CHAIN RIGIDITY OF SUBSTITUTED AROMATIC POLYAMIDES

Static and dynamic light scattering and viscosity techniques were used for a dilute solution study of fractions of two aromatic polyamides having a phenyl substituent on every fourth ring. With respect to their unsubstituted analogues, poly(p-benzamide) (PBA) and poly(p-phenylenediamine - terephthalic acid) (PPTA) respectively, PA-2 and PA-4 show increased solubility in organic solvents and smaller persistence length, q. The average value of q for PA-2 in DMAC + 1 % LiCl is 123 +/- 52 angstrom, whereas for PBA q ranges from 240 to 750 angstrom. The average value of q for PA-4 in DMAC + 4 % LiCl is 85 +/- 26 A, as opposed to a range from 150 to 290 angstrom for PPTA. We conclude that even one phenyl substituent on every four rings has a significant effect in making the chain more flexible. This occurs when substitution is either on the terephthalic acid or on the diamine moiety. The chains still retain the ability to form a mesophase in concentrated solutions.