Molecular design and property prediction of sterically confined polyimides for thermally stable and transparent materials

To meet the demand for next-generation flexible optoelectronic devices, it is crucial to accurately establish the chemical structure-property relationships of new optical polymer films from a theoretical point of view, prior to production. In the current study, computer-aided simulations of newly designed poly(ester imide)s (PEsIs) with various side groups (-H, -CH₃, and -CF₃) and substituted positions were employed to study substituent-derived steric effects on their optical and thermal properties. From calculations of the dihedral angle distribution of the model compounds, it was found that the torsion angle of the C-N imide bonds was effectively constrained by the judicious introduction of di-, tetra-, and hexa-substituted aromatic diamines with -CF₃ groups. A high degree of fluorination of the PEsI repeating units resulted in weaker intra- and intermolecular conjugations. Their behavior was consistent with the molecular orbital energies obtained using density functional theory (DFT). In addition, various potential energy components of the PEsIs were investigated, and their role in glass-transition behavior was studied. The van derWaals energy (EvdW) played a crucial role in the segmental chain motion, which had an abrupt change near glass-transition temperature (T_g). The more effective steric effect caused by -CF₃ substituents at the 3-position of the 4-aminophenyl group significantly improved the chain rigidity, and showed high thermal stability (T_g > 731 K) when compared with the -CH₃ substituent at the same position, by highly distorting (89.7°) the conformation of the main chain.