Clarifying the effect of moisture absorption and high-temperature thermal aging on structure and properties of polyimide film at molecular dynamic level

Abstract

A series of pyromellitic dianhydride (PMDA)/4,4′-oxydianiline (ODA) polyimide (PI) films with different moisture content were experimentally prepared and corresponding PI/H2O composite models were constructed and analyzed based on molecular dynamic (MD) simulation. The elongation at break increased from 47.2% to 59.7% when the moisture content of the PI film reached 2%, which was attributed to the presence of water molecules that increased the mobility of molecular chains. The fractional free volume and XRD patterns of the composite models illustrated that the average packing distance of PI chains increased with moisture absorption and weakened intermolecular charge-transfer complex formation, therefore endowing the wet PI films better transmittance. The thermal decomposition mechanism of PMDA/ODA, 3,3′,4,4′-biphenyldianhydride (BPDA)/ODA and BPDA/p-phenylene diamine (PDA) based PI films were also investigated by thermogravimetric-infrared radiation (TG-IR) and MD simulation. The IR spectra of gas products indicated that the thermal decomposition of PI mainly generated CO2, CO, NH3 and HCN, and the PI with different chemical structures had the same thermal decomposition products. The MD results showed that the C–O bond in the ODA structural unit was broken first during the thermal decomposition, which could well explain that the BPDA/PDA exhibited better thermal stability compared with PMDA/ODA and BPDA/ODA. This work paved the way for the investigation of the macromolecular structural evolution of PI materials during moisture absorption and thermal aging process.