Correlation between the High-Temperature Local Mobility of Heterocyclic Polyimides and Their Mechanical Properties

The present study provides insights into the changes of the mechanical properties of heterocyclic polymers which are directly connected to their local segmental mobility above the glass-transition point. By performing both fully atomistic molecular dynamic simulations and physical experimentation, we, for the first time, focus on the mechanical behavior of the thermoplastic polyimide R-BAPS with the repeating unit consisting of 1,3-bis(3′,4-dicarboxyphenoxy)benzene (dianhydride R) and 4,4′-bis(4″-aminophenoxy)biphenyl sulfone (diamine BAPS). The previous computer simulations of this polyimide established the significant role of the partial charges to interpret the experimental thermal properties of R-BAPS. The present study determines the influence of the electrostatic interactions on the local mobility of R-BAPS, which, in turn, is to a large extent responsible for its mechanical behavior in the glassy state. It is demonstrated that accounting for partial charges increases the average translational and orientational relaxation times by approximately 2 orders of magnitude as compared to the systems without partial charges. We show that this segmental mobility reduction above the glass transition leads to the improved polyimide mechanical properties in the glassy state. With proper accounting for partial charges in the simulations, the R-BAPS yield stress increases, and the Poisson's ratio is reduced, as compared to the systems without partial charges. At the same time, all the simulated samples show similar dependence of mechanical properties on the cooling and deformation rates. The Eyring theory formalism has been used to assess the plastic deformation-related kinetic properties. The interrelation between the activation energy during the plastic deformation and the thermal history (cooling rate) of the simulated samples is shown.