Synthesis and properties of poly(benzoxazole imide)s derived from two isomeric diamines containing a benzoxazole moiety

Two isomeric diamine monomers, 2-(3-aminophenyl)benzo[d]oxazol-5-amine (2a) and 2-(3-aminophenyl)benzo[d]oxazol-6-amine (2b), were designed and synthesized. The corresponding novel series of poly(benzoxazole imide)s (PBOPIs) were prepared with commercial tetracarboxylic dianhydrides via thermal imidization. These series of PBOPIs showed great Tgs ranging from 285 to 363 °C, excellent thermal stability when performed at 5% weight loss temperatures (Td5%s) of 510–564 °C in N2 and good mechanical properties (tensile strengths of 103–126 MPa, tensile moduli of 2.9–3.7 GPa and elongations at break of 3.0–6.5%). Furthermore, controlled molecular weight (Mw) polymers PI-3a-PA and PI-3b-PA derived from 4,4′-oxydiphthalic anhydride (ODPA) with 2a and 2b demonstrated good melt processability with minimum complex viscosities of 2258 and 3444 Pa·s at 420 °C and low melt viscosity ratios of 1.29 and 1.93 at 400 °C, respectively. The PI-b series showed higher Tg & Td5% and lower transparency compared with the PI-a series due to the isomeric effect.