Thermal analysis and infrared spectroscopic studies were performed to investigate the strength of intermolecular interactions of the polyimides derived from 5,4'-diamino-2-phenyl benzimidazole (DAPBI) and 5,4'-diamino-2-phenyl benzoxazole (DAPBO). Polyimide films were prepared based on biphenyltetracarboxylic dianhydride (BPDA) isomers, difference of glass transition temperatures (Delta T-g) between DAPBI and DAPBO type polyimides decreased consistently as the steric impedance increased, implying that polyimides containing benzimidazole group had higher degree of intermolecular interactions. Infrared spectra of polyimide films displayed that the imide carbonyl (C = O) stretching band for DAPBI derived polyimides shifted to lower frequency compared to the DAPBO counterpart. In the IR spectra of the copolyimides based on DAPBI and DAPBO, significant red shift was observed for C = O stretching band and T-g increased as a function of DAPBI content. This suggests the presence of strong intermolecular interactions due to the hydrogen bonding for the DAPBI based polyimides. (C) 2013 Elsevier Ltd. All rights reserved.