Revealing the way of how modification of the chemical structure of a polymer affects its macroscopic physical properties offers an opportunity to develop novel polymer materials with pre-defined characteristics. To address this problem two thermoplastic polyimides, ULTEM and EXTEM, were simulated with small difference in chemical structures of monomer units, namely, the phenyl ring in ULTEM was replaced by the diphenylsulphone group in EXTEM. It is shown that such a small modification results in a drastic difference of the thermal properties: the glass transition temperature of EXTEM is higher than that of ULTEM. Our molecular-dynamics simulations clearly demonstrated that it is the electrostatic interactions that are responsible for the observed difference in thermal properties of ULTEM and EXTEM: large partial charges of the sulphone group in the EXTEM lead to strong dipole-dipole intra- and intermolecular interactions and correspondingly to an elevated glass transition temperature. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014, 52, 640-646