Carbon Dioxide Diffusion and Plasticization in Fluorinated Polyimides

Abstract

Extensive molecular dynamics (MD) simulations of 6FDA−6FpDA, 6FDA−6FmDA, and 6FDA−DAM polyimides with CO 2 weight percentages up to ∼30%, were carried out to characterize the atomic level features associated with CO 2 diffusivity in these glassy matrices. The fluorinated polyimide models were first loaded with CO 2 in increments of 2% in order to mimic the experimental procedure of progressive loading and to avoid the necessity of artificially preswelling the simulation boxes. The sorption phase was then followed by a progressive desorption phase in decrements of 2%. This work covered nominal CO 2 concentrations up to ∼200 cm 3 (STP) cm −3 and amounted to a total of more than 80 simulations of 5000 ps each at 308 K, as well as an additional 20 simulations at higher temperatures. In all cases, CO 2 trajectories display the basic hopping-type mechanism, i.e. a combination of oscillations within available free volumes in the polymer matrix associated with occasional jumps from one site to another. There are no long-lived interactions with either the polymer or with the other penetrants, and thus, CO 2 is a very mobile penetrant free to access any part of the matrix free-volume. Diffusion coefficients, D CO 2, at 308 K were estimated from a novel trajectory-extending kinetic Monte Carlo (TEKMC) method, which, based on the actual CO 2 trajectories during the MD production runs, allowed us to extend them by more than 3 orders of magnitude. These estimates of D CO 2 compare very well with those obtained by a high-temperature Arrhenius extrapolation approach and with experimental evidence. Activation energies for diffusion are also validated by experimental data. All three polyimide models are able to reproduce both experimental penetrant-induced plasticization and sorption−desorption hysteresis during the few nanoseconds time scale available to MD simulations. The D CO 2 are found to be very closely linked to the volume swelling−contraction behavior. They tend to remain low up to the start of plasticization and to be directly correlated to the gradual transition to an almost linear increase in volume at higher concentrations. The sorption−desorption hysteresis can be related to a fairly limited increase in polymer local mobility upon volume dilation, which means that the system is not able to come back to its initial structure upon desorption.