Effect of structure on the thermal stability of polyimides

Polyimides with different proportions of m‐phenylene and p‐phenylene (or p,p′‐biphenylene) were prepared by polymerizing different molar ratios of m‐phenylene diamine and p‐phenylene diamine (or p,p′‐diaminobiphenyl) with pyromellitic dianhydride in dimethylformamide at 0°C. Chemical cyclodehydration of polyamic acids resulted in the corresponding polyimides. Polymers were characterized by infrared (IR), viscosity, and density measurements. Viscosity and density of polymers decreased with an increase on m‐phenylene groups in the backbone. The thermal and thermooxidative stabilities were investigated by dynamic thermogravimetry. Stability decreased when m‐phenylene groups were introduced in the backbone.