Aromatic polyhydrazide fiber of high modulus and high tenacity

High modulus and high tenacity aromatic polyhydrazide fiber composed of a copolymer, at least 85 mol % of the polymer chain of which consists essentially of repeating units from the group consisting of p-phenylene terephthalamide (PT), terethphal hydrazide (HT) and para-benzamide (A) in amounts within the range surrounded by a hexagon obtained by connecting with straight lines the points R, S, T, U, V and W in the triangular coordinates in the FIGURE, which is a ternary diagram of compositions consisting of (HT), (PT) and (A) and the points R, S, T, U, V and W therein represent mol % compositions as follows:

• (HT) 75 60 42.5 30 60 70 (PT) 0 0 15 40 40 30 (A) 25 40 42.5 30 0 0
• said copolymer also including not more than 15 mol % of units selected from the group consisting of (Ar.sub.1 --Z.sub.1) and (Ar.sub.1 --Z.sub.1 --Ar.sub.2 --Z.sub.2), wherein Ar.sub.1 and Ar.sub.2 are groups selected from the class consisting of ##SPC12##
• and a group obtained by replacing an H atom of these groups with an alkyl group or halogen, and Ar.sub.1 and Ar.sub.2 may be the same or different;
• Z.sub.1 and Z.sub.2 are groups selected from the class consisting of --CONH--, --NHCONH--and --CONH.NHCO--, and Z.sub.1 and Z.sub.2 may be the same or different.