Rigid-chain polyamides

B. Jingsheng, Y. Anji, Z. Shengqing, Z. Shufan and R Chang, Studies on the semi-rigid chain polyamide - poly(l,4-phenylene terephthalamide), J. Appl. Polym. Sci., 26 (1981) 1211-1220. 

Concentrated sulfuric acid or some other acids act as a universal solvent for aromatic rigid-chain polyamides. These acids form compounds of the salt type with the amide groups of the polymer, which also results in passage of the polymer into solution. 

A specific feature of rigid-chain polyamides related to the possibility of dissolution of these polymers only due to the very energetic interaction of the elementary units of the polymer with the solvent molecules is manifested in these systems. At low temperatures, salt compounds crystallize out in the form of crystal solvates [42] with a constant polymer-sulfuric acid molar ratio. At high temperatures, the crystal solvate melts and equilibrium involving a liquid-crystalline phase is attained. The polymer-acid complex in this example is thermally unstable and decomposes at relatively low temperatures. Melting of the compound with decomposition— incongruent melting—results in the appear-... 

The advent of th motropic LC polymers was stimulated by the creation of high-strength polyamide fibers in the formation of rigid-chain polyamides fi om... 

A major impetus was given to work, both academic and industrial, in the field of lyotropic systems by the development by duPont of commercial fibres having exceptionally high tensile strength and modulus through use of nematic anisotropic solutions of relatively rigid-chain aromatic polyamides. The earliest product to appear, Fibre B, was based upon poly (p-benzamide) (I)10), but was replaced by the fully commercial product, Kevlar, based upon poly (p-phenylene terephthalamide) (II) U). Arenka, from Akzo, also has the latter chemical repeating unit. 

The most important representatives of this group are the fully aromatic rigid chains, such as polyphenylenes, the fully aromatic polyesters ("arylates") and the fully aromatic polyamides ("aramides"). 

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 ... 

One of the basic problems confronting molecular composites is the difficulty of finding miscible combinations of rigid rod polymers with flexible chain polymers. Poly(p-phenylene benzobisthiazole)/poly(-2,5(6)-benzimidazole block copolymers have been reported by Tsai et al. [1985] and are noted to exhibit better processability and mechanical properties than the simple blends of the block copolymer constituents. Chang and Lee [1993] prepared poly(p-benzamide)/Pl block copolymers and reported on the liquid crystalline behavior. Such approaches would appear to have future implications. As an example PA e.g., PA-66) block copolymers with rigid rod polyamides could be prepared and used in blends with PA-66 to yield the desired molecular composite. 

Finally we mention that draw-induced mesophases have also been observed in some other semi-rigid chain polymers. Examples of such studies are cold drawing of glassy PEN [108,109] and oriented crystallization of PET/PEN copolymers [110, 111]. In all cases a smectic mesophase has been reported. In the absence of crystallization it can attain a degree of stability that resists decay from chain relaxation. This supports the view that the mesophase represents a thermodynamic state [101]. We expect that similar mesophases based on semi-rigid monomers can be observed in other polymers with intrinsic rigid building blocks, such as other aromatic polyesters and polyamides. 

In general, polymers with flexible chains, such as polypropylene and polyamide-6,6, flow easily, whereas those with rigid chains exhibit higher viscosity. Blending two polymers can significantly alter poly-... 

Our approach has been to look for amorphous fully aromatic polyamides, to optimize both T (by virtue of their rigid chains) and solvent resistance (by virtue of their high polarity and chain-chain interaction). It turned out that random copolymerization employing at least four monomers (in the four-comonomer case in close to equimolar amounts) was necessary and sufficient to render the aramid polyemer amorphous - the monomers had to be selected to an extent of at least half the total mass, from the group that would lead to some form of kinking in the chain. 

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