Investigation on cyclization process of co-polyimides containing 2-(4-aminophenyl)-5-aminobenzimidazole units

As one type of high-performance polymers, polyimides have been extensively used in various fields. The cyclization reaction plays a significant role in the change of the polymeric chemical structure and physical properties. In this work, a series of random co-poly(amic acid)s (PAAs) were synthesized by introducing an aromatic heterocyclic diamine monomer (2-(4-aminophenyl)-5-aminobenzimidazole (BIA)) into the homopolyimide backbones of pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA). Fourier transform infrared (FTIR) spectroscopy was employed to study the cyclization kinetics in the temperature range of 180–300°C. The results indicate that the addition of BIA decreases the cyclization rates, which is mainly attributed to the rigid-rod structure of the BIA. The mechanism of thermal imidization of the solid state co-PAA was also studied by FTIR using two-dimensional correlation spectroscopy method. The signs of the cross peaks in asynchronous spectra suggested that the imide-related modes changed prior to the amide mode, which indicates that cyclization occurred before the amide proton was abstracted. The result illustrates the truth that the introduction of BIA units into PMDA-ODA segments obviously increases the barrier for the meta-segment of PAA into corresponding polyimide, which is the essential reason for the decreasing cyclization rate.