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

Abstract

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 T_gs ranging from 285 to 363 °C, excellent thermal stability when performed at 5% weight loss temperatures (T_d5%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 (M_w) 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 T_g & T_d5% and lower transparency compared with the PI-a series due to the isomeric effect.