Synthesis and properties of poly(imide-benzoxazole) fibers from 4,4′-oxydiphthalic dianhydride in polyphosphoric acid

A series of copolyimides containing benzoxazole moieties in the main chain are synthesized through a one-pot method with aromatic dianhydrides and benzoxazole diamines in polyphosphoric acid. The benzoxazole diamines are prepared in advance and react with aromatic dianhydrides without separating polyphosphoric acid. The feasibility of this one-pot method for polymerization is confirmed by a model reaction between benzoxazole diamines and phthalic anhydride. The resulting compounds and copolymers are characterized by Fourier Translation Infrared Spectroscopy (FT-IR), ¹H nuclear magnetic resonance (¹H NMR) and thermogravimetric analyses (TGA). All copolymer were obtained at high yields. The high molecular weight was also obtained with the inherent viscosities (η_inh) ranging from 5.60 to 6.98 dL/g. Fibers spun from the copolymers solution are strong, and the tensile strengths are up to 2.45-3.12 GPa. TGA results show that the thermal degradation of the copolyimides takes place at around 570 °C in nitrogen. It confirms that the introduction of the benzoxazole moieties has improved the thermal stability and the mechanical property of the copolymers. The copolyimides containing benzoxazole moieties are ideal candidates for high performance fibers.