Thermal curing mechanism of acetylene-terminated polyimides: A combination of density functional theory computation and microkinetic analysis

Abstract

The mechanical and thermal properties of acetylene-terminated polyimides are highly dependent on the cross-linked structures. However, the cross-links formed from arylethynyl groups are still elusive, which causes difficulty in tuning the performance of final resins. Therefore, fundamentally understanding the curing mechanisms and predicting the cured products of acetylene-terminated polyimides are of great importance for developing new high-temperature resins. We present quantum chemical computations together with microkinetic analysis on the thermal curing reaction of acetylene-terminated polyimides. The initial pathways of forming the naphthalenic dimer, benzenic trimer, and monoradicals were put forward, and then the radical polymerization of ethynyl groups initiated by monoradicals was investigated. Finally, an ab initio based microkinetic analysis was performed by using a kinetic Monte Carlo simulation. The computational results indicated that a higher curing temperature and lower initial concentration of arylethynyl groups result in a higher percentage of benzenic trimer and a decrease of polyenic chains, and the naphthalenic dimer is present in a minor fraction. The cyclobutenyl and cyclohexadienyl radicals formed by intramolecular cyclization of trans and cis growing chains, respectively, play an essential role in determining the composition of cured products. The results about the effect of curing conditions on cross-links are in good agreement with the existing experimental results. The work can also provide guidance for tuning the structure of final cured resins.