A theoretical study of internal rotation in fragments of aromatic polyimide chains, containing ether and sulphide groups

Abstract

A theoretical examination has been carried out of internal rotation in fragments of polyimide chains containing phenylene-oxide-phenylene and phenylene-sulphide-phenylene groupings. Diphenyl ether (DPE) and diphenyl sulphide (DPS) were taken as models. For analysis of internal rotation the classical conformational method at different values of the torsional potential U OC about the OC bonds was used, as well as the EHT, MINDO/2, CNDO/2 and PCILO quantum chemical methods. The results are presented in the form of potential maps describing the dependence of the energy of DPE on the angles of internal rotation around the OC bonds. The potential maps take different forms depending on the value of U OC or the quantum chemical method used, at some values of U OC the classical maps being similar to the quantum chemical maps. Comparison of the potential maps with one another and with experimental measurements showed that the most realistic results are given by the CNDO/2 and EHT methods and the corresponding classical barriers. According to these maps the “propeller” equilibrium conformation is characteristic of DPE, in which internal rotation is fairly free. Potential maps were calculated for DPS by the EHT method and by the classical method for various values of U SC. The calculations showed that DPS also has the equilibrium propeller conformation, but hindrance to free rotation is greater in DPS than in DPE.