Poly[N,N'-(1,3-phenylene)isophthalamide]

Poly(m-phenylene isophthalamide) has an extremely high melting point (380-390°C) and cannot be melt processed by the usual means. Commercial material is supplied as fibre and as a paper and is used directly in these forms. Fibre is prepared by extruding a solution of the polymer in a mixture of dimethylformamide and lithium chloride into hot air. The aromatic polyamide papers are produced from a combinati~n of chopped fibres and chopped film (prepared continuously by interfacial polymerization).
At ordinary temperatures, poly(m-phenylene isophthalamide) has mechanical properties comparable to those of the aliphatic nylons but at elevated temperatures the aromatic polymer is greatly superior. Broadly, the mechanical properties of poly(m-phenylene isophthalamide) show little change up to about 200°C. The aromatic polymer also shows little change in electrical insulation properties over a similar temperature range. The polymer resists ignition and is free of after-glow. The chemical resistance of poly(m-phenylene isophthalamide) appears to be generally similar to that of the aliphatic nylons except that resistance to mineral acids is rather better. The fibre is used for heat and flame protective clothing. The paper has found use in electrical insulating applications where resistance to elevated temperatures is required.

Poly[N,N′-(1,3-phenylene)isophthalamide] is used in heat and flame protective clothing. Electrical insulating paper requiring high temperature stability.

Poly(m-phenylene isophthalamide is produced by reaction of isophthaloyl chloride and m-phenylenediamine:

This reaction occurs rapidly in the presence of an acid acceptor under mild conditions. The conventional melt polymerization techniques, as used for the preparation of nylons, cannot be applied to aromatic polyamides since the melting points of the polymers are too high. Polymerization is therefore conducted either in solution (e.g. in methylene chloride) or in suspension. In the latter case, the diamine is dissolved in water, together with an acid acceptor (e.g. sodium carbonate) and the diacid chloride is dissolved in a solvent which is immiscible with water (e.g. carbon tetrachloride or cyclohexanone). The two solutions are then subjected to intensive mixing. Rapid reaction occurs at the liquid interface or just inside the solvent boundary and this technique is therefore commonly termed interfacial polymerization.