Para-aromatic polyamides

The range of polymers which were found to be able to form liquid crystalhne systems has been considerably extended. Poly(Y-benzyl-L-glutamate) and its analogs, as well as para-aromatic polyamides, exhibit this property in solutions, which served the basis for relating them to lyotropic liquid crystals (see Sect. 2.1). Subsequently, the classes of polymers were found which exhibited such a transition during a change of temperature thermotropic liquid crystals). 

Theoretical calculations of dimensions of the chains of para-aromatic polyamides were made by Birshtein Persistence length was estimated within the limits of... 

For para-aromatic polyamides, the experimental results have also been obtained that confirm the existence of phase transitions from isotropic solutions to anisotropic ones via the region of coexistence of the two phases. This is illustrated by Fig. 5 (see also... 

In the heterophase region LC -f- C the system goes over to the solid state typical of crystalline substances. Usually this solidification of the system is treated as a limited solubility of a polymer. For para-aromatic polyamides the solid state lying... 

At the present time the range of rigid-chain polymers has considerably broadened. Now it includes helix-forming polypeptides, para-aromatic polyamides and polyesters, cellulose and its derivatives, polyisocyanates, and some other polymers. 

It should be taken into account that in very rigid chains, such as those of poly-(alkyl isocyanate)s and para-aromatic polyamides, apart from rotation about valence bonds another mechanism can contribute to flexibility the deformation of valence angles and bonds during thermal chain motion just as it should occur in ladder structures (p. 100). When several flexibility mechanisms exist, the resulting rigidity of the homopolymer chain can be evaluated if the flexibilities, resulting from different mechanisms, are considered to be additive and the following equations are used ... 

The decrease in the equilibrium rigidity of a para-aromatic polyamide molecule through incorporation of aromatic rings into the mete-position in the chain is also shown in Fig. 44. The points denote the experimental values of ln]/[r) for some high molecular weight copolymers... 

Hence, the tremendously high equilibrium rigidity and the ordered structure of para aromatic polyamides favouring the formation of mesophases in the concentrated polymer solutions and permitting their use for the manufacture of ultrahigh-modulus fibers is ensured by both the frans-structure of the amide groups and the para position of the phenyl rings. 

The values of a for some copolymers of meta- and para-aromatic polyamides for which the experimental values of In]/(r)l are shown in Fig. 44 can be determined similarly. If the simplifying assumption of the uniformity of structure of all repeating units in the copolymer chain is made (Fig. 45), each repeating unit can be replaced by two virtual bonds A = Aj/Z and 6 = 6f/Z where Aj and 6 j ate virtual bonds of the PMPhIPhA chain (Fig. 43). 

Kwolek SL, Memeger W, Van Trump JE (1987) In Lewin M (ed) Liquid crystalline para aromatic polyamides. VCH, New York, p 421... 

M. Iwama and T. Takahashi. Process for preparing para-aromatic polyamide paper. US Patent 6942757, assigned to Teijin Twaron B.V. (Arnhem, NL), September 13, 2005. 

Among polymer molecules exhibiting lyotropic mesomorphism the properties of synthetic polypeptides, poly-alkylisocyanates, cellulose derivatives and para-aromatic polyamides have been investigated in greatest detail. 

In the first conformation (Fig. 7) all possible axes of rotation are parallel as in the crankshaft structure characteristic of para-aromatic polyamides. This molecular conformation would ensure very high rigidity of the entire molecule and high optical anisotropy. In the second conformation (Fig. 7b), the axes of rotation are placed at an angle to each other which would lead to high flexibility of the molecule and a lower value of... 

These theoretical considerations have led to the following view on a nematic solution, in particular of a solution of a para-aromatic polyamide in sulfuric acid [38]. In a quiescent solution of a lyotropic polymer the chains are more or less aligned parallel inside domains of microscopic size, see Fig. 3. The degree of orientation inside the domain, as represented by the order parameter (Pj), is determined by the concentration and temperature. The excluded volume entropy term leads to the formation of oriented blobs with a size of the order of Lp, the persistence length. These blobs line up due to their anisotropic polarizability, which implies that the formation of the anisotropic phase is governed by a dipole-dipole type of interaction, immediately leading to the Maier-Saupe mean-field potential. The entropy or excluded volume interaction merely tells us... 

Conio et al. studied the fiber formation of PpBA from an organic solvent, viz. JV,JV -dimethylacetamide containing 3% LiCl, but in these experiments an air gap was hardly used [86]. This may be the reason that they arrived at somewhat different conclusions concerning the spinning of para-aromatic polyamide solutions. 

Fibers are identified here by their chemical names. In this respect it is useful to know that Twaron and Kevlar are the trade names for PpPTA fibers, and that PRD-49 is a fiber made from PpBA. The para-aromatic polyamide fibers PpPTA, PpBAT and PpBA are often called aramid fibers. 

A distinctive structural feature of PpPTA and other para-aromatic polyamide fibres is the pleated sheet, which is shown in Fig. 13c, 13d and 13e. It was for the first time observed by Ballou in dark-field images taken from meridional reflections of the ED pattern [132]. This phenomenon has been extensively studied by Dobb et al. [125]. The pleated sheet consists of parallel-oriented chains, which are hydrogen bonded along the direction of the crystallographic b-axis. They are usually oriented perpendicular to the surface of the filament, i.e., in the circular cross-section the b-axes are directed preferably along the radius, as shown in Fig. 14. The angle between adjacent planes of the pleat is about 170° and the distanee between two pleats is about 250 nm, but both values may vary along the radius of the fibre cross-section. 

Table 2. Observed refractive indices of para-aromatic polyamide fibers and a PBO fiber, radially lateral, n, tangentially lateral, Hji lateral, n longitudinal. The results of Weeda were obtained by matching immersion liquids under the interference microscope, or from measuring the retardation [139]. For PBO fiber ii was estimated from a birefringence determination...

Table 3. Modulus (E), strength (O],), elongation at break (8],) of filaments and the density (q) of para-aromatic polyamide, PBO and PBT fibers...

As discussed previously para-aromatic polyamide fibers show a pleated sheet, which can also be described as a sinusoidal undulation of the fibrils. The effective orientation parameter determining the modulus of the fiber may then be approximated by... 

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