Abstract
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 der Waals energy (EvdW) played a crucial role in the segmental chain motion, which had an abrupt change near glass-transition temperature (Tg). 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 (Tg > 731 K) when compared with the –CH3 substituent at the same position, by highly distorting (89.7°) the conformation of the main chain.
Highlights
In recent years, interest grew in the development of thin, lightweight, and unbreakable new-generation optoelectronic devices, with transparent flexible plastic substrates for portable devices, and roll-up and conformable displays [1,2]
The flexible plastic substrate must be able to withstand the high temperatures of the thin-film transistor (TFT)-driven active matrix manufacturing processes, which can exceed 623 K for liquid crystal display (LCD), or active matrix organic light-emitting display (AMOLED) devices
The mean-square displacements, potential energies, and torsion angle distributions of the C–N imide bonds of poly(ester imide)s (PEsIs) models were calculated to explain the influence of the substituents on the thermo-chemical characteristics of the PEsI system
Summary
Interest grew in the development of thin, lightweight, and unbreakable new-generation optoelectronic devices, with transparent flexible plastic substrates for portable devices, and roll-up and conformable displays [1,2]. Wholly aromatic polyimides (PIs) have outstanding thermal stability (Tg > 523 K), are mechanically tough, and exhibit high solvent resistance and good electrical properties [7,8,9,10,11,12] They have dark colors (pale yellow to brown) and poor optical transparency in optoelectronic applications [13,14]. Various approaches were developed to inhibit the CT interactions of conventional aromatic PIs for optoelectronic device applications, including the use of cycloaliphatic moieties, bulky pendant groups, electron-rich bridges (such as –O–), electron-withdrawing groups, and asymmetrical structures in their backbones [16,17,18,19] These methods generally result in poor thermal and mechanical properties.
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