Clarifying the effect of chemical structure on high-temperature resistance of polyimides based on DFT and ReaxFF based molecular dynamic simulation

In this work, the pyrolysis route and high-temperature resistance of 3,3', 4,4'-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine (p-PDA), BPDA/m-phenylenediamine (m-PDA), BPDA/3,5-diaminobenzoic acid (DABA), BPDA/4,4' -oxydianiline (ODA), pyromellitic dianhydride (PMDA)/ODA, and 4,4' -oxydiphthalic anhydride (ODPA)/ODA were studied by molecular simulation and thermogravimetric analysis (TGA). Simulation based on density functional theory (DFT) was used for isothermal pyrolysis simulation of single-chain block copolymer models; molecular dynamics (MD) based on reaction force field (ReaxFF) was used for isothermal pyrolysis simulation of condensed state models. Two novel and facile methods, named as fragment counting method and total substances counting method, respectively, were proposed to evaluate the high-temperature resistance and explore the pyrolysis mechanisms of PIs. The results show that the BPDA/p-PDA PI presents the best thermostability. Compared with p-PDA, introducing m-PDA and DABA promotes the cleavages of C-N bonds and benzene rings to impair the high-temperature stability of PIs molecular chains. In addition, carboxyl groups in DABA lead to the release of large amounts of CO2 during pyrolysis process. The cleavage of ether bonds in ODA and ODPA is observed at the early stage of pyrolysis, therefore leading to significant decrease of hightemperature resistance of PIs. PMDA slightly reduces the high-temperature resistance of PIs compared with BPDA when the diamine unit is ODA. Besides, 2 kinds of pyrolysis mechanisms are revealed by analyzing the proportion of volatiles in the by-products. In DABA-containing PIs, the pyrolysis process is dominated by the generation of volatiles and molecular chains breakage, and presents two-stage weight loss, which is considered as the first pyrolysis mechanism. The pyrolysis process of DABA-free PIs is dominated by molecular chains cleavage, and displays only one-stage weight loss, which is considered as the second mechanism. The above simulation results are in good agreement with the experimental results from TGA, proving the reliability of the simulation. Mayer bond order (MBO) analysis of the target PI models was performed, and it is found that varying the chemical structure will lead to significant changes of bond strength, which is suggested to be one of the important reasons affecting the high-temperature stability and pyrolysis process of PIs. This work is believed to be of great help to the molecular design of high-temperature resistance PIs.