Preparation of polyamides via the phosphorylation reaction. II. Modification of wholly aromatic polyamides with trifunctional monomers



The trifunctional monomers trimesic acid (TMA) and 3,5-diaminobenzoic acid (DAB), each separately and in combination, were used in amounts of 2 to 5 mole-% in order to increase the molecular weight of aromatic polyamides prepared via the Yamazaki phosphorylation reaction. In general, under comparable conditions the use of DAB led to polymer having higher inherent viscosity (IV) values than did TMA. A typical polymer was the polyisophthalamide of 4,4′-methylenedianiline, MDA-I, prepared by the reaction at 100°C of 4,4′-methylenedianiline (MDA) with isophthalic acid (I) in N-methylpyrrolidione (containing 5% dissolved lithium chloride) and employing triphenyl phosphite as condensation agent with pyridine as catalyst. For molar substitutions of 0.0, 2.0, 2.5, and 3.0% of DAB, MDA–I having IV values respectively of 1.1, 1.3, 1.9, and 2.3 was obtained. The latter two samples have IV values in the same range as those obtained for MDA–I prepared from MDA and isophthaloyl chloride in dimethylacetamide (DMAc) via the low-temperature polycondensation method, which prior experience has shown yields polymers that are quite suitable for the spinning of good fibers. At 5 mole-% substitution of DAB, an IV of 3.7 was obtained but a large quantity of gel particles was observed on dissolving the sample in DMAc containing 5% LiCl, indicating that considerable crosslinking had probably occurred. The rod-like polymer poly-p-benzamide (PPB) was prepared in similar fashion to MDA-I from p-aminobenzoic acid and IV values of respectively 1.6, 2.3, 3.8, and 3.9 were obtained when 0.0, 2.0, 2.5, and 3.0 mole-% of mixed trifunctional monomers were present. A solution of the latter sample dissolved in concentrated sulfuric acid contained considerable gel, indicating that crosslinked polymer was probably produced. It would appear that the chain branching approach for producing high molecular weight PPB for spinning to fibers will not prove useful because PPB having an IV value of about 3 to 4 is considered only marginal for the production of commercial-quality fibers (even though PPB of IV value 1.6 can be spun to high-strength/high-modulus fibers) and because the requisite balance of strength and modulus to elongation to break are only obtained for fibers from PPB having an IV value above about 3.5, and preferably about 5 to 6.