Synthesis, characterisation and properties of aramid copolymers prepared from AB and AB2 monomers

Abstract

Abstract Aromatic polyamide (aramid) copolymers having a highly branched architecture were prepared by direct polycondensation of an AB2 monomer (5-(4-aminobenzoylamino) isophthalic acid) with an AB monomer (N-(4-aminophenyl)terephthalamic acid). Two synthetic routes have been followed for copolymer preparation, differing from each other in the overall monomer content and the type of salts added to the reaction medium (LiCl or LiCl + CaCl2). Otherwise, both methods used the same conditions and the same condensing agent (triphenyl phosphite). The feed ratio of the monomers affected the copolymer solubility in aprotic polar solvents such as N,N-dimethylformamide, N-methylpyrrolidone and dimethyl sulfoxide. Only high AB2/AB monomer ratios allowed obtaining completely soluble polymers in the above media. Also copolymer intrinsic viscosity was influenced by that ratio as well as by the polymerisation routes. IR measurements gave an indication of polymer structure evolution as a function of the monomer feed ratio, while 1H NMR experiments verified the actual monomer composition in the synthesized polymers. The monomer feed ratio turned out to influence polymer properties, such as degree of crystallinity, thermal degradation temperature and liquid crystalline behaviour. Some of the characterisation techniques we used (1H and 31P NMR, IR, SEM-EDS) revealed the presence of phosphorus derivatives in the copolymers, despite extensive sample purification. This presence is coming from derivatives of the condensing agent, and is directly related to the content of AB2-type structures present in the copolymers.