens of Ph-HQ/HQ/BB with m/n = 50/50 exhibited a much higher modulus than those of Ph-HQ/BB due to the increased rigidity and packing density of the polymer chain upon copolymerization with HQ. However, the F-value of injection molded specimens of Ph-HQ/HQ/BB (m/n = 50/50) was not much higher than those of Ph-HQ/BB, as shown in Table 19.4. 

Thus, the influential factors in obtaining high moduli for injection molded specimens are as follows (1) the stability of the liquid crystalline state, (2) the rigidity of the polymer chain, and (3) the packing density of the polymer chain. 

For a specific polymer, critical concentrations and temperatures depend on the solvent. In Fig. 15.42b the concentration condition has already been illustrated on the basis of solution viscosity. Much work has been reported on PpPTA in sulphuric acid and of PpPBA in dimethylacetamide/lithium chloride. Besides, Boerstoel (1998), Boerstoel et al. (2001) and Northolt et al. (2001) studied liquid crystalline solutions of cellulose in phosphoric acid. In Fig. 16.27 a simple example of the phase behaviour of PpPTA in sulphuric acid (see also Chap. 19) is shown (Dobb, 1985). In this figure it is indicated that a direct transition from mesophase to isotropic liquid may exist. This is not necessarily true, however, as it has been found that in some solutions the nematic mesophase and isotropic phase coexist in equilibrium (Collyer, 1996). Such behaviour was found by Aharoni (1980) for a 50/50 copolymer of //-hexyl and n-propylisocyanate in toluene and shown in Fig. 16.28. Clearing temperatures for PpPTA (Twaron or Kevlar , PIPD (or M5), PABI and cellulose in their respective solvents are illustrated in Fig. 16.29. The rigidity of the polymer chains increases in the order of cellulose, PpPTA, PIPD. The very rigid PIPD has a LC phase already at very low concentrations. Even cellulose, which, in principle, is able to freely rotate around the ether bond, forms a LC phase at relatively low concentrations. 

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. ... 

The effect of polymer-solvent interactions on the mesophase can be derived from the rigidity of the polymer chain, the critical concentration to form liquid crystalline phase, and relaxation studies. After shearing a rigid polymeric liquid crystal, a banded texture is formed in which the direction of the bands... 

However, the rigidity of the polymer chain is not the only factor to determine the pitch of the mesophase. Guo and Gray found that for acetylethylcellulose... 

The moduli of as-spun fibers of polyaiylates depend highly upon the stability of die liquid ciystalline state, die rigidity of the polymer chain, and the degree of elongational flow orientation (T -value). On die other hand, the moduli of injection molded specimens of polyai ylates depend upon die stability of the hquid ci-ystalline state, and the rigidity and packing density of the polymer chain. 

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